Nix: support passing ghcVersion to shell.nix

domenkozar · domenkozar · commit 3a9932c75aa2 · 2021-02-27T11:39:13.000Z
This is handy for two reasons:

- ghc currently passed relies on importing nixpkgs from $NIX_PATH
  that is against best practices
- one might want to refer to a compiler outside nixpkgs, for example
  haskell.nix
diff --git a/ChangeLog.md b/ChangeLog.md
@@ -23,6 +23,9 @@ Behavior changes:
 
 Other enhancements:
 
+* Nix integration now passes `ghcVersion` (in addition to existing `ghc`) to 
+  `shell-file` as an identifier that can be looked up in a compiler attribute set.
+
 * `stack list` is a new command to list package versions in a snapshot.
   See [#5431](https://github.com/commercialhaskell/stack/pull/5431)
 
diff --git a/src/Stack/Config/Nix.hs b/src/Stack/Config/Nix.hs
@@ -5,6 +5,7 @@
 module Stack.Config.Nix
        (nixOptsFromMonoid
        ,nixCompiler
+       ,nixCompilerVersion
        ,StackNixException(..)
        ) where
 
@@ -77,6 +78,16 @@ nixCompiler compilerVersion =
     WCGhcjs{} -> Left $ stringException "Only GHC is supported by stack --nix"
     WCGhcGit{} -> Left $ stringException "Only GHC is supported by stack --nix"
 
+nixCompilerVersion :: WantedCompiler -> Either StringException T.Text
+nixCompilerVersion compilerVersion =
+  case compilerVersion of
+    WCGhc version ->
+      case T.split (== '.') (fromString $ versionString version) of
+        x : y : minor -> Right $ "ghc" <> T.concat (x : y : minor)
+        _ -> Left $ stringException "GHC major version not specified"
+    WCGhcjs{} -> Left $ stringException "Only GHC is supported by stack --nix"
+    WCGhcGit{} -> Left $ stringException "Only GHC is supported by stack --nix"
+
 -- Exceptions thown specifically by Stack.Nix
 data StackNixException
   = NixCannotUseShellFileAndPackagesException
diff --git a/src/Stack/Nix.hs b/src/Stack/Nix.hs
@@ -18,7 +18,7 @@ import           Data.Version (showVersion)
 import           Path.IO
 import qualified Paths_stack as Meta
 import           Stack.Config (getInContainer, withBuildConfig)
-import           Stack.Config.Nix (nixCompiler)
+import           Stack.Config.Nix (nixCompiler, nixCompilerVersion)
 import           Stack.Constants (platformVariantEnvVar,inNixShellEnvVar,inContainerEnvVar)
 import           Stack.Types.Config
 import           Stack.Types.Docker
@@ -56,14 +56,16 @@ runShellAndExit = do
      compilerVersion <- withBuildConfig $ view wantedCompilerVersionL
 
      ghc <- either throwIO return $ nixCompiler compilerVersion
+     ghcVersion <- either throwIO return $ nixCompilerVersion compilerVersion
      let pkgsInConfig = nixPackages (configNix config)
          pkgs = pkgsInConfig ++ [ghc, "git", "gcc", "gmp"]
          pkgsStr = "[" <> T.intercalate " " pkgs <> "]"
          pureShell = nixPureShell (configNix config)
          addGCRoots = nixAddGCRoots (configNix config)
          nixopts = case mshellFile of
-           Just fp -> [toFilePath fp, "--arg", "ghc"
-                      ,"with (import <nixpkgs> {}); " ++ T.unpack ghc]
+           Just fp -> [toFilePath fp
+                      ,"--arg", "ghc", "with (import <nixpkgs> {}); " ++ T.unpack ghc
+                      ,"--argstr", "ghcVersion", T.unpack ghcVersion]
            Nothing -> ["-E", T.unpack $ T.concat
                               ["with (import <nixpkgs> {}); "
                               ,"let inputs = ",pkgsStr,"; "
