Stack/Nix used with a shell file now expects a function

Addresses commercialhaskell#2243

The shell file should expect a `{ghc}` argument which
should be passed to the `buildInputs`, so the stack environment
contains the right GHC
If the shell file doesn't define a function, then the ghc passed
by stack is just ignored, but the doc doesn't mention it in
order to encourage this new good practise.

doc/nix_integration.md

@@ -200,20 +200,24 @@ equivalent of the configuration used in
 2015-03-05):
 
 ```nix
+{ghc}:
 with (import <nixpkgs> {});
 
 haskell.lib.buildStackProject {
   name = "myEnv";
-  buildInputs = [ glpk pcre ];
+  buildInputs = [ ghc glpk pcre ];
 }
 ```
 
-Defining manually a `shell.nix` file gives you the possibility to
-override some Nix derivations ("packages"), for instance to change
-some build options of the libraries you use, or to set additional
-environment variables. See the [Nix manual][nix-manual-exprs] for
-more. The `buildStackProject` utility function is documented in the
-[Nixpkgs manual][nixpkgs-manual-haskell].
+Defining manually a `shell.nix` file gives you the possibility to override some
+Nix derivations ("packages"), for instance to change some build options of the
+libraries you use, or to set additional environment variables. See the
+[Nix manual][nix-manual-exprs] for more. The `buildStackProject` utility
+function is documented in the [Nixpkgs manual][nixpkgs-manual-haskell].  In such
+case, stack expect this file to define a function of exactly one argument that
+should be called `ghc` (as arguments within a set are non-positional), which you
+should give to `buildInputs`. This is the ghc from the resolver you set in the
+`stack.yaml`.
 
 And now for the `stack.yaml` file:
 

src/Stack/Nix.hs

@@ -84,10 +84,12 @@ runShellAndExit mprojectRoot maresolver mcompiler getCmdArgs = do
          traverse (resolveFile (fromMaybeProjectRoot mprojectRoot)) $
          nixInitFile (configNix config)
      let pkgsInConfig = nixPackages (configNix config)
-         pkgs = pkgsInConfig ++ [nixCompiler config mresolver mcompiler]
+         ghc = nixCompiler config mresolver mcompiler
+         pkgs = pkgsInConfig ++ [ghc]
          pureShell = nixPureShell (configNix config)
          nixopts = case mshellFile of
-           Just fp -> [toFilePath fp]
+           Just fp -> [toFilePath fp, "--arg", "ghc"
+                      ,"with (import <nixpkgs> {}); " ++ T.unpack ghc]
            Nothing -> ["-E", T.unpack $ T.intercalate " " $ concat
                               [["with (import <nixpkgs> {});"
                                ,"runCommand \"myEnv\" {"