diff --git a/tidepool/flake.lock b/tidepool/flake.lock
index 138d5f2..f783170 100644
--- a/tidepool/flake.lock
+++ b/tidepool/flake.lock
@@ -8,10 +8,10 @@
       },
       "locked": {
         "dir": "foundation",
-        "dirtyRev": "cdc90a46565dadea3b3c8f673b8b55d4ff75ea92-dirty",
-        "dirtyShortRev": "cdc90a4-dirty",
-        "lastModified": 1718356339,
-        "narHash": "sha256-BSiyT1q5xSlO1DyNJlkRWI82WWxVtQ1Xy8zPOqdqUlU=",
+        "dirtyRev": "b3f9fe574e6ff545e4a6fe6ad5e90fc71baeece3-dirty",
+        "dirtyShortRev": "b3f9fe5-dirty",
+        "lastModified": 1718366115,
+        "narHash": "sha256-/ktNtlY/usvE1YaathJ0f6y60cv3gQjc32SMKApsp7Y=",
         "type": "git",
         "url": "file:../?dir=foundation"
       },
@@ -24,10 +24,10 @@
     "lib": {
       "locked": {
         "dir": "lib",
-        "dirtyRev": "cdc90a46565dadea3b3c8f673b8b55d4ff75ea92-dirty",
-        "dirtyShortRev": "cdc90a4-dirty",
-        "lastModified": 1718356339,
-        "narHash": "sha256-BSiyT1q5xSlO1DyNJlkRWI82WWxVtQ1Xy8zPOqdqUlU=",
+        "dirtyRev": "b3f9fe574e6ff545e4a6fe6ad5e90fc71baeece3-dirty",
+        "dirtyShortRev": "b3f9fe5-dirty",
+        "lastModified": 1718366115,
+        "narHash": "sha256-/ktNtlY/usvE1YaathJ0f6y60cv3gQjc32SMKApsp7Y=",
         "type": "git",
         "url": "file:../?dir=lib"
       },
diff --git a/tidepool/src/builders/basic.nix b/tidepool/src/builders/basic.nix
new file mode 100644
index 0000000..ae09616
--- /dev/null
+++ b/tidepool/src/builders/basic.nix
@@ -0,0 +1,73 @@
+{
+  lib,
+  config,
+}: let
+  cfg = config.builders.basic;
+
+  lib' = config.lib;
+
+  inherit (config) foundation;
+in {
+  config.builders = {
+    basic = {
+      executable = "${foundation.stage2-bash}/bin/bash";
+
+      build = package: let
+        phases = package.phases;
+        sorted = lib.dag.sort.topographic phases;
+
+        script =
+          lib.strings.concatMapSep "\n" (
+            entry:
+              if builtins.isFunction entry.value
+              then entry.value package
+              else entry.value
+          )
+          sorted.result;
+
+        system = package.platform.build;
+
+        deps = lib'.packages.dependencies.resolve package.deps system;
+      in
+        (builtins.trace script)
+        builtins.derivation (package.env
+          // {
+            inherit (package) name;
+            inherit script system;
+
+            passAsFile = ["script"];
+
+            SHELL = cfg.executable;
+
+            PATH = lib.paths.bin (
+              (lib'.packages.dependencies.getPackages "build.host" deps)
+              ++ [
+                foundation.stage2-bash
+                foundation.stage2-coreutils
+              ]
+            );
+
+            builder = cfg.executable;
+
+            args = [
+              "-e"
+              (builtins.toFile "bash-builder.sh" ''
+                export CONFIG_SHELL=$SHELL
+
+                # Normalize the NIX_BUILD_CORES variable. The value might be 0, which
+                # means that we're supposed to try and auto-detect the number of
+                # available CPU cores at run-time.
+                NIX_BUILD_CORES="''${NIX_BUILD_CORES:-1}"
+                if ((NIX_BUILD_CORES <= 0)); then
+                  guess=$(nproc 2>/dev/null || true)
+                  ((NIX_BUILD_CORES = guess <= 0 ? 1 : guess))
+                fi
+                export NIX_BUILD_CORES
+
+                bash -eux $scriptPath
+              '')
+            ];
+          });
+    };
+  };
+}
diff --git a/tidepool/src/builders/default.nix b/tidepool/src/builders/default.nix
new file mode 100644
index 0000000..7cb7012
--- /dev/null
+++ b/tidepool/src/builders/default.nix
@@ -0,0 +1,18 @@
+{
+  lib,
+  config,
+}: let
+  lib' = config.lib;
+in {
+  includes = [
+    ./basic.nix
+  ];
+
+  options = {
+    builders = lib.options.create {
+      description = "A set of builders that can be used to build packages.";
+      type = lib.types.attrs.of lib'.types.builder;
+      default.value = {};
+    };
+  };
+}
diff --git a/tidepool/src/export.nix b/tidepool/src/export.nix
index 560f6f9..1d879f8 100644
--- a/tidepool/src/export.nix
+++ b/tidepool/src/export.nix
@@ -4,11 +4,16 @@
   lib,
   config,
 }: let
+  lib' = config.lib;
 in {
   config = {
     exports = {
       lib = config.lib;
       modules = import ./modules.nix;
+
+      packages = {
+        example = lib'.packages.export "example.x";
+      };
     };
   };
 }
diff --git a/tidepool/src/lib/packages.nix b/tidepool/src/lib/packages.nix
index ab92d10..617a400 100644
--- a/tidepool/src/lib/packages.nix
+++ b/tidepool/src/lib/packages.nix
@@ -6,6 +6,47 @@
 in {
   config = {
     lib.packages = {
+      dependencies = {
+        getPackages = path: dependencies: let
+          resolved =
+            if builtins.isList path
+            then path
+            else lib.strings.split "." path;
+
+          attrs = lib.attrs.select resolved {} dependencies;
+        in
+          lib.attrs.mapToList (name: value: value.package) attrs;
+
+        resolve = dependencies: system:
+          builtins.mapAttrs
+          # Note that this does not correspond to the "host" and "target" platforms, but rather
+          # where the code is used and where it is intended to end up.
+          (
+            host: platforms:
+              builtins.mapAttrs
+              (
+                target: deps:
+                  builtins.mapAttrs
+                  (
+                    name: dependency:
+                      assert lib.errors.trace (dependency.namespace != null) "Namespace unknown for dependency `${name}`."; let
+                        targeted = lib'.packages.export [dependency.namespace name] system;
+                      in
+                        if targeted == null
+                        then builtins.throw "Dependency `${dependency.namespace}.${name}` does not support system `${system}`."
+                        else targeted
+                  )
+                  deps
+              )
+              platforms
+          )
+          dependencies;
+      };
+
+      ## Get a package from the package set by its path. The path can be either
+      ## a string or a list of strings that is used to access `config.packages.generic`.
+      ##
+      ## @type String | List String -> Package
       get = path: let
         resolved =
           if builtins.isList path
@@ -14,12 +55,16 @@ in {
 
         package = lib.attrs.selectOrThrow resolved config.packages.generic;
       in
-        assert lib.errors.trace (builtins.length resolved > 1) "Packages must have a namespace specified.";
+        assert lib.errors.trace (builtins.length resolved > 1) "Cannot get package without a namespace.";
           package
           // {
             namespace = lib.modules.override 99 (builtins.head resolved);
           };
 
+      ## Export a package by its path. Use this function with the `config.exports.packages.*`
+      ## options.
+      ##
+      ## @type String | List String -> String -> Package
       export = path: system: let
         resolved =
           if builtins.isList path
diff --git a/tidepool/src/lib/types.nix b/tidepool/src/lib/types.nix
index f068a29..c1c34c0 100644
--- a/tidepool/src/lib/types.nix
+++ b/tidepool/src/lib/types.nix
@@ -11,40 +11,40 @@ in {
           options = {
             name = {
               full = lib.options.create {
-                type = lib.types.string;
                 description = "The full name of the license.";
+                type = lib.types.string;
               };
 
               short = lib.options.create {
-                type = lib.types.string;
                 description = "The short name of the license.";
+                type = lib.types.string;
               };
             };
 
             spdx = lib.options.create {
+              description = "The SPDX identifier for the license.";
               type = lib.types.nullish lib.types.string;
               default.value = null;
-              description = "The SPDX identifier for the license.";
             };
 
             url = lib.options.create {
-              type = lib.types.nullish lib.types.string;
               description = "The URL for the license.";
+              type = lib.types.nullish lib.types.string;
             };
 
             free = lib.options.create {
+              description = "Whether the license is free.";
               type = lib.types.bool;
               default.value = true;
-              description = "Whether the license is free.";
             };
 
             redistributable = lib.options.create {
+              description = "Whether the license is allows redistribution.";
               type = lib.types.bool;
               default = {
                 text = "config.free";
                 value = config.free;
               };
-              description = "Whether the license is allows redistribution.";
             };
           };
         });
@@ -54,51 +54,51 @@ in {
       meta = lib.types.submodule {
         options = {
           description = lib.options.create {
+            description = "The description for the package.";
             type = lib.types.nullish lib.types.string;
             default.value = null;
-            description = "The description for the package.";
           };
 
           homepage = lib.options.create {
+            description = "The homepage for the package.";
             type = lib.types.nullish lib.types.string;
             default.value = null;
-            description = "The homepage for the package.";
           };
 
           license = lib.options.create {
+            description = "The license for the package.";
             type = lib.types.nullish config.lib.types.license;
             default.value = null;
-            description = "The license for the package.";
           };
 
           free = lib.options.create {
+            description = "Whether the package is free.";
             type = lib.types.bool;
             default.value = true;
-            description = "Whether the package is free.";
           };
 
           insecure = lib.options.create {
+            description = "Whether the package is insecure.";
             type = lib.types.bool;
             default.value = false;
-            description = "Whether the package is insecure.";
           };
 
           broken = lib.options.create {
+            description = "Whether the package is broken.";
             type = lib.types.bool;
             default.value = false;
-            description = "Whether the package is broken.";
           };
 
           main = lib.options.create {
+            description = "The main entry point for the package.";
             type = lib.types.nullish lib.types.string;
             default.value = null;
-            description = "The main entry point for the package.";
           };
 
           platforms = lib.options.create {
+            description = "The platforms the package supports.";
             type = lib.types.list.of lib.types.string;
             default.value = [];
-            description = "The platforms the package supports.";
           };
         };
       };
@@ -108,6 +108,7 @@ in {
 
         options = {
           build = lib.options.create {
+            description = "The build function which takes a package definition and creates a derivation.";
             type = lib.types.function lib.types.derivation;
           };
         };
@@ -120,6 +121,12 @@ in {
           ]);
         }));
 
+        versioned = lib.types.attrs.of (lib.types.submodule ({name}: {
+          freeform = lib.types.attrs.of (lib.types.submodule [
+            lib'.types.package.versioned'
+          ]);
+        }));
+
         # packages.<system>
         targeted = lib.types.attrs.of (lib.types.submodule ({name}: let
           system = name;
@@ -148,6 +155,7 @@ in {
           # packages.<system>.<cross>
           options.cross = lib.attrs.generate lib'.systems.doubles.all (cross:
             lib.options.create {
+              description = "A cross-compiling package set.";
               default.value = {};
               # packages.<system>.<cross>.<namespace>
               type = lib.types.attrs.of (lib.types.submodule (
@@ -177,7 +185,6 @@ in {
 
       dependencies = {
         generic = lib.types.attrs.of (lib.types.nullish lib'.types.package.generic);
-
         targeted = lib.types.attrs.of (lib.types.nullish lib'.types.package.targeted);
       };
 
@@ -188,13 +195,17 @@ in {
           name ? assert false; null,
           config,
         }: {
+          freeform = lib.types.any;
+
           options = {
             namespace = lib.options.create {
+              description = "The namespace for the package.";
               type = lib.types.nullish lib.types.string;
               default.value = null;
             };
 
             pname = lib.options.create {
+              description = "The program name for the package";
               type = lib.types.string;
               default = {
                 text = "<name> or \"unknown\"";
@@ -206,6 +217,139 @@ in {
             };
 
             version = lib.options.create {
+              description = "The version for the package.";
+              type = lib.types.nullish lib.types.version;
+              default.value = null;
+            };
+
+            meta = lib.options.create {
+              description = "Metadata for the package.";
+              type = lib'.types.meta;
+              default.value = {
+                name = config.pname;
+              };
+            };
+
+            phases = lib.options.create {
+              description = "The phases for the package.";
+              type = lib.types.dag.of (
+                lib.types.either
+                lib.types.string
+                (lib.types.function lib.types.string)
+              );
+              default.value = {};
+            };
+
+            env = lib.options.create {
+              description = "The environment for the package.";
+              type = lib.types.attrs.of lib.types.string;
+              default.value = {};
+            };
+
+            builder = lib.options.create {
+              description = "The builder for the package.";
+              type = lib'.types.builder;
+            };
+
+            deps = {
+              build = {
+                only = lib.options.create {
+                  description = "Dependencies which are only used in the build environment.";
+                  type = lib'.types.dependencies.generic;
+                  default.value = {};
+                };
+
+                build = lib.options.create {
+                  description = "Dependencies which are created in the build environment and are executed in the build environment.";
+                  type = lib'.types.dependencies.generic;
+                  default.value = {};
+                };
+
+                host = lib.options.create {
+                  description = "Dependencies which are created in the build environment and are executed in the host environment.";
+                  type = lib'.types.dependencies.generic;
+                  default.value = {};
+                };
+
+                target = lib.options.create {
+                  description = "Dependencies which are created in the build environment and are executed in the target environment.";
+                  type = lib'.types.dependencies.generic;
+                  default.value = {};
+                };
+              };
+
+              host = {
+                only = lib.options.create {
+                  description = "Dependencies which are only used in the host environment.";
+                  type = lib'.types.dependencies.generic;
+                  default.value = {};
+                };
+
+                host = lib.options.create {
+                  description = "Dependencies which are executed in the host environment.";
+                  type = lib'.types.dependencies.generic;
+                  default.value = {};
+                };
+
+                target = lib.options.create {
+                  description = "Dependencies which are executed in the host environment which produces code for the target environment.";
+                  type = lib'.types.dependencies.generic;
+                  default.value = {};
+                };
+              };
+
+              target = {
+                only = lib.options.create {
+                  description = "Dependencies which are only used in the target environment.";
+                  type = lib'.types.dependencies.generic;
+                  default.value = {};
+                };
+
+                target = lib.options.create {
+                  description = "Dependencies which are executed in the target environment.";
+                  type = lib'.types.dependencies.generic;
+                  default.value = {};
+                };
+              };
+            };
+
+            versions = lib.options.create {
+              description = "Available package versions.";
+              type = lib.types.attrs.of lib'.types.package.versioned;
+              default.value = {};
+            };
+          };
+        };
+
+        versioned = lib.types.submodule lib'.types.package.versioned';
+
+        versioned' = args @ {
+          name ? assert false; null,
+          config,
+        }: {
+          freeform = lib.types.any;
+
+          options = {
+            namespace = lib.options.create {
+              description = "The namespace for the package.";
+              type = lib.types.nullish lib.types.string;
+              default.value = null;
+            };
+
+            pname = lib.options.create {
+              description = "The program name for the package";
+              type = lib.types.string;
+              default = {
+                text = "<name> or \"unknown\"";
+                value =
+                  if args ? name
+                  then args.name
+                  else "unknown";
+              };
+            };
+
+            version = lib.options.create {
+              description = "The version for the package.";
               type = lib.types.nullish lib.types.version;
               default.value = null;
             };
@@ -218,6 +362,7 @@ in {
             };
 
             phases = lib.options.create {
+              description = "The phases for the package.";
               type = lib.types.dag.of (
                 lib.types.either
                 lib.types.string
@@ -227,32 +372,38 @@ in {
             };
 
             env = lib.options.create {
+              description = "The environment for the package.";
               type = lib.types.attrs.of lib.types.string;
               default.value = {};
             };
 
             builder = lib.options.create {
+              description = "The builder for the package.";
               type = lib'.types.builder;
             };
 
             deps = {
               build = {
                 only = lib.options.create {
+                  description = "Dependencies which are only used in the build environment.";
                   type = lib'.types.dependencies.generic;
                   default.value = {};
                 };
 
                 build = lib.options.create {
+                  description = "Dependencies which are created in the build environment and are executed in the build environment.";
                   type = lib'.types.dependencies.generic;
                   default.value = {};
                 };
 
                 host = lib.options.create {
+                  description = "Dependencies which are created in the build environment and are executed in the host environment.";
                   type = lib'.types.dependencies.generic;
                   default.value = {};
                 };
 
                 target = lib.options.create {
+                  description = "Dependencies which are created in the build environment and are executed in the target environment.";
                   type = lib'.types.dependencies.generic;
                   default.value = {};
                 };
@@ -260,16 +411,19 @@ in {
 
               host = {
                 only = lib.options.create {
+                  description = "Dependencies which are only used in the host environment.";
                   type = lib'.types.dependencies.generic;
                   default.value = {};
                 };
 
                 host = lib.options.create {
+                  description = "Dependencies which are executed in the host environment.";
                   type = lib'.types.dependencies.generic;
                   default.value = {};
                 };
 
                 target = lib.options.create {
+                  description = "Dependencies which are executed in the host environment which produces code for the target environment.";
                   type = lib'.types.dependencies.generic;
                   default.value = {};
                 };
@@ -277,11 +431,13 @@ in {
 
               target = {
                 only = lib.options.create {
+                  description = "Dependencies which are only used in the target environment.";
                   type = lib'.types.dependencies.generic;
                   default.value = {};
                 };
 
                 target = lib.options.create {
+                  description = "Dependencies which are executed in the target environment.";
                   type = lib'.types.dependencies.generic;
                   default.value = {};
                 };
@@ -289,7 +445,8 @@ in {
             };
 
             versions = lib.options.create {
-              type = lib.types.attrs.of lib'.types.packages.generic;
+              description = "Available package versions.";
+              type = lib.types.attrs.of lib'.types.package.versioned;
               default.value = {};
             };
           };
@@ -302,7 +459,10 @@ in {
           config,
         }: {
           options = {
+            freeform = lib.types.any;
+
             name = lib.options.create {
+              description = "The name of the package.";
               type = lib.types.string;
               default = {
                 text = "\${namespace}-\${pname}-\${version} or \${pname}-\${version}";
@@ -320,11 +480,13 @@ in {
             };
 
             namespace = lib.options.create {
+              description = "The namespace for the package.";
               type = lib.types.nullish lib.types.string;
               default.value = null;
             };
 
             pname = lib.options.create {
+              description = "The program name for the package.";
               type = lib.types.string;
               default = {
                 text = "<name> or \"unknown\"";
@@ -336,11 +498,13 @@ in {
             };
 
             version = lib.options.create {
+              description = "The version for the package.";
               type = lib.types.nullish lib.types.version;
               default.value = null;
             };
 
             meta = lib.options.create {
+              description = "Metadata for the package.";
               type = lib'.types.meta;
               default.value = {
                 name = config.pname;
@@ -349,22 +513,26 @@ in {
 
             platform = {
               build = lib.options.create {
+                description = "The build platform for the package.";
                 type = lib.types.nullish lib.types.string;
                 default.value = null;
               };
 
               host = lib.options.create {
+                description = "The host platform for the package.";
                 type = lib.types.nullish lib.types.string;
                 default.value = null;
               };
 
               target = lib.options.create {
+                description = "The target platform for the package.";
                 type = lib.types.nullish lib.types.string;
                 default.value = null;
               };
             };
 
             phases = lib.options.create {
+              description = "Build phases for the package.";
               type = lib.types.dag.of (
                 lib.types.either
                 lib.types.string
@@ -374,36 +542,43 @@ in {
             };
 
             env = lib.options.create {
+              description = "The environment for the package.";
               type = lib.types.attrs.of lib.types.string;
               default.value = {};
             };
 
             builder = lib.options.create {
+              description = "The builder for the package.";
               type = lib'.types.builder;
             };
 
             package = lib.options.create {
+              description = "The built derivation.";
               type = lib.types.derivation;
             };
 
             deps = {
               build = {
                 only = lib.options.create {
+                  description = "Dependencies which are only used in the build environment.";
                   type = lib'.types.dependencies.targeted;
                   default.value = {};
                 };
 
                 build = lib.options.create {
+                  description = "Dependencies which are created in the build environment and are executed in the build environment.";
                   type = lib'.types.dependencies.targeted;
                   default.value = {};
                 };
 
                 host = lib.options.create {
+                  description = "Dependencies which are created in the build environment and are executed in the host environment.";
                   type = lib'.types.dependencies.targeted;
                   default.value = {};
                 };
 
                 target = lib.options.create {
+                  description = "Dependencies which are created in the build environment and are executed in the target environment.";
                   type = lib'.types.dependencies.targeted;
                   default.value = {};
                 };
@@ -411,16 +586,19 @@ in {
 
               host = {
                 only = lib.options.create {
+                  description = "Dependencies which are only used in the host environment.";
                   type = lib'.types.dependencies.targeted;
                   default.value = {};
                 };
 
                 host = lib.options.create {
+                  description = "Dependencies which are executed in the host environment.";
                   type = lib'.types.dependencies.targeted;
                   default.value = {};
                 };
 
                 target = lib.options.create {
+                  description = "Dependencies which are executed in the host environment which produces code for the target environment.";
                   type = lib'.types.dependencies.targeted;
                   default.value = {};
                 };
@@ -428,11 +606,13 @@ in {
 
               target = {
                 only = lib.options.create {
+                  description = "Dependencies which are only used in the target environment.";
                   type = lib'.types.dependencies.targeted;
                   default.value = {};
                 };
 
                 target = lib.options.create {
+                  description = "Dependencies which are executed in the target environment.";
                   type = lib'.types.dependencies.targeted;
                   default.value = {};
                 };
@@ -440,7 +620,8 @@ in {
             };
 
             versions = lib.options.create {
-              type = lib.types.attrs.of lib'.types.packages.targeted;
+              description = "Available package versions.";
+              type = lib.types.attrs.of lib'.types.package.versioned;
               default.value = {};
             };
           };
diff --git a/tidepool/src/modules.nix b/tidepool/src/modules.nix
index e2b06d9..0b97ad5 100644
--- a/tidepool/src/modules.nix
+++ b/tidepool/src/modules.nix
@@ -1,4 +1,5 @@
 {
+  builders = ./builders;
   exports = ./exports;
   lib = ./lib;
   packages = ./packages;
diff --git a/tidepool/src/packages/aux/foundation.nix b/tidepool/src/packages/aux/foundation.nix
index ddab66d..5470462 100644
--- a/tidepool/src/packages/aux/foundation.nix
+++ b/tidepool/src/packages/aux/foundation.nix
@@ -6,8 +6,6 @@
   lib' = config.lib;
 in {
   config = {
-    exports.packages.example-x = lib'.packages.export "example.x";
-
     packages = {
       generic = {
         example = {
@@ -15,12 +13,7 @@ in {
             meta.platforms = ["i686-linux" "x86_64-linux"];
             version = "1.0.0";
 
-            builder.build = package:
-              derivation {
-                name = package.name;
-                builder = "/bin/sh";
-                system = package.platform.build;
-              };
+            builder = config.builders.basic;
 
             phases = {
               build = package: ''
@@ -31,6 +24,10 @@ in {
                 make install DESTDIR=$out
               '';
             };
+
+            versions = {
+              "latest" = config.packages.generic.example.x;
+            };
           };
         };
       };