diff --git a/lib/README.md b/lib/README.md
index 7de6e61..b14e1ab 100644
--- a/lib/README.md
+++ b/lib/README.md
@@ -75,12 +75,40 @@ in
 }
 ```
 
-Successful tests will return `true` while failing test will resolve with `false`.
+Successful tests will return `true` while failing test will resolve with `false`. You can run
+all tests with the following command:
+
+```shell
+./test.sh
+```
+
+If you want to run a specific test suite, you can run the command, specifying the directory
+to the tests file:
+
+```shell
+./test.sh $namespace
+```
+
+For example, to run the tests for only `attrs`, use the following command:
+
+```shell
+./test.sh attrs
+```
 
 ### Formatting
 
+> **Note:** To keep this flake light and keep its inputs empty we do not include a package
+> set which would provide a formatter. Instead please run `nix run nixpkgs#nixfmt-rfc-style`
+> until an improved solution is available.
+
 All code in this project must be formatted using the provided formatter in the `flake.nix`
-file. You can run this formatter using the command `nix fmt`.
+file. You can run this formatter using the command `nix fmt` (not currently available).
+
+### Code Quality
+
+In order to keep the project approachable and easy to maintain, certain patterns are not allowed.
+In particular, the use of `with` and `rec` are not allowed. Additionally, you should prefer the
+fully qualified name of a variable rather than creating intermediate ones using `inherit`.
 
 ### Adding Functionality
 
diff --git a/lib/default.test.nix b/lib/default.test.nix
new file mode 100644
index 0000000..9b7861e
--- /dev/null
+++ b/lib/default.test.nix
@@ -0,0 +1,107 @@
+let
+  lib = import ./default.nix;
+
+  root = ./.;
+
+  files = [
+    ./src/attrs/default.test.nix
+    ./src/bools/default.test.nix
+    ./src/errors/default.test.nix
+    ./src/fp/default.test.nix
+    ./src/generators/default.test.nix
+    ./src/importers/default.test.nix
+    ./src/lists/default.test.nix
+    ./src/math/default.test.nix
+    ./src/modules/default.test.nix
+    ./src/numbers/default.test.nix
+    ./src/options/default.test.nix
+    ./src/packages/default.test.nix
+    ./src/paths/default.test.nix
+    ./src/points/default.test.nix
+    ./src/strings/default.test.nix
+    ./src/types/default.test.nix
+    ./src/versions/default.test.nix
+  ];
+
+  resolve = file: let
+    imported = import file;
+    value =
+      if builtins.isFunction imported
+      then imported {inherit lib;}
+      else imported;
+    relative = lib.strings.removePrefix (builtins.toString root) (builtins.toString file);
+  in {
+    inherit file value;
+    relative =
+      if lib.strings.hasPrefix "/" relative
+      then "." + relative
+      else relative;
+    namespace = getNamespace file;
+  };
+
+  resolved = builtins.map resolve files;
+
+  getNamespace = path: let
+    relative = lib.strings.removePrefix (builtins.toString root) (builtins.toString path);
+    parts = lib.strings.split "/" relative;
+  in
+    if builtins.length parts > 2
+    then builtins.elemAt parts 2
+    else relative;
+
+  results = let
+    getTests = file: prefix: suite: let
+      nested = lib.attrs.mapToList (name: value: getTests file (prefix ++ [name]) value) suite;
+      relative = lib.strings.removePrefix (builtins.toString root) (builtins.toString file);
+    in
+      if builtins.isAttrs suite
+      then builtins.concatLists nested
+      else [
+        {
+          inherit prefix file;
+          name = builtins.concatStringsSep " > " prefix;
+          value = suite;
+          relative =
+            if lib.strings.hasPrefix "/" relative
+            then "." + relative
+            else relative;
+        }
+      ];
+
+    base =
+      builtins.map (entry: getTests entry.file [entry.namespace] entry.value) resolved;
+  in
+    builtins.concatLists base;
+
+  successes = builtins.filter (test: test.value) results;
+  failures = builtins.filter (test: !test.value) results;
+
+  total = "${builtins.toString (builtins.length successes)} / ${builtins.toString (builtins.length results)}";
+in
+  if failures == []
+  then let
+    message =
+      lib.strings.concatMapSep "\n"
+      (test: "✅ ${test.name}")
+      successes;
+  in ''
+    SUCCESS (${total})
+
+    ${message}
+  ''
+  else let
+    successMessage =
+      lib.strings.concatMapSep "\n"
+      (test: "✅ ${test.name}")
+      successes;
+    failureMessage =
+      lib.strings.concatMapSep "\n\n"
+      (test:
+        "❎ ${test.name}\n"
+        + "  -> ${test.relative}")
+      failures;
+  in ''
+    FAILURE (${total})
+
+    ${failureMessage}
+  ''
diff --git a/lib/src/attrs/default.nix b/lib/src/attrs/default.nix
index 9f1fcd6..4d47802 100644
--- a/lib/src/attrs/default.nix
+++ b/lib/src/attrs/default.nix
@@ -13,16 +13,18 @@ lib: {
     mergeRecursiveUntil = predicate: x: y: let
       process = path:
         builtins.zipAttrsWith (
-          key: values: let
-            currentPath = path ++ [key];
+          name: values: let
+            currentPath = path ++ [name];
             isSingleValue = builtins.length values == 1;
             isComplete =
               predicate currentPath
               (builtins.elemAt values 1)
               (builtins.elemAt values 0);
           in
-            if isSingleValue || isComplete
+            if isSingleValue
             then builtins.elemAt values 0
+            else if isComplete
+            then builtins.elemAt values 1
             else process currentPath values
         );
     in
@@ -43,13 +45,13 @@ lib: {
     ##
     ## @type (List String) -> a -> Attrs -> a | b
     select = path: fallback: target: let
-      key = builtins.head path;
+      name = builtins.head path;
       rest = builtins.tail path;
     in
       if path == []
       then target
-      else if target ? ${key}
-      then lib.attrs.select rest fallback target.${key}
+      else if target ? ${name}
+      then lib.attrs.select rest fallback target.${name}
       else fallback;
 
     ## Get a value from an attribute set by a path. If the path does not exist,
@@ -61,10 +63,12 @@ lib: {
       error = builtins.throw "Path not found in attribute set: ${pathAsString}";
     in
       if lib.attrs.has path target
-      then lib.attrs.select path target
+      then lib.attrs.select path null target
       else error;
 
-    # TODO: Document this.
+    ## Create a nested attribute set with a value as the leaf node.
+    ##
+    ## @type (List String) -> a -> Attrs
     set = path: value: let
       length = builtins.length path;
       process = depth:
@@ -80,13 +84,13 @@ lib: {
     ##
     ## @type (List String) -> Attrs -> Bool
     has = path: target: let
-      key = builtins.head path;
+      name = builtins.head path;
       rest = builtins.tail path;
     in
       if path == []
       then true
-      else if target ? ${key}
-      then lib.attrs.has rest target.${key}
+      else if target ? ${name}
+      then lib.attrs.has rest target.${name}
       else false;
 
     ## Depending on a given condition, either use the given value or an empty
@@ -98,43 +102,46 @@ lib: {
       then value
       else {};
 
-    ## Map an attribute set's keys and values to a list.
+    ## Map an attribute set's names and values to a list.
     ##
     ## @type Any a => (String -> Any -> a) -> Attrs -> List a
     mapToList = f: target:
-      builtins.map (key: f key target.${key}) (builtins.attrNames target);
+      builtins.map (name: f name target.${name}) (builtins.attrNames target);
 
-    # TODO: Document this.
+    ## Map an attribute set recursively. Only non-set leaf nodes will be mapped.
+    ##
+    ## @type (List String -> Any -> Any) -> Attrs -> Attrs
     mapRecursive = f: target:
       lib.attrs.mapRecursiveWhen (lib.fp.const true) f target;
 
-    # TODO: Document this.
+    ## Map an attribute set recursively when a given predicate returns true.
+    ## Only leaf nodes according to the predicate will be mapped.
+    ##
+    ## @type (Attrs -> Bool) -> (List String -> Any -> Any) -> Attrs -> Attrs
     mapRecursiveWhen = predicate: f: target: let
       process = path:
         builtins.mapAttrs (
-          key: value:
+          name: value:
             if builtins.isAttrs value && predicate value
-            then process (path ++ [key]) value
-            else f (path ++ [key]) value
+            then process (path ++ [name]) value
+            else f (path ++ [name]) value
         );
     in
       process [] target;
 
-    # TODO: Document this.
+    ## Filter an attribute set by a given predicate. The filter is only performed
+    ## on the base level of the attribute set.
+    ##
+    ## @type (String -> Any -> Bool) -> Attrs -> Attrs
     filter = predicate: target: let
-      keys = builtins.attrNames target;
-      process = key: let
-        value = target.${key};
+      names = builtins.attrNames target;
+      process = name: let
+        value = target.${name};
       in
-        if predicate key value
-        then [
-          {
-            name = key;
-            value = value;
-          }
-        ]
+        if predicate name value
+        then [{inherit name value;}]
         else [];
-      valid = builtins.concatMap process keys;
+      valid = builtins.concatMap process names;
     in
       builtins.listToAttrs valid;
   };
diff --git a/lib/src/attrs/default.test.nix b/lib/src/attrs/default.test.nix
index 00be372..301d74d 100644
--- a/lib/src/attrs/default.test.nix
+++ b/lib/src/attrs/default.test.nix
@@ -1,6 +1,79 @@
 let
   lib = import ./../default.nix;
 in {
+  "merge" = {
+    "merges two shallow sets" = let
+      expected = {
+        x = 1;
+        y = 2;
+      };
+
+      actual = lib.attrs.merge {x = 1;} {y = 2;};
+    in
+      expected == actual;
+
+    "overwrites values from the first set" = let
+      expected = {
+        x = 2;
+      };
+      actual = lib.attrs.merge {x = 1;} {x = 2;};
+    in
+      actual == expected;
+
+    "does not merge nested sets" = let
+      expected = {
+        x.y = 2;
+      };
+      actual = lib.attrs.merge {x.z = 1;} {x.y = 2;};
+    in
+      actual == expected;
+  };
+
+  "mergeRecursiveUntil" = {
+    "merges with predicate" = let
+      expected = {
+        x.y.z = 1;
+      };
+      actual =
+        lib.attrs.mergeRecursiveUntil
+        (path: x: y: lib.lists.last path == "z")
+        {x.y.z = 2;}
+        {x.y.z = 1;};
+    in
+      actual == expected;
+
+    "handles shallow merges" = let
+      expected = {
+        x.y.z = 1;
+      };
+      actual =
+        lib.attrs.mergeRecursiveUntil
+        (path: x: y: true)
+        {
+          x = {
+            y.z = 2;
+
+            a = false;
+          };
+        }
+        {x.y.z = 1;};
+    in
+      actual == expected;
+  };
+
+  "mergeRecursive" = {
+    "merges two sets deeply" = let
+      expected = {
+        x.y.z = 1;
+      };
+      actual =
+        lib.attrs.mergeRecursive
+        {x.y.z = 2;}
+        {x.y.z = 1;};
+    in
+      actual == expected;
+  };
+
   "select" = {
     "selects a nested value" = let
       expected = "value";
@@ -13,5 +86,244 @@ in {
         };
     in
       actual == expected;
+
+    "handles empty path" = let
+      expected = {
+        x = {
+          y = {
+            z = 1;
+          };
+        };
+      };
+      actual =
+        lib.attrs.select
+        []
+        null
+        {
+          x = {
+            y = {
+              z = 1;
+            };
+          };
+        };
+    in
+      actual == expected;
+
+    "handles fallback value" = let
+      expected = "fallback";
+      actual =
+        lib.attrs.select
+        ["x" "y" "z"]
+        expected
+        {};
+    in
+      actual == expected;
+  };
+
+  "selectOrThrow" = {
+    "selects a nested value" = let
+      expected = "value";
+      actual =
+        lib.attrs.selectOrThrow
+        ["x" "y" "z"]
+        {
+          x.y.z = expected;
+        };
+    in
+      actual == expected;
+
+    "handles empty path" = let
+      expected = {
+        x = {
+          y = {
+            z = 1;
+          };
+        };
+      };
+      actual =
+        lib.attrs.selectOrThrow
+        []
+        {
+          x = {
+            y = {
+              z = 1;
+            };
+          };
+        };
+    in
+      actual == expected;
+
+    "throws on nonexistent path" = let
+      actual =
+        lib.attrs.selectOrThrow
+        ["x" "y" "z"]
+        {};
+
+      evaluated = builtins.tryEval (builtins.deepSeq actual actual);
+    in
+      !evaluated.success;
+  };
+
+  "set" = {
+    "creates a nested set" = let
+      expected = {
+        x = {
+          y = {
+            z = 1;
+          };
+        };
+      };
+      actual = lib.attrs.set ["x" "y" "z"] 1;
+    in
+      actual == expected;
+
+    "handles empty path" = let
+      expected = 1;
+      actual = lib.attrs.set [] 1;
+    in
+      actual == expected;
+  };
+
+  "has" = {
+    "returns true for a nested value" = let
+      exists = lib.attrs.has ["x" "y" "z"] {x.y.z = 1;};
+    in
+      exists;
+
+    "returns false for a nonexistent value" = let
+      exists = lib.attrs.has ["x" "y" "z"] {};
+    in
+      !exists;
+
+    "handles empty path" = let
+      exists = lib.attrs.has [] {};
+    in
+      exists;
+  };
+
+  "when" = {
+    "returns the value when condition is true" = let
+      expected = "value";
+      actual = lib.attrs.when true expected;
+    in
+      actual == expected;
+
+    "returns an empty set when condition is false" = let
+      expected = {};
+      actual = lib.attrs.when false "value";
+    in
+      actual == expected;
+  };
+
+  "mapToList" = {
+    "converts a set to a list" = let
+      expected = [
+        {
+          name = "x";
+          value = 1;
+        }
+        {
+          name = "y";
+          value = 2;
+        }
+      ];
+      actual =
+        lib.attrs.mapToList
+        (name: value: {inherit name value;})
+        {
+          x = 1;
+          y = 2;
+        };
+    in
+      actual == expected;
+  };
+
+  "mapRecursiveWhen" = {
+    "maps a set recursively" = let
+      expected = {
+        x = {
+          y = {
+            z = 2;
+          };
+        };
+      };
+      actual =
+        lib.attrs.mapRecursiveWhen
+        (value: true)
+        (path: value: value + 1)
+        {
+          x = {
+            y = {
+              z = 1;
+            };
+          };
+        };
+    in
+      actual == expected;
+
+    "maps a set given a condition" = let
+      expected = {
+        x = {
+          y = {
+            z = 1;
+          };
+        };
+      };
+      actual =
+        lib.attrs.mapRecursiveWhen
+        (value: !(value ? z))
+        (path: value:
+          # We map before we get to a non-set value
+            if builtins.isAttrs value
+            then value
+            else value + 1)
+        {
+          x = {
+            y = {
+              z = 1;
+            };
+          };
+        };
+    in
+      actual == expected;
+  };
+
+  "mapRecursive" = {
+    "maps a set recursively" = let
+      expected = {
+        x = {
+          y = {
+            z = 2;
+          };
+        };
+      };
+      actual =
+        lib.attrs.mapRecursive
+        (path: value: value + 1)
+        {
+          x = {
+            y = {
+              z = 1;
+            };
+          };
+        };
+    in
+      actual == expected;
+  };
+
+  "filter" = {
+    "filters a set" = let
+      expected = {
+        y = 2;
+      };
+      actual =
+        lib.attrs.filter
+        (name: value: name == "y")
+        {
+          x = 1;
+          y = 2;
+        };
+    in
+      actual == expected;
   };
 }
diff --git a/lib/src/bools/default.nix b/lib/src/bools/default.nix
index 8455249..f15cf71 100644
--- a/lib/src/bools/default.nix
+++ b/lib/src/bools/default.nix
@@ -1,10 +1,21 @@
 lib: {
   bools = {
     into = {
+      ## Convert a boolean value into a string.
+      ##
+      ## @type Bool -> String
       string = value:
         if value
         then "true"
         else "false";
+
+      ## Convert a boolean into either the string "yes" or "no".
+      ##
+      ## @type Bool -> String
+      yesno = value:
+        if value
+        then "yes"
+        else "no";
     };
 
     ## Choose between two values based on a condition. When true, the first value
diff --git a/lib/src/bools/default.test.nix b/lib/src/bools/default.test.nix
new file mode 100644
index 0000000..89f2313
--- /dev/null
+++ b/lib/src/bools/default.test.nix
@@ -0,0 +1,161 @@
+let
+  lib = import ./../default.nix;
+in {
+  "into" = {
+    "string" = {
+      "handles true" = let
+        expected = "true";
+        actual = lib.bools.into.string true;
+      in
+        actual == expected;
+
+      "handles false" = let
+        expected = "false";
+        actual = lib.bools.into.string false;
+      in
+        actual == expected;
+    };
+
+    "yesno" = {
+      "handles true" = let
+        expected = "yes";
+        actual = lib.bools.into.yesno true;
+      in
+        actual == expected;
+      "handles false" = let
+        expected = "no";
+        actual = lib.bools.into.yesno false;
+      in
+        actual == expected;
+    };
+  };
+
+  "when" = {
+    "returns first value when true" = let
+      expected = "foo";
+      actual = lib.bools.when true expected "bar";
+    in
+      actual == expected;
+
+    "returns second value when false" = let
+      expected = "bar";
+      actual = lib.bools.when false "foo" expected;
+    in
+      actual == expected;
+  };
+
+  "and" = {
+    "returns true when both are true" = let
+      expected = true;
+      actual = lib.bools.and true true;
+    in
+      actual == expected;
+    "returns false when first is false" = let
+      expected = false;
+      actual = lib.bools.and false true;
+    in
+      actual == expected;
+    "returns false when second is false" = let
+      expected = false;
+      actual = lib.bools.and true false;
+    in
+      actual == expected;
+    "returns false when both are false" = let
+      expected = false;
+      actual = lib.bools.and false false;
+    in
+      actual == expected;
+  };
+
+  "and'" = let
+    getTrue = _: true;
+    getFalse = _: false;
+  in {
+    "returns true when both are true" = let
+      expected = true;
+      actual = lib.bools.and' getTrue getTrue null;
+    in
+      actual == expected;
+
+    "returns false when first is false" = let
+      expected = false;
+      actual = lib.bools.and' getFalse getTrue null;
+    in
+      actual == expected;
+
+    "returns false when second is false" = let
+      expected = false;
+      actual = lib.bools.and' getTrue getFalse null;
+    in
+      actual == expected;
+
+    "returns false when both are false" = let
+      expected = false;
+      actual = lib.bools.and' getFalse getFalse null;
+    in
+      actual == expected;
+  };
+
+  "or" = {
+    "returns true when both are true" = let
+      expected = true;
+      actual = lib.bools.or true true;
+    in
+      actual == expected;
+    "returns true when first is true" = let
+      expected = true;
+      actual = lib.bools.or true false;
+    in
+      actual == expected;
+    "returns true when second is true" = let
+      expected = true;
+      actual = lib.bools.or false true;
+    in
+      actual == expected;
+    "returns false when both are false" = let
+      expected = false;
+      actual = lib.bools.or false false;
+    in
+      actual == expected;
+  };
+
+  "or'" = let
+    getTrue = _: true;
+    getFalse = _: false;
+  in {
+    "returns true when both are true" = let
+      expected = true;
+      actual = lib.bools.or' getTrue getTrue null;
+    in
+      actual == expected;
+    "returns true when first is true" = let
+      expected = true;
+      actual = lib.bools.or' getTrue getFalse null;
+    in
+      actual == expected;
+    "returns true when second is true" = let
+      expected = true;
+      actual = lib.bools.or' getFalse getTrue null;
+    in
+      actual == expected;
+    "returns false when both are false" = let
+      expected = false;
+      actual = lib.bools.or' getFalse getFalse null;
+    in
+      actual == expected;
+  };
+
+  "not" = {
+    "returns false when true" = let
+      expected = false;
+      actual = lib.bools.not true;
+    in
+      actual == expected;
+
+    "returns true when false" = let
+      expected = true;
+      actual = lib.bools.not false;
+    in
+      actual == expected;
+  };
+}
diff --git a/lib/src/default.nix b/lib/src/default.nix
index 36e696f..33eb8e4 100644
--- a/lib/src/default.nix
+++ b/lib/src/default.nix
@@ -40,8 +40,8 @@ let
   mergeAttrsRecursiveUntil = predicate: x: y: let
     process = path:
       builtins.zipAttrsWith (
-        key: values: let
-          currentPath = path ++ [key];
+        name: values: let
+          currentPath = path ++ [name];
           isSingleValue = builtins.length values == 1;
           isComplete =
             predicate currentPath
diff --git a/lib/src/errors/default.nix b/lib/src/errors/default.nix
index d024b63..9522f78 100644
--- a/lib/src/errors/default.nix
+++ b/lib/src/errors/default.nix
@@ -1,5 +1,10 @@
 lib: {
   errors = {
+    ## Prints a message if the condition is not met. The result of
+    ## the condition is returned.
+    ##
+    ## @notest
+    ## @type Bool -> String -> Bool
     trace = condition: message:
       if condition
       then true
diff --git a/lib/src/errors/default.test.nix b/lib/src/errors/default.test.nix
new file mode 100644
index 0000000..00ebf8d
--- /dev/null
+++ b/lib/src/errors/default.test.nix
@@ -0,0 +1,3 @@
+let
+  lib = import ./../default.nix;
+in {}
diff --git a/lib/src/fp/default.nix b/lib/src/fp/default.nix
index 57fd5a2..152160c 100644
--- a/lib/src/fp/default.nix
+++ b/lib/src/fp/default.nix
@@ -21,7 +21,7 @@ lib: {
     ##
     ## @type (List (Any -> Any)) -> Any -> Any
     pipe = fs: (
-      x: builtins.foldl' (value: f: f x) x fs
+      x: builtins.foldl' (value: f: f value) x fs
     );
 
     ## Reverse the order of arguments to a function that has two parameters.
@@ -37,13 +37,20 @@ lib: {
     ## @type (a -> b -> c -> d -> e) -> d -> c -> b -> a -> e
     flip4 = f: a: b: c: d: f d c b a;
 
-    # TODO: Document this.
+    ## Get the arguments of a function or functor.
+    ## An attribute set is returned with the arguments as keys. The values
+    ## are `true` when the argument has a default value specified and `false`
+    ## when it does not.
+    ##
+    ## @type Function -> Attrs
     args = f:
       if f ? __functor
-      then f.__args__ or lib.fp.args (f.__functor f)
+      then f.__args__ or (lib.fp.args (f.__functor f))
       else builtins.functionArgs f;
 
-    # TODO: Document this.
+    ## Create a function that is called with only the arguments it specifies.
+    ##
+    ## @type Attrs a => (a -> b) -> a -> b
     withDynamicArgs = f: args: let
       fArgs = lib.fp.args f;
       common = builtins.intersectAttrs fArgs args;
diff --git a/lib/src/fp/default.test.nix b/lib/src/fp/default.test.nix
new file mode 100644
index 0000000..d1d352d
--- /dev/null
+++ b/lib/src/fp/default.test.nix
@@ -0,0 +1,139 @@
+let
+  lib = import ./../default.nix;
+in {
+  "id" = {
+    "returns its argument" = let
+      expected = "foo";
+      actual = lib.fp.id expected;
+    in
+      actual == expected;
+  };
+
+  "const" = {
+    "creates a function that returns its argument" = let
+      expected = "foo";
+      actual = lib.fp.const expected "bar";
+    in
+      actual == expected;
+  };
+
+  "compose" = {
+    "composes two functions" = let
+      f = x: x + 1;
+      g = x: x * 2;
+      expected = 5;
+      actual = lib.fp.compose f g 2;
+    in
+      actual == expected;
+  };
+
+  "pipe" = {
+    "pipes two functions" = let
+      f = x: x + 1;
+      g = x: x * 2;
+      expected = 5;
+      actual = lib.fp.pipe [g f] 2;
+    in
+      actual == expected;
+  };
+
+  "flip2" = {
+    "flips the arguments of a binary function" = let
+      f = a: b: a - b;
+      expected = 1;
+      actual = lib.fp.flip2 f 1 2;
+    in
+      actual == expected;
+  };
+
+  "flip3" = {
+    "flips the arguments of a ternary function" = let
+      f = a: b: c: a - b - c;
+      expected = 0;
+      actual = lib.fp.flip3 f 1 2 3;
+    in
+      actual == expected;
+  };
+
+  "flip4" = {
+    "flips the arguments of a quaternary function" = let
+      f = a: b: c: d: a - b - c - d;
+      expected = -2;
+      actual = lib.fp.flip4 f 1 2 3 4;
+    in
+      actual == expected;
+  };
+
+  "args" = {
+    "gets a functions attr set arguments" = let
+      expected = {
+        x = false;
+        y = true;
+      };
+      actual = lib.fp.args ({
+        x,
+        y ? null,
+      }:
+        null);
+    in
+      actual == expected;
+
+    "returns an empty set if the function has no attrs arguments" = let
+      expected = {};
+      actual = lib.fp.args (args: null);
+    in
+      actual == expected;
+
+    "supports functors" = let
+      expected = {
+        x = false;
+        y = true;
+      };
+
+      actual = lib.fp.args {
+        __functor = self: {
+          x,
+          y ? null,
+        }:
+          null;
+      };
+    in
+      actual == expected;
+
+    "supports cached functor arguments" = let
+      expected = {
+        x = false;
+        y = true;
+      };
+      actual = lib.fp.args {
+        __args__ = {
+          x = false;
+          y = true;
+        };
+        __functor = self: args:
+          null;
+      };
+    in
+      actual == expected;
+  };
+
+  "withDynamicArgs" = {
+    "applies a function with dynamic arguments" = let
+      expected = {x = true;};
+      actual = lib.fp.withDynamicArgs (args @ {x}: args) {
+        x = true;
+        y = true;
+      };
+    in
+      actual == expected;
+
+    "applies all arguments if none are specified" = let
+      expected = {
+        x = true;
+        y = true;
+      };
+      actual = lib.fp.withDynamicArgs (args: args) expected;
+    in
+      actual == expected;
+  };
+}
diff --git a/lib/src/generators/default.test.nix b/lib/src/generators/default.test.nix
new file mode 100644
index 0000000..2ef515e
--- /dev/null
+++ b/lib/src/generators/default.test.nix
@@ -0,0 +1,113 @@
+let
+  lib = import ./../default.nix;
+in {
+  "withRecursion" = {
+    "evaluates within a given limit" = let
+      expected = {
+        x = 1;
+      };
+      actual = lib.generators.withRecursion {limit = 100;} expected;
+    in
+      expected == actual;
+
+    "fails when the limit is reached" = let
+      expected = {
+        x = 1;
+      };
+      actual = lib.generators.withRecursion {limit = -1;} expected;
+      evaluated = builtins.tryEval (builtins.deepSeq actual actual);
+    in
+      !evaluated.success;
+
+    "does not fail when throw is disabled" = let
+      expected = {
+        x = "<unevaluated>";
+      };
+      actual =
+        lib.generators.withRecursion {
+          limit = -1;
+          throw = false;
+        }
+        {x = 1;};
+      evaluated = builtins.tryEval (builtins.deepSeq actual actual);
+    in
+      evaluated.success
+      && evaluated.value == expected;
+  };
+
+  "pretty" = {
+    "formats with defaults" = let
+      expected = ''
+        {
+          attrs = { };
+          bool = true;
+          float = 0.0;
+          function = <function>;
+          int = 0;
+          list = [ ];
+          string = "string";
+        }'';
+      actual = lib.generators.pretty {} {
+        attrs = {};
+        bool = true;
+        float = 0.0;
+        function = x: x;
+        int = 0;
+        list = [];
+        string = "string";
+        # NOTE: We are not testing `path` types because they can return out of store
+        # values which are not deterministic.
+        # path = ./.;
+      };
+    in
+      actual == expected;
+
+    "formats with custom prettifiers" = let
+      expected = ''
+        {
+          attrs = { };
+          bool = true;
+          custom = <custom>;
+          float = 0.0;
+          function = <function>;
+          int = 0;
+          list = [ ];
+          string = "string";
+        }'';
+      actual =
+        lib.generators.pretty {
+          allowCustomPrettifiers = true;
+        } {
+          attrs = {};
+          bool = true;
+          float = 0.0;
+          function = x: x;
+          int = 0;
+          list = [];
+          string = "string";
+          custom = {
+            value = 0;
+            __pretty__ = value: "<custom>";
+          };
+        };
+    in
+      actual == expected;
+
+    "formats with multiline disabled" = let
+      expected = "{ attrs = { }; bool = true; float = 0.0; function = <function>; int = 0; list = [ ]; string = \"string\"; }";
+      actual =
+        lib.generators.pretty {
+          multiline = false;
+        } {
+          attrs = {};
+          bool = true;
+          float = 0.0;
+          function = x: x;
+          int = 0;
+          list = [];
+          string = "string";
+        };
+    in
+      actual == expected;
+  };
+}
diff --git a/lib/src/importers/default.nix b/lib/src/importers/default.nix
index 833372d..8fe0967 100644
--- a/lib/src/importers/default.nix
+++ b/lib/src/importers/default.nix
@@ -2,11 +2,13 @@ lib: {
   importers = {
     ## Import a JSON file as a Nix value.
     ##
+    ## @notest
     ## @type Path -> a
     json = file: builtins.fromJSON (builtins.readFile file);
 
     ## Import a TOML file as a Nix value.
     ##
+    ## @notest
     ## @type Path -> a
     toml = file: builtins.fromTOML (builtins.readFile file);
   };
diff --git a/lib/src/importers/default.test.nix b/lib/src/importers/default.test.nix
new file mode 100644
index 0000000..00ebf8d
--- /dev/null
+++ b/lib/src/importers/default.test.nix
@@ -0,0 +1,3 @@
+let
+  lib = import ./../default.nix;
+in {}
diff --git a/lib/src/lists/default.nix b/lib/src/lists/default.nix
index 8a0f696..1d86b15 100644
--- a/lib/src/lists/default.nix
+++ b/lib/src/lists/default.nix
@@ -1,7 +1,11 @@
 lib: {
   lists = {
     from = {
-      # TODO: Document this.
+      ## Convert a value to a list. If the value is already a list,
+      ## it will be returned as-is. If the value is not a list, it
+      ## will be wrapped in a list.
+      ##
+      ## @type a | (List a) -> List a
       any = value:
         if builtins.isList value
         then value
@@ -11,7 +15,7 @@ lib: {
     sort = {
       ## Perform a natural sort on a list of strings.
       ##
-      ## @type List -> List
+      ## @type List String -> List String
       natural = list: let
         vectorize = string: let
           serialize = part:
@@ -27,21 +31,29 @@ lib: {
         builtins.map (x: builtins.elemAt x 1) (builtins.sort isLess prepared);
     };
 
-    # TODO: Document this.
+    ## Map a list using both the index and value of each item. The
+    ## index starts at 0.
+    ##
+    ## @type (Int -> a -> b) -> List a -> List b
     mapWithIndex = f: list:
       builtins.genList
       (i: f i (builtins.elemAt list i))
       (builtins.length list);
 
-    # TODO: Document this.
+    ## Map a list using both the index and value of each item. The
+    ## index starts at 1.
+    ##
+    ## @type (Int -> a -> b) -> List a -> List b
     mapWithIndex1 = f: list:
       builtins.genList
       (i: f (i + 1) (builtins.elemAt list i))
       (builtins.length list);
 
-    ## Compare two lists.
+    ## Compare two lists using a custom compare function. The compare
+    ## function is called for each element in the lists that need to
+    ## be compared.
     ##
-    ## @type (a -> b -> Int) -> List a -> List b -> Int
+    ## @type (a -> b -> -1 | 0 | 1) -> List a -> List b -> Int
     compare = compare: a: b: let
       result = compare (builtins.head a) (builtins.head b);
     in
@@ -60,7 +72,7 @@ lib: {
     ##
     ## @type List a -> a
     last = list:
-      assert lib.assertMsg (list != []) "List cannot be empty";
+      assert lib.errors.trace (list != []) "List cannot be empty";
         builtins.elemAt list (builtins.length list - 1);
 
     ## Slice part of a list to create a new list.
@@ -137,7 +149,9 @@ lib: {
         else [value]
       else [];
 
-    # TODO: Document this.
+    ## Count the number of items in a list that satisfy a given predicate.
+    ##
+    ## @type (a -> Bool) -> List a -> Int
     count = predicate: list:
       builtins.foldl' (
         total: value:
@@ -148,7 +162,9 @@ lib: {
       0
       list;
 
-    # TODO: Document this.
+    ## Remove duplicate items from a list.
+    ##
+    ## @type List -> List
     unique = list: let
       filter = result: value:
         if builtins.elem value result
diff --git a/lib/src/lists/default.test.nix b/lib/src/lists/default.test.nix
new file mode 100644
index 0000000..a4460e4
--- /dev/null
+++ b/lib/src/lists/default.test.nix
@@ -0,0 +1,174 @@
+let
+  lib = import ./../default.nix;
+in {
+  "from" = {
+    "any" = {
+      "returns a list containing the value" = let
+        expected = [1];
+        actual = lib.lists.from.any 1;
+      in
+        actual == expected;
+
+      "returns the value if the value was already a list" = let
+        expected = [1];
+        actual = lib.lists.from.any expected;
+      in
+        actual == expected;
+    };
+  };
+
+  "sort" = {
+    "natural" = {
+      "sorts a list of strings" = let
+        expected = ["1" "a" "a0" "a1" "b" "c"];
+        actual = lib.lists.sort.natural ["c" "a" "b" "a1" "a0" "1"];
+      in
+        actual == expected;
+    };
+  };
+
+  "mapWithIndex" = {
+    "maps a list using index 0" = let
+      expected = ["0: a" "1: b" "2: c"];
+      actual = lib.lists.mapWithIndex (i: v: "${builtins.toString i}: ${v}") ["a" "b" "c"];
+    in
+      actual == expected;
+  };
+
+  "mapWithIndex1" = {
+    "maps a list using index 1" = let
+      expected = ["1: a" "2: b" "3: c"];
+      actual = lib.lists.mapWithIndex1 (i: v: "${builtins.toString i}: ${v}") ["a" "b" "c"];
+    in
+      actual == expected;
+  };
+
+  "compare" = {
+    "compares two lists" = {
+      "returns -1 if the first list is smaller" = let
+        expected = -1;
+        actual = lib.lists.compare lib.numbers.compare [1 2 3] [1 2 4];
+      in
+        actual == expected;
+      "returns 1 if the first list is larger" = let
+        expected = 1;
+        actual = lib.lists.compare lib.numbers.compare [1 2 4] [1 2 3];
+      in
+        actual == expected;
+      "returns 0 if the lists are equal" = let
+        expected = 0;
+        actual = lib.lists.compare lib.numbers.compare [1 2 3] [1 2 3];
+      in
+        actual == expected;
+    };
+  };
+
+  "last" = {
+    "returns the last element of a list" = let
+      expected = 3;
+      actual = lib.lists.last [1 2 3];
+    in
+      actual == expected;
+
+    "fails if the list is empty" = let
+      actual = lib.lists.last [];
+      evaluated = builtins.tryEval actual;
+    in
+      !evaluated.success;
+  };
+
+  "slice" = {
+    "slices a list" = {
+      "slices a list from the start" = let
+        expected = [1 2];
+        actual = lib.lists.slice 0 2 [1 2 3];
+      in
+        actual == expected;
+      "slices a list from the end" = let
+        expected = [2 3];
+        actual = lib.lists.slice 1 3 [1 2 3];
+      in
+        actual == expected;
+      "slices a list from the middle" = let
+        expected = [2];
+        actual = lib.lists.slice 1 1 [1 2 3];
+      in
+        actual == expected;
+    };
+  };
+
+  "take" = {
+    "takes the first n elements" = let
+      expected = [1 2];
+      actual = lib.lists.take 2 [1 2 3];
+    in
+      actual == expected;
+  };
+
+  "drop" = {
+    "drops the first n elements" = let
+      expected = [3];
+      actual = lib.lists.drop 2 [1 2 3];
+    in
+      actual == expected;
+  };
+
+  "reverse" = {
+    "reverses a list" = let
+      expected = [3 2 1];
+      actual = lib.lists.reverse [1 2 3];
+    in
+      actual == expected;
+  };
+
+  "intersperse" = {
+    "intersperses a list with a separator" = let
+      expected = [1 "-" 2 "-" 3];
+      actual = lib.lists.intersperse "-" [1 2 3];
+    in
+      actual == expected;
+
+    "handles lists with less than 2 elements" = let
+      expected = [1];
+      actual = lib.lists.intersperse "-" [1];
+    in
+      actual == expected;
+  };
+
+  "range" = {
+    "returns a range of numbers" = let
+      expected = [1 2 3 4 5];
+      actual = lib.lists.range 1 5;
+    in
+      actual == expected;
+  };
+
+  "when" = {
+    "returns the list if the condition is true" = let
+      expected = [1 2 3];
+      actual = lib.lists.when true [1 2 3];
+    in
+      actual == expected;
+    "returns an empty list if the condition is false" = let
+      expected = [];
+      actual = lib.lists.when false [1 2 3];
+    in
+      actual == expected;
+  };
+
+  "count" = {
+    "counts the number of elements in a list" = let
+      expected = 2;
+      actual = lib.lists.count (value: value < 3) [1 2 3];
+    in
+      actual == expected;
+  };
+
+  "unique" = {
+    "removes duplicate elements" = let
+      expected = [1 2 3];
+      actual = lib.lists.unique [1 2 3 1 2 3];
+    in
+      actual == expected;
+  };
+}
diff --git a/lib/src/math/default.nix b/lib/src/math/default.nix
index c7486e3..45331a3 100644
--- a/lib/src/math/default.nix
+++ b/lib/src/math/default.nix
@@ -1,12 +1,16 @@
 lib: {
   math = {
-    # TODO: Document this.
+    ## Return the smaller of two numbers.
+    ##
+    ## @type Int -> Int -> Int
     min = x: y:
       if x < y
       then x
       else y;
 
-    # TODO: Document this.
+    ## Return the larger of two numbers.
+    ##
+    ## @type Int -> Int -> Int
     max = x: y:
       if x > y
       then x
diff --git a/lib/src/math/default.test.nix b/lib/src/math/default.test.nix
new file mode 100644
index 0000000..c9aa0cd
--- /dev/null
+++ b/lib/src/math/default.test.nix
@@ -0,0 +1,19 @@
+let
+  lib = import ./../default.nix;
+in {
+  "min" = {
+    "returns the smaller number" = let
+      expected = 1;
+      actual = lib.math.min 1 2;
+    in
+      actual == expected;
+  };
+
+  "max" = {
+    "returns the larger number" = let
+      expected = 2;
+      actual = lib.math.max 1 2;
+    in
+      actual == expected;
+  };
+}
diff --git a/lib/src/modules/default.nix b/lib/src/modules/default.nix
index b956596..ffe807a 100644
--- a/lib/src/modules/default.nix
+++ b/lib/src/modules/default.nix
@@ -3,6 +3,7 @@ lib: {
     from = {
       ## Create a module from a JSON file.
       ##
+      ## @notest
       ## @type Path -> Module
       json = file: {
         __file__ = file;
@@ -11,6 +12,7 @@ lib: {
 
       ## Create a module from a TOML file.
       ##
+      ## @notest
       ## @type Path -> Module
       toml = file: {
         __file__ = file;
@@ -19,7 +21,12 @@ lib: {
     };
 
     apply = {
-      # TODO: Document this.
+      ## Apply custom definitions such as `merge` and `when` to a definition.
+      ## Note that this function does not perform actiosn like `merge`, but
+      ## instead pulls out the merge contents to be processed by the module
+      ## system.
+      ##
+      ## @type Definition -> List (Definition | (List Definition))
       properties = definition:
         if lib.types.is "merge" definition
         then builtins.concatMap lib.modules.apply.properties definition.content
@@ -32,7 +39,11 @@ lib: {
           else []
         else [definition];
 
-      # TODO: Document this.
+      ## Apply overrides for a definition. This uses the priority system
+      ## to determine which definition to use. The most important (lowest
+      ## priority) choice will be used.
+      ##
+      ## @type List Definition -> { highestPriority :: Int, values :: List (Definition | (List Definition)) }
       overrides = definitions: let
         getPriority = definition:
           if lib.types.is "override" definition.value
@@ -64,7 +75,9 @@ lib: {
           definitions;
       };
 
-      # TODO: Document this.
+      ## Apply ordering for prioritized definitions.
+      ##
+      ## @type List Definition -> List Definition
       order = definitions: let
         normalize = definition:
           if lib.types.is "order" definition
@@ -80,7 +93,10 @@ lib: {
       in
         builtins.sort compare normalized;
 
-      # TODO: Document this.
+      ## Normalize the type of an option. This will set a default type if none
+      ## was provided.
+      ##
+      ## @type List String -> Option -> Option
       fixup = location: option:
         if option.type.getSubModules or null == null
         then
@@ -95,32 +111,43 @@ lib: {
             options = [];
           };
 
-      # TODO: Document this.
+      ## Invert the structure of `merge`, `when`, and `override` definitions so
+      ## that they apply to each individual attribute in their respective sets.
+      ## Note that this function _only_ supports attribute sets within specialized
+      ## definitions such as `when` and `override`. Other values like lists will
+      ## throw a type error.
+      ##
+      ## @type Definition -> List Definition
       invert = config:
         if lib.types.is "merge" config
         then builtins.concatMap lib.modules.apply.invert config.content
         else if lib.types.is "when" config
         then
           builtins.map
-          (builtins.mapAttrs (key: value: lib.modules.when config.condition value))
+          (builtins.mapAttrs (name: value: lib.modules.when config.condition value))
           (lib.modules.apply.invert config.content)
         else if lib.types.is "override" config
         then
           builtins.map
-          (builtins.mapAttrs (key: value: lib.modules.override config.priority value))
+          (builtins.mapAttrs (name: value: lib.modules.override config.priority value))
           (lib.modules.apply.invert config.content)
         else [config];
     };
 
     validate = {
-      # TODO: Document this.
+      ## Check that a module only specifies supported attributes.
+      ##
+      ## @type Attrs -> Bool
       keys = module: let
         invalid = builtins.removeAttrs module lib.modules.VALID_KEYS;
       in
         invalid == {};
     };
 
-    # TODO: Document this.
+    ## Modules only support certain keys at the root level. This list determines
+    ## the valid attributes that users can supply.
+    ##
+    ## @type List String
     VALID_KEYS = [
       "__file__"
       "__key__"
@@ -132,7 +159,10 @@ lib: {
       "meta"
     ];
 
-    # TODO: Document this.
+    ## Normalize a module to a standard structure. All other information will be
+    ## lost in the conversion.
+    ##
+    ## @type String -> String -> Attrs -> Module
     normalize = file: key: module: let
       invalid = builtins.removeAttrs module lib.modules.VALID_KEYS;
       invalidKeys = builtins.concatStringsSep ", " (builtins.attrNames invalid);
@@ -159,7 +189,11 @@ lib: {
       }
       else builtins.throw "Module `${key}` has unsupported attribute(s): ${invalidKeys}";
 
-    # TODO: Document this.
+    ## Convert a module that is either a function or an attribute set into
+    ## a resolved attribute set. If the module was a function then it will
+    ## be evaluated and the result will be returned.
+    ##
+    ## @type String -> Attrs | (Attrs -> Attrs) -> Attrs -> Attrs
     resolve = key: module: args: let
       dynamicArgs =
         builtins.mapAttrs
@@ -175,24 +209,38 @@ lib: {
       then lib.fp.withDynamicArgs module (args // dynamicArgs)
       else module;
 
-    # TODO: Document this.
+    ## The default priority to set for values that do not have one provided.
+    ##
+    ## @type Int
     DEFAULT_PRIORITY = 100;
 
     ## Allow for sorting the values provided to a module by priority. The
     ## most important value will be used.
     ##
-    ## @type Int -> a -> Priority a
+    ## @notest
+    ## @type Int -> a -> Attrs
     order = priority: value: {
       __type__ = "order";
       inherit priority value;
     };
 
     orders = {
-      # TODO: Document this.
+      ## Order a value before others.
+      ##
+      ## @notest
+      ## @type a -> Attrs
       before = lib.modules.order 500;
-      # TODO: Document this.
+
+      ## Use the default ordering for a value.
+      ##
+      ## @notest
+      ## @type a -> Attrs
       default = lib.modules.order 1000;
-      # TODO: Document this.
+
+      ## Order a value after others.
+      ##
+      ## @notest
+      ## @type a -> Attrs
       after = lib.modules.order 1500;
     };
 
@@ -201,36 +249,70 @@ lib: {
     ## @type List Module -> List (String | Path)
     getFiles = builtins.map (module: module.__file__);
 
-    # TODO: Document this.
+    ## Create a conditional value which is only used when the condition's
+    ## value is `true`.
+    ##
+    ## @notest
+    ## @type Bool -> a -> When a
     when = condition: content: {
       __type__ = "when";
       inherit condition content;
     };
 
-    # TODO: Document this.
+    ## Merge module attribute sets when evaluated in the module system.
+    ##
+    ## @notest
+    ## @type List Attrs -> Attrs
     merge = content: {
       __type__ = "merge";
       inherit content;
     };
 
-    # TODO: Document this.
+    ## Create a value override which can replace other definitions if it
+    ## has the higher priority.
+    ##
+    ## @notest
+    ## @type Int -> a -> Attrs
     override = priority: content: {
       __type__ = "override";
       inherit priority content;
     };
 
     overrides = {
-      # TODO: Document this.
+      ## Create an override used for setting the `default.value` from an
+      ## option. This uses the lowest priority of all predefined overrides.
+      ##
+      ## @notest
+      ## @type a -> Attrs
       option = lib.modules.override 1500;
-      # TODO: Document this.
+
+      ## Create a default override for a value. This uses a very low priority
+      ## so that it can easily be overridden.
+      ##
+      ## @notest
+      ## @type a -> Attrs
       default = lib.modules.override 1000;
-      # TODO: Document this.
+
+      ## Create a high priority override for a value. This is not the highest
+      ## priority possible, but it will override nearly everything else except
+      ## for very explicit cases.
+      ##
+      ## @notest
+      ## @type a -> Attrs
       force = lib.modules.override 50;
-      # TODO: Document this.
+
+      ## Create a high priority override intended to be used only for VM targets.
+      ## This allows for forcing certain values even if a user has otherwise
+      ## specified `lib.modules.overrides.force`.
+      ##
+      ## @notest
+      ## @type a -> Attrs
       vm = lib.modules.override 10;
     };
 
-    # TODO: Document this.
+    ## Combine multiple modules together.
+    ##
+    ## @type List String -> List Module -> { matched :: Attrs, unmatched :: List Definition }
     combine = prefix: modules: let
       getConfig = module:
         builtins.map
@@ -246,16 +328,15 @@ lib: {
         modules;
 
       process = prefix: options: configs: let
-        # TODO: Document this.
         byName = attr: f: modules:
           builtins.zipAttrsWith
-          (lib.fp.const builtins.concatLists)
+          (key: value: builtins.concatLists value)
           (builtins.map (
               module: let
                 subtree = module.${attr};
               in
                 if builtins.isAttrs subtree
-                then builtins.mapAttrs (key: f module) subtree
+                then builtins.mapAttrs (name: f module) subtree
                 else builtins.throw "Value for `${builtins.concatStringsSep "." prefix} is of type `${builtins.typeOf subtree}` but an attribute set was expected."
             )
             modules);
@@ -348,7 +429,7 @@ lib: {
         matched = builtins.mapAttrs (key: value: value.matched) resultsByName;
 
         unmatched =
-          builtins.mapAttrs (key: value: value.unmatched) resultsByName
+          builtins.mapAttrs (name: value: value.unmatched) resultsByName
           // builtins.removeAttrs definitionsByName' (builtins.attrNames matched);
       in {
         inherit matched;
@@ -374,7 +455,10 @@ lib: {
     in
       process prefix modules configs;
 
-    # TODO: Document this.
+    ## Run a set of modules. Custom arguments can also be supplied which will
+    ## be provided to all modules statically as they are not modifiable.
+    ##
+    ## @type { modules? :: List (Attrs | (Attrs -> Attrs)), args? :: Attrs, prefix? :: List String } -> { type :: Module, extend :: lib.modules.run, options :: Attrs, config :: Attrs, __module__ :: Attrs }
     run = settings @ {
       modules ? [],
       args ? {},
@@ -395,7 +479,6 @@ lib: {
           prefix = extensions.prefix or settings.prefix or [];
         };
 
-      # TODO: Document this.
       collect = let
         load = args: file: key: module: let
           moduleFromValue = lib.modules.normalize file key (lib.modules.resolve key module args);
@@ -532,7 +615,7 @@ lib: {
         declared =
           lib.attrs.mapRecursiveWhen
           (value: !(lib.types.is "option" value))
-          (key: value: value.value)
+          (name: value: value.value)
           options;
 
         freeform = let
diff --git a/lib/src/modules/default.test.nix b/lib/src/modules/default.test.nix
index 6ce3c10..b92d74b 100644
--- a/lib/src/modules/default.test.nix
+++ b/lib/src/modules/default.test.nix
@@ -1,7 +1,315 @@
 let
   lib = import ./../default.nix;
 in {
-  examples = {
+  "apply" = {
+    "properties" = {
+      "handles normal values" = let
+        expected = [{}];
+        actual = lib.modules.apply.properties {};
+      in
+        actual == expected;
+
+      "handles merge" = let
+        expected = [{x = 1;} {x = 2;}];
+        actual = lib.modules.apply.properties (lib.modules.merge [{x = 1;} {x = 2;}]);
+      in
+        actual
+        == expected;
+
+      "handles when" = let
+        expected = [[{x = 1;}]];
+        actual = lib.modules.apply.properties (lib.modules.when true [{x = 1;}]);
+      in
+        actual == expected;
+    };
+
+    "overrides" = {
+      "handles normal values" = let
+        expected = {
+          highestPriority = 100;
+          values = [
+            {
+              value = 1;
+            }
+          ];
+        };
+        actual = lib.modules.apply.overrides [
+          {
+            value = 1;
+          }
+        ];
+      in
+        actual == expected;
+
+      "handles overrides" = let
+        expected = {
+          highestPriority = 100;
+          values = [
+            {value = "world";}
+          ];
+        };
+        actual = lib.modules.apply.overrides [
+          {value = "world";}
+          {value = lib.modules.override 101 "hello";}
+        ];
+      in
+        actual == expected;
+    };
+
+    "order" = {
+      "handles normal values" = let
+        expected = [{}];
+        actual = lib.modules.apply.order [{}];
+      in
+        actual == expected;
+
+      "handles priority" = let
+        expected = [
+          {
+            value = 1;
+            priority = 10;
+          }
+          {
+            value = 3;
+            priority = 50;
+          }
+          {value = 2;}
+        ];
+        actual = lib.modules.apply.order [
+          {
+            value = 1;
+            priority = 10;
+          }
+          {value = 2;}
+          {
+            value = 3;
+            priority = 50;
+          }
+        ];
+      in
+        actual == expected;
+    };
+
+    "fixup" = {
+      "sets default type for option" = let
+        actual = lib.modules.apply.fixup [] (
+          lib.options.create {}
+        );
+      in
+        actual.type.name == "Unspecified";
+    };
+
+    "invert" = {
+      "inverts merge" = let
+        expected = [{x = 1;} {x = 2;}];
+        actual =
+          lib.modules.apply.invert (lib.modules.merge [{x = 1;} {x = 2;}]);
+      in
+        actual
+        == expected;
+
+      "inverts when" = let
+        expected = [
+          {
+            x = {
+              __type__ = "when";
+              condition = true;
+              content = 1;
+            };
+            y = {
+              __type__ = "when";
+              condition = true;
+              content = 2;
+            };
+          }
+        ];
+        actual = lib.modules.apply.invert (lib.modules.when true {
+          x = 1;
+          y = 2;
+        });
+      in
+        actual == expected;
+
+      "inverts overrides" = let
+        expected = [
+          {
+            x = {
+              __type__ = "override";
+              priority = 100;
+              content = 1;
+            };
+            y = {
+              __type__ = "override";
+              priority = 100;
+              content = 2;
+            };
+          }
+        ];
+        actual = lib.modules.apply.invert (lib.modules.override 100 {
+          x = 1;
+          y = 2;
+        });
+      in
+        actual == expected;
+    };
+  };
+
+  "validate" = {
+    "keys" = {
+      "handles an empty set" = let
+        value = lib.modules.validate.keys {};
+      in
+        value;
+
+      "handles a valid module" = let
+        value = lib.modules.validate.keys {
+          __file__ = "virtual:aux/example";
+          __key__ = "aux/example";
+          includes = [];
+          excludes = [];
+          options = {};
+          config = {};
+          freeform = null;
+          meta = {};
+        };
+      in
+        value;
+
+      "handles an invalid module" = let
+        value = lib.modules.validate.keys {
+          invalid = null;
+        };
+      in
+        !value;
+    };
+  };
+
+  "normalize" = {
+    "handles an empty set" = let
+      expected = {
+        __file__ = "/aux/example.nix";
+        __key__ = "example";
+        config = {};
+        excludes = [];
+        includes = [];
+        options = {};
+      };
+      actual = lib.modules.normalize "/aux/example.nix" "example" {};
+    in
+      actual == expected;
+
+    "handles an example module" = let
+      expected = {
+        __file__ = "myfile.nix";
+        __key__ = "mykey";
+        config = {
+          x = true;
+        };
+        excludes = [];
+        includes = [];
+        options = {};
+      };
+      actual = lib.modules.normalize "/aux/example.nix" "example" {
+        __file__ = "myfile.nix";
+        __key__ = "mykey";
+        config.x = true;
+      };
+    in
+      actual == expected;
+  };
+
+  "resolve" = {
+    "handles an attribute set" = let
+      expected = {config.x = 1;};
+      actual = lib.modules.resolve "example" {config.x = 1;} {};
+    in
+      actual == expected;
+
+    "handles a function" = let
+      expected = {config.x = 1;};
+      actual = lib.modules.resolve "example" (lib.fp.const {config.x = 1;}) {};
+    in
+      actual == expected;
+
+    "handles a function with arguments" = let
+      expected = {config.x = 1;};
+      actual = lib.modules.resolve "example" (args: {config.x = args.x;}) {x = 1;};
+    in
+      actual == expected;
+  };
+
+  "getFiles" = {
+    "gets the files for a list of modules" = let
+      expected = ["/aux/example.nix"];
+      actual = lib.modules.getFiles [{__file__ = "/aux/example.nix";}];
+    in
+      actual
+      == expected;
+  };
+
+  "combine" = {
+    "handles empty modules" = let
+      expected = {
+        matched = {};
+        unmatched = [];
+      };
+      actual = lib.modules.combine [] [
+        (lib.modules.normalize "/aux/example.nix" "example" {})
+      ];
+    in
+      actual == expected;
+
+    "handles a single module" = let
+      expected = {
+        matched = {};
+        unmatched = [
+          {
+            __file__ = "/aux/example.nix";
+            prefix = ["x"];
+            value = 1;
+          }
+        ];
+      };
+      actual = lib.modules.combine [] [
+        (lib.modules.normalize "/aux/example.nix" "example" {
+          config = {
+            x = 1;
+          };
+        })
+      ];
+    in
+      actual == expected;
+
+    "handles multiple modules" = let
+      unmatched = [
+        {
+          __file__ = "/aux/example2.nix";
+          prefix = ["y"];
+          value = 2;
+        }
+      ];
+      actual = lib.modules.combine [] [
+        (lib.modules.normalize "/aux/example1.nix" "example2" {
+          options = {
+            x = lib.options.create {};
+          };
+
+          config = {
+            x = 1;
+          };
+        })
+        (lib.modules.normalize "/aux/example2.nix" "example2" {
+          config = {
+            y = 2;
+          };
+        })
+      ];
+    in
+      (actual.unmatched == unmatched)
+      && actual.matched ? x;
+  };
+
+  "run" = {
     "empty" = let
       evaluated = lib.modules.run {
         modules = [
@@ -73,5 +381,47 @@ in {
       actual = evaluated.config.aux.message;
     in
       actual == expected;
+
+    "conditional" = let
+      expected = "Hello, World!";
+      evaluated = lib.modules.run {
+        modules = [
+          {
+            options.aux = {
+              message = lib.options.create {
+                type = lib.types.string;
+              };
+            };
+            config = {
+              aux = {
+                message = lib.modules.when true expected;
+              };
+            };
+          }
+        ];
+      };
+    in
+      evaluated.config.aux.message == expected;
+
+    "conditional list" = let
+      expected = ["Hello, World!"];
+      evaluated = lib.modules.run {
+        modules = [
+          {
+            options.aux = {
+              message = lib.options.create {
+                type = lib.types.list.of lib.types.string;
+              };
+            };
+            config = {
+              aux = {
+                message = lib.modules.when true expected;
+              };
+            };
+          }
+        ];
+      };
+    in
+      evaluated.config.aux.message == expected;
   };
 }
diff --git a/lib/src/numbers/default.nix b/lib/src/numbers/default.nix
index 84f6965..7921cd1 100644
--- a/lib/src/numbers/default.nix
+++ b/lib/src/numbers/default.nix
@@ -49,7 +49,7 @@ lib: {
           then "F"
           else builtins.throw "Invalid hex digit.";
       in
-        lib.strings.concatMapSep
+        lib.strings.concatMap
         serialize
         (lib.numbers.into.base 16 value);
     };
diff --git a/lib/src/numbers/default.test.nix b/lib/src/numbers/default.test.nix
new file mode 100644
index 0000000..45ba2b8
--- /dev/null
+++ b/lib/src/numbers/default.test.nix
@@ -0,0 +1,57 @@
+let
+  lib = import ./../default.nix;
+in {
+  "into" = {
+    "string" = {
+      "converts an int into a string" = let
+        expected = "1";
+        actual = lib.numbers.into.string 1;
+      in
+        actual == expected;
+
+      "converts a float into a string" = let
+        expected = "1.0";
+        actual = lib.numbers.into.string 1.0;
+      in
+        actual == expected;
+    };
+
+    "base" = {
+      "converts a number into a given base" = let
+        expected = [1 0 0];
+        actual = lib.numbers.into.base 2 4;
+      in
+        actual == expected;
+    };
+
+    "hex" = {
+      "converts a number into a hex string" = let
+        expected = "64";
+        actual = lib.numbers.into.hex 100;
+      in
+        (builtins.trace actual)
+        actual
+        == expected;
+    };
+  };
+
+  "compare" = {
+    "returns -1 when first is less than second" = let
+      expected = -1;
+      actual = lib.numbers.compare 1 2;
+    in
+      actual == expected;
+
+    "returns 0 when first is equal to second" = let
+      expected = 0;
+      actual = lib.numbers.compare 1 1;
+    in
+      actual == expected;
+
+    "returns 1 when first is greater than second" = let
+      expected = 1;
+      actual = lib.numbers.compare 2 1;
+    in
+      actual == expected;
+  };
+}
diff --git a/lib/src/options/default.nix b/lib/src/options/default.nix
index 7d56b8d..c6fdd48 100644
--- a/lib/src/options/default.nix
+++ b/lib/src/options/default.nix
@@ -33,7 +33,9 @@ lib: {
         # TODO: Improve this error message to show the location and definitions for the option.
         else builtins.throw "Cannot merge definitions.";
 
-      # TODO: Document this.
+      ## Merge multiple option definitions together.
+      ##
+      ## @type Location -> Type -> List Definition
       definitions = location: type: definitions: let
         identifier = lib.options.getIdentifier location;
         resolve = definition: let
@@ -81,11 +83,14 @@ lib: {
         };
       };
 
+      ## Merge multiple option declarations together.
+      ##
+      ## @type Location -> List Option
       declarations = location: options: let
         merge = result: option: let
           mergedType = result.type.mergeType option.options.type.functor;
           isTypeMergeable = mergedType != null;
-          shared = key: option.options ? ${key} && result ? ${key};
+          shared = name: option.options ? ${name} && result ? ${name};
           typeSet = lib.attrs.when ((shared "type") && isTypeMergeable) {
             type = mergedType;
           };
@@ -212,9 +217,12 @@ lib: {
     ## @type List String -> String
     getIdentifier = location: let
       special = [
-        "<name>" # attrsOf (submodule {})
-        "*" # listOf (submodule {})
-        "<function body>" # functionTo
+        # lib.types.attrs.of (lib.types.submodule {})
+        "<name>"
+        # lib.types.list.of (submodule {})
+        "*"
+        # lib.types.function
+        "<function body>"
       ];
       escape = part:
         if builtins.elem part special
@@ -258,7 +266,9 @@ lib: {
     in
       lib.strings.concatMap serialize definitions;
 
-    # TODO: Document this.
+    ## Run a set of definitions, calculating the resolved value and associated information.
+    ##
+    ## @type Location -> Option -> List Definition -> String & { value :: Any, highestPriority :: Int, isDefined :: Bool, files :: List String, definitions :: List Any, definitionsWithLocations :: List Definition }
     run = location: option: definitions: let
       identifier = lib.options.getIdentifier location;
 
diff --git a/lib/src/options/default.test.nix b/lib/src/options/default.test.nix
new file mode 100644
index 0000000..00ebf8d
--- /dev/null
+++ b/lib/src/options/default.test.nix
@@ -0,0 +1,3 @@
+let
+  lib = import ./../default.nix;
+in {}
diff --git a/lib/src/packages/default.nix b/lib/src/packages/default.nix
index 94ee27a..da3a089 100644
--- a/lib/src/packages/default.nix
+++ b/lib/src/packages/default.nix
@@ -12,7 +12,7 @@ lib: {
         if builtins.stringLength value <= 207 && validate value != null
         then builtins.unsafeDiscardStringContext value
         else
-          lib.fp.pipe value [
+          lib.fp.pipe [
             # Get rid of string context. This is safe under the assumption that the
             # resulting string is only used as a derivation name
             builtins.unsafeDiscardStringContext
@@ -36,6 +36,7 @@ lib: {
               if builtins.stringLength x == 0
               then "unknown"
               else x)
-          ];
+          ]
+          value;
   };
 }
diff --git a/lib/src/packages/default.test.nix b/lib/src/packages/default.test.nix
new file mode 100644
index 0000000..00ebf8d
--- /dev/null
+++ b/lib/src/packages/default.test.nix
@@ -0,0 +1,3 @@
+let
+  lib = import ./../default.nix;
+in {}
diff --git a/lib/src/paths/default.test.nix b/lib/src/paths/default.test.nix
new file mode 100644
index 0000000..00ebf8d
--- /dev/null
+++ b/lib/src/paths/default.test.nix
@@ -0,0 +1,3 @@
+let
+  lib = import ./../default.nix;
+in {}
diff --git a/lib/src/points/default.nix b/lib/src/points/default.nix
index f51520b..153fc4b 100644
--- a/lib/src/points/default.nix
+++ b/lib/src/points/default.nix
@@ -17,10 +17,7 @@ lib: {
     ## definitions. Unlike `fix`, the resulting value is also given a `__unfix__`
     ## attribute that is set to the original function passed to `fix'`.
     ##
-    ## FIXME: The below type annotation should include a mention of the `__unfix__`
-    ## value.
-    ##
-    ## @type (a -> a) -> a
+    ## @type (a -> a) -> a & { __unfix__ :: (a -> a) }
     fix' = f: let
       x =
         f x
diff --git a/lib/src/points/default.test.nix b/lib/src/points/default.test.nix
new file mode 100644
index 0000000..00ebf8d
--- /dev/null
+++ b/lib/src/points/default.test.nix
@@ -0,0 +1,3 @@
+let
+  lib = import ./../default.nix;
+in {}
diff --git a/lib/src/strings/default.test.nix b/lib/src/strings/default.test.nix
new file mode 100644
index 0000000..00ebf8d
--- /dev/null
+++ b/lib/src/strings/default.test.nix
@@ -0,0 +1,3 @@
+let
+  lib = import ./../default.nix;
+in {}
diff --git a/lib/src/types/default.nix b/lib/src/types/default.nix
index f83bdb8..5d5bd28 100644
--- a/lib/src/types/default.nix
+++ b/lib/src/types/default.nix
@@ -151,7 +151,7 @@ lib: {
     # TODO: Document this.
     unspecified = lib.types.create {
       name = "Unspecified";
-      description = "unspecified value";
+      description = "unspecified type";
     };
 
     # TODO: Document this.
@@ -388,20 +388,20 @@ lib: {
           merge = location: definitions: let
             normalize = definition:
               builtins.mapAttrs
-              (key: value: {
+              (name: value: {
                 __file__ = definition.__file__;
                 value = value;
               })
               definition.value;
             normalized = builtins.map normalize definitions;
-            zipper = key: definitions:
-              (lib.options.merge.definitions (location ++ [key]) type definitions).optional;
+            zipper = name: definitions:
+              (lib.options.merge.definitions (location ++ [name]) type definitions).optional;
             filtered =
               lib.attrs.filter
-              (key: value: value ? value)
+              (name: value: value ? value)
               (builtins.zipAttrsWith zipper normalized);
           in
-            builtins.mapAttrs (key: value: value.value) filtered;
+            builtins.mapAttrs (name: value: value.value) filtered;
           getSubOptions = prefix: type.getSubOptions (prefix ++ ["<name>"]);
           getSubModules = type.getSubModules;
           withSubModules = modules: lib.types.attrs.of (type.withSubModules modules);
@@ -421,14 +421,14 @@ lib: {
           merge = location: definitions: let
             normalize = definition:
               builtins.mapAttrs
-              (key: value: {
+              (name: value: {
                 __file__ = definition.__file__;
                 value = value;
               })
               definition.value;
             normalized = builtins.map normalize definitions;
-            zipper = key: definitions: let
-              merged = lib.options.merge.definitions (location ++ [key]) type definitions;
+            zipper = name: definitions: let
+              merged = lib.options.merge.definitions (location ++ [name]) type definitions;
             in
               merged.optional.value or type.fallback.value or merged.merged;
           in
@@ -494,7 +494,14 @@ lib: {
                     j: value: let
                       resolved =
                         lib.options.merge.definitions
-                        (location ++ ["[definition ${builtins.toString i}-entry ${j}]"]);
+                        (location ++ ["[definition ${builtins.toString i}-entry ${j}]"])
+                        type
+                        [
+                          {
+                            file = definition.file;
+                            value = value;
+                          }
+                        ];
                     in
                       resolved.optional
                   )
@@ -503,7 +510,7 @@ lib: {
               definitions;
             merged = builtins.concatLists result;
             filtered = builtins.filter (definition: definition ? value) merged;
-            values = lib.optiosn.getDefinitionValues filtered;
+            values = lib.options.getDefinitionValues filtered;
           in
             values;
           getSubOptions = prefix: type.getSubOptions (prefix ++ ["*"]);
diff --git a/lib/src/types/default.test.nix b/lib/src/types/default.test.nix
new file mode 100644
index 0000000..00ebf8d
--- /dev/null
+++ b/lib/src/types/default.test.nix
@@ -0,0 +1,3 @@
+let
+  lib = import ./../default.nix;
+in {}
diff --git a/lib/src/versions/default.test.nix b/lib/src/versions/default.test.nix
new file mode 100644
index 0000000..00ebf8d
--- /dev/null
+++ b/lib/src/versions/default.test.nix
@@ -0,0 +1,3 @@
+let
+  lib = import ./../default.nix;
+in {}
diff --git a/lib/test.sh b/lib/test.sh
new file mode 100755
index 0000000..b6e1592
--- /dev/null
+++ b/lib/test.sh
@@ -0,0 +1,18 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash -p jq
+
+set -euo pipefail
+
+namespace=${1:-}
+
+if [ -z "$namespace" ]; then
+	nix eval -f ./default.test.nix --show-trace --raw
+else
+	if [ -d "./src/$namespace" ]; then
+		nix eval -f "./src/$namespace/default.test.nix" --show-trace --json | jq
+	else
+		echo "Namespace $namespace not found"
+		exit 1
+	fi
+fi
+