snack.nix: allow more overrides

README.md

@@ -67,6 +67,12 @@ _See the [Hacking](#hacking) section if you want to hack on snack_
 Assuming that [Nix][nix] is installed on your machine, clone this repo
 and run:
 
+``` shell
+$ ./script/install
+```
+
+which is equivalent to
+
 ``` shell
 $ nix-env -f ./default.nix -iA snack-exe
 ```
@@ -75,8 +81,8 @@ The _snack_ executable is now in your `PATH`:
 
 ``` shell
 $ snack --help
-Usage: snack [-l|--lib DIR] [-j|--cores INT] ([-s|--package-nix PATH] |
-             [-p|--package-yaml PATH]) COMMAND
+Usage: snack [-l|--lib DIR] [-b|--snack-nix PATH] [-j|--cores INT]
+             ([-s|--package-nix PATH] | [-p|--package-yaml PATH]) COMMAND
 
 Available options:
   -l,--lib DIR             Path to the directory to use as the Nix library
@@ -221,8 +227,50 @@ $ snack run # looks for a file called package.nix by default
 Alternatively, use `$ snack build` or `$ snack ghci` if you only want to build,
 or fire up `ghci`, respectively.
 
-### Advanced Nix Example
+### Using other versions of GHC and nixpkgs
 
+The _snack_ executable comes with a [bundled version of
+nixpkgs](./nix/nixpkgs/nixpkgs-src.json) and uses the GHC executable provided
+by `haskell.packages.ghc822.ghcWithPackages`. You may override those default by
+providing a `snack.nix`:
+
+``` shell
+$ snack --snack-nix ./snack.nix build
+```
+
+This file looks like the following:
+
+
+``` nix
+rec {
+  # If you only wish to change the version of GHC being used, set
+  # `ghc-version`. The following versions are currently available:
+  #  * ghc7103
+  #  * ghc7103Binary
+  #  * ghc802
+  #  * ghc821Binary
+  #  * ghc822
+  #  * ghc841
+  #  * ghc842
+  #  * ghcHEAD
+  #  * ghcjs
+  #  * ghcjsHEAD
+  #  * integer-simple
+  # NOTE: not all versions have been tested with snack.
+  ghc-version = "ghc802";
+
+  # Alternatively you can provide you own `ghcWithPackages`, which should have
+  # the same structure as that provided by
+  # `pkgs.haskell.packages.<version>.ghcWithPackages:
+  ghcWithPackages = pkgs.haskellPackages.ghcWithPackages;
+
+  # Finally you can provide your own set of Nix packages, which should evaluate
+  # to an attribute set:
+  pkgs = import ./nix;
+}
+```
+
+### Advanced Nix Example
 
 You may want custom builds that involve things such as [archiving and base64
 encoding entire

bin/Snack.hs

@@ -280,9 +280,10 @@ nixBuild snackCfg extraNixArgs nixExpr =
         { #{ intercalate "," funArgs } }:
         let
           spec = builtins.fromJSON specJson;
-          pkgs = import #{ pkgsSrc } {};
+          config = #{ pkgsSrc };
+          pkgs = config.pkgs;
           libDir = #{ libDir };
-          snack = (import libDir) { inherit pkgs; };
+          snack = (import libDir) config;
         in #{ T.unpack $ unNixExpr $ nixExpr }
       |])
       "nix-build"
@@ -291,13 +292,17 @@ nixBuild snackCfg extraNixArgs nixExpr =
     pkgsSrc :: String
     pkgsSrc =  case snackNix snackCfg of
       Just (SnackNix fp) ->
-        [i|(import #{ fp }).pkgs|]
+        [i|(import #{ fp })|]
       Nothing ->
         [i|
-        (builtins.fetchTarball
-            { url = "https://github.com/${spec.owner}/${spec.repo}/archive/${spec.rev}.tar.gz";
-              sha256 = spec.sha256;
-            })
+        { pkgs = import
+            (
+            builtins.fetchTarball
+               { url = "https://github.com/${spec.owner}/${spec.repo}/archive/${spec.rev}.tar.gz";
+                 sha256 = spec.sha256;
+               }
+            ) {} ;
+        }
         |]
     libDir :: String
     libDir = case snackLib snackCfg of

script/install

@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+nix-env -f ./default.nix -iA snack-exe

snack-lib/default.nix

@@ -2,6 +2,8 @@
 # TODO: currently single out derivations prepend the PWD to the path
 # TODO: make sure that filters for "base" are airtight
 { pkgs
+, ghc-version ? "ghc822"
+, ghcWithPackages ? pkgs.haskell.packages.${ghc-version}.ghcWithPackages
 }:
 
 with pkgs;
@@ -16,7 +18,7 @@ with (callPackage ./package-spec.nix {});
 with (callPackage ./hpack.nix {});
 
 let
-  ghcWith = deps: haskellPackages.ghcWithPackages
+  ghcWith = deps: ghcWithPackages
     (ps: map (p: ps.${p}) deps);
 
   # Assumes the package description describes an executable

tests/extended-config/nix/default.nix

@@ -1,4 +1,3 @@
-{}:
 let
   spec = builtins.fromJSON (builtins.readFile ../../../nix/nixpkgs/nixpkgs-src.json);
   nixpkgs =

tests/extended-config/snack.nix

@@ -1,3 +1,4 @@
-{
-  pkgs = ./nix;
+rec {
+  pkgs = import ./nix;
+  ghcWithPackages = pkgs.haskellPackages.ghcWithPackages;
 }

tests/extended-config/test

@@ -16,6 +16,6 @@ test() {
   rm $TMP_FILE
 }
 
-SNACK="snack -j4 -b snack.nix -s package.nix" test
-SNACK="snack -j4 -b snack.nix" test
-SNACK="snack -j4 -b snack.nix --package-yaml package.yaml" test
+SNACK="snack -j4 --snack-nix snack.nix --package-nix package.nix" test
+SNACK="snack -j4 --snack-nix snack.nix" test
+SNACK="snack -j4 --snack-nix snack.nix --package-yaml package.yaml" test