Get rid of cabal2nix

The `callCabal2nix` invocation caused `cabal2nix` to be downloaded on
every install of `niv`. This introduces `foo`, a Nix function that does
exactly the same.

default.nix

@@ -51,7 +51,7 @@ with rec
             pkgs.haskell.lib.disableExecutableProfiling (
               pkgs.haskell.lib.disableLibraryProfiling (
                 pkgs.haskell.lib.generateOptparseApplicativeCompletion "niv" (
-                  haskellPackages.callCabal2nix "niv" niv-source {}
+                  (pkgs.callPackage ./foo {}).buildPackage { root = ./.; src = niv-source; }
                 )
               )
             )

foo/default.nix

@@ -0,0 +1,83 @@
+{ haskellPackages
+, stdenv
+, lib
+}:
+
+{
+  buildPackage =
+    attrs:
+      let
+        src = if !lib.isDerivation attrs && lib.isAttrs attrs then attrs.src else attrs;
+        root = if !lib.isDerivation attrs && lib.isAttrs attrs then attrs.root else attrs;
+        nubdeps = ds: lib.lists.sort (x: y: x < y) (
+          lib.unique (
+            map (d: lib.head (lib.splitString " " d)) ds
+          )
+        );
+        spec = builtins.fromJSON (builtins.readFile (root + "/package.yaml"));
+        commonDeps = spec.dependencies;
+
+        libraryExtraDeps =
+          lib.optionals
+            (spec ? library && spec.library ? dependencies)
+            spec.library.dependencies;
+        libraryDeps = nubdeps (commonDeps ++ libraryExtraDeps);
+
+        exeExtraDeps = lib.optionals (spec ? executables) (
+          lib.concatMap
+            (
+              exe: lib.optionals
+                (exe ? dependencies) exe.dependencies
+            )
+            (builtins.attrValues spec.executables)
+        );
+        exeDeps =
+          nubdeps
+            (
+              builtins.filter (x: x != spec.name)
+                (commonDeps ++ exeExtraDeps)
+            );
+
+        testExtraDeps = lib.optionals (spec ? tests) (
+          lib.concatMap
+            (
+              test: lib.optionals
+                (test ? dependencies) test.dependencies
+            )
+            (builtins.attrValues spec.tests)
+        );
+        testDeps = nubdeps (builtins.filter (x: x != spec.name) (commonDeps ++ testExtraDeps));
+
+        depsFor = depType:
+          map (
+            d:
+              if ! builtins.hasAttr d haskellPackages
+              then throw "haskellPackages does not contain dependency '${d}' needed for '${depType}'"
+              else
+                haskellPackages.${d}
+          );
+
+      in
+        haskellPackages.callPackage (
+          { mkDerivation }:
+            haskellPackages.mkDerivation {
+              pname = spec.name;
+              version = spec.version;
+              inherit src;
+              isLibrary = builtins.hasAttr "library" spec;
+              isExecutable = builtins.hasAttr "executables" spec;
+              enableSeparateDataOutput = true;
+              libraryHaskellDepends = depsFor "libraryHaskellDepends" libraryDeps;
+              libraryToolDepends = [ haskellPackages.hpack ];
+              executableHaskellDepends = depsFor "executableHaskellDepends" exeDeps;
+              testHaskellDepends = depsFor "testHaskellDepends" testDeps;
+              prePatch = "hpack";
+              homepage = "https://github.com/${spec.github}#readme";
+              description = spec.synopsis;
+              license =
+                if builtins.hasAttr "license" spec && spec.license == "MIT"
+                then stdenv.lib.licenses.mit
+                else throw "Don't know how to handle license: ${builtins.toJSON spec.license}";
+            }
+        ) {};
+}

package.yaml

@@ -1,67 +1,73 @@
-name: niv
+{
-version: 0.2.13
+  "name": "niv",
-license: MIT
+  "version": "0.2.13",
-author: Nicolas Mattia <nicolas@nmattia.com>
+  "license": "MIT",
-maintainer: Nicolas Mattia <nicolas@nmattia.com>
+  "author": "Nicolas Mattia <nicolas@nmattia.com>",
-copyright: (c) 2019 Nicolas Mattia
+  "maintainer": "Nicolas Mattia <nicolas@nmattia.com>",
-category: Development
+  "copyright": "(c) 2019 Nicolas Mattia",
-github: nmattia/niv
+  "category": "Development",
-synopsis: Easy dependency management for Nix projects
+  "github": "nmattia/niv",
-description: Easy dependency management for Nix projects.
+  "synopsis": "Easy dependency management for Nix projects",
-
+  "description": "Easy dependency management for Nix projects.",
-ghc-options:
+  "ghc-options": [
-    - -Wall
+    "-Wall",
-
+    "-optP-Wno-nonportable-include-path"
-    # For macOS: https://github.com/gibiansky/IHaskell/issues/942
+  ],
-    - -optP-Wno-nonportable-include-path
+  "data-files": [
-
+    "nix/sources.nix"
-data-files:
+  ],
-    - nix/sources.nix
+  "extra-source-files": [
-
+    "README.md"
-extra-source-files:
+  ],
-    - README.md
+  "dependencies": [
-
+    "aeson",
-dependencies:
+    "aeson-pretty",
-    - aeson
+    "ansi-terminal",
-    - aeson-pretty
+    "base < 5",
-    - ansi-terminal
+    "bytestring",
-    - base < 5
+    "directory",
-    - bytestring
+    "file-embed",
-    - directory
+    "filepath",
-    - file-embed
+    "hashable",
-    - filepath
+    "http-conduit",
-    - hashable
+    "mtl",
-    - http-conduit
+    "optparse-applicative",
-    - mtl
+    "process",
-    - optparse-applicative
+    "profunctors",
-    - process
+    "pureMD5",
-    - profunctors
+    "string-qq",
-    - pureMD5
+    "text",
-    - string-qq
+    "unliftio",
-    - text
+    "unordered-containers"
-    - unliftio
+  ],
-    - unordered-containers
+  "library": {
-
+    "source-dirs": [
-library:
+      "src"
-    source-dirs:
+    ],
-        - src
+    "dependencies": [
-    dependencies:
+      "aeson",
-        - aeson
+      "tasty",
-        - tasty
+      "tasty-hunit",
-        - tasty-hunit
+      "unordered-containers"
-        - unordered-containers
+    ]
-
+  },
-executables:
+  "executables": {
-    niv:
+    "niv": {
-        main: Niv.main
+      "main": "Niv.main",
-        source-dirs: app
+      "source-dirs": "app",
-        dependencies:
+      "dependencies": [
-            - niv
+        "niv"
-
+      ]
-tests:
+    }
-    unit:
+  },
-        main: NivTest.main
+  "tests": {
-        source-dirs: app
+    "unit": {
-        dependencies:
+      "main": "NivTest.main",
-            - tasty
+      "source-dirs": "app",
-            - niv
+      "dependencies": [
+        "tasty",
+        "niv"
+      ]
+    }
+  }
+}