Add callStackToNix function (#116)

Allow building a stack project directly without needing to generate the scaffolding.

```nix
{ mkStackPkgSet, callStackToNix, ... }:
let
  pkgSet = mkStackPkgSet {
    stack-pkgs = callStackToNix { src = ./.; };
    pkg-def-extras = [];
    modules = [];
  };
in pkgSet.config.hsPkgs
```

call-stack-to-nix.nix

@@ -0,0 +1,20 @@
+/* The function obtained when this is applied to a package set calls
+ * the stack-to-nix tool on a supplied source set and then
+ * imports the resulting pkgs.nix. The application of this function
+ * to a source path can thus be used directly as the input to mkStackPackageSet
+ */
+{ nix-tools, pkgs }:
+{ src, stackYaml ? null }:
+let
+  pkgsNix = pkgs.stdenv.mkDerivation {
+    name = "pkgs-nix";
+    inherit src;
+    nativeBuildInputs = [ nix-tools pkgs.nix-prefetch-git ];
+    installPhase = ''
+      export LANG=C.utf8 # Needed or stack-to-nix will die on unicode inputs
+      mkdir -p $out
+      stack-to-nix --stack-yaml=$src/${if stackYaml == null then "stack.yaml" else stackYaml} -o $out
+      mv $out/pkgs.nix $out/default.nix
+    '';
+  };
+in import pkgsNix

default.nix

@@ -106,8 +106,12 @@ let
       };
 
     # Programs for generating Nix expressions from Cabal and Stack
-    # files.
-    nix-tools = self.callPackage ./nix-tools { inherit fetchExternal; };
+    # files. We need to make sure we build this from the buildPackages,
+    # we never want to actually cross compile nix-tools on it's own.
+    nix-tools = pkgs.buildPackages.callPackage ./nix-tools { inherit fetchExternal; inherit (self) mkCabalProjectPkgSet; };
+
+    # Function to call stackToNix
+    callStackToNix = self.callPackage ./call-stack-to-nix.nix {};
 
     # Snapshots of Hackage and Stackage, converted to Nix expressions,
     # regularly updated.

test/callStackToNix/.gitignore

@@ -0,0 +1,3 @@
+.stack-work/
+stack-simple.cabal
+*~

test/callStackToNix/Setup.hs

@@ -0,0 +1,2 @@
+import Distribution.Simple
+main = defaultMain

test/callStackToNix/app/Main.hs

@@ -0,0 +1,6 @@
+module Main where
+
+import Lib
+
+main :: IO ()
+main = someFunc

test/callStackToNix/default.nix

@@ -0,0 +1,33 @@
+{ stdenv, mkStackPkgSet, callStackToNix }:
+
+with stdenv.lib;
+
+let
+  pkgSet = mkStackPkgSet {
+    stack-pkgs = callStackToNix { src = ./.; };
+    pkg-def-extras = [];
+    modules = [];
+  };
+
+  packages = pkgSet.config.hsPkgs;
+
+in
+  stdenv.mkDerivation {
+    name = "callStackToNix-test";
+
+    buildCommand = ''
+      exe="${packages.stack-simple.components.exes.stack-simple-exe}/bin/stack-simple-exe"
+
+      printf "checking whether executable runs... " >& 2
+      $exe
+
+      touch $out
+    '';
+
+    meta.platforms = platforms.all;
+
+    passthru = {
+      # Attributes used for debugging with nix repl
+      inherit pkgSet packages;
+    };
+  }

test/callStackToNix/package.yaml

@@ -0,0 +1,48 @@
+name:                stack-simple
+version:             0.1.0.0
+github:              "githubuser/stack-simple"
+license:             BSD3
+author:              "Author name here"
+maintainer:          "example@example.com"
+copyright:           "2019 Author name here"
+
+extra-source-files:
+- README.md
+- ChangeLog.md
+
+# Metadata used when publishing your package
+# synopsis:            Short description of your package
+# category:            Web
+
+# To avoid duplicated efforts in documentation and dealing with the
+# complications of embedding Haddock markup inside cabal files, it is
+# common to point users to the README.md file.
+description:         Please see the README on GitHub at <https://github.com/githubuser/stack-simple#readme>
+
+dependencies:
+- base >= 4.7 && < 5
+
+library:
+  source-dirs: src
+
+executables:
+  stack-simple-exe:
+    main:                Main.hs
+    source-dirs:         app
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
+    dependencies:
+    - stack-simple
+
+tests:
+  stack-simple-test:
+    main:                Spec.hs
+    source-dirs:         test
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
+    dependencies:
+    - stack-simple

test/callStackToNix/src/Lib.hs

@@ -0,0 +1,6 @@
+module Lib
+    ( someFunc
+    ) where
+
+someFunc :: IO ()
+someFunc = putStrLn "someFunc"

test/callStackToNix/stack.yaml

@@ -0,0 +1,66 @@
+# This file was automatically generated by 'stack init'
+#
+# Some commonly used options have been documented as comments in this file.
+# For advanced use and comprehensive documentation of the format, please see:
+# https://docs.haskellstack.org/en/stable/yaml_configuration/
+
+# Resolver to choose a 'specific' stackage snapshot or a compiler version.
+# A snapshot resolver dictates the compiler version and the set of packages
+# to be used for project dependencies. For example:
+#
+# resolver: lts-3.5
+# resolver: nightly-2015-09-21
+# resolver: ghc-7.10.2
+#
+# The location of a snapshot can be provided as a file or url. Stack assumes
+# a snapshot provided as a file might change, whereas a url resource does not.
+#
+# resolver: ./custom-snapshot.yaml
+# resolver: https://example.com/snapshots/2018-01-01.yaml
+resolver: lts-13.6
+
+# User packages to be built.
+# Various formats can be used as shown in the example below.
+#
+# packages:
+# - some-directory
+# - https://example.com/foo/bar/baz-0.0.2.tar.gz
+# - location:
+#    git: https://github.com/commercialhaskell/stack.git
+#    commit: e7b331f14bcffb8367cd58fbfc8b40ec7642100a
+# - location: https://github.com/commercialhaskell/stack/commit/e7b331f14bcffb8367cd58fbfc8b40ec7642100a
+#  subdirs:
+#  - auto-update
+#  - wai
+packages:
+- .
+# Dependency packages to be pulled from upstream that are not in the resolver
+# using the same syntax as the packages field.
+# (e.g., acme-missiles-0.3)
+extra-deps:
+- process-1.6.5.0
+- transformers-0.5.6.2
+
+# Override default flag values for local packages and extra-deps
+# flags: {}
+
+# Extra package databases containing global packages
+# extra-package-dbs: []
+
+# Control whether we use the GHC we find on the path
+# system-ghc: true
+#
+# Require a specific version of stack, using version ranges
+# require-stack-version: -any # Default
+# require-stack-version: ">=1.9"
+#
+# Override the architecture used by stack, especially useful on Windows
+# arch: i386
+# arch: x86_64
+#
+# Extra directories used by stack for building
+# extra-include-dirs: [/path/to/dir]
+# extra-lib-dirs: [/path/to/dir]
+#
+# Allow a newer minor version of GHC than the snapshot specifies
+# compiler-check: newer-minor

test/callStackToNix/test/Spec.hs

@@ -0,0 +1,2 @@
+main :: IO ()
+main = putStrLn "Test suite not yet implemented"

test/default.nix

@@ -15,6 +15,7 @@ in {
   with-packages = haskell.callPackage ./with-packages { inherit util; };
   builder-haddock = haskell.callPackage ./builder-haddock {};
   stack-simple = haskell.callPackage ./stack-simple {};
+  callStackToNix = haskell.callPackage ./callStackToNix {};
 
   # Run unit tests with: nix-instantiate --eval --strict -A unit
   # An empty list means success.

test/tests.sh

@@ -12,7 +12,7 @@ rm -rvf */cabal.project.local */.ghc.environment* */dist */dist-newstyle */.stac
 echo >& 2
 
 printf "*** Running the nix-build tests...\n" >& 2
-nix-build $NIX_BUILD_ARGS --no-out-link --keep-going ./default.nix
+nix build $NIX_BUILD_ARGS --no-link --keep-going -f ./default.nix
 echo >& 2
 
 printf "*** Running the unit tests... " >& 2