haskellPackages.mkDerivation: use emcc as C compiler for ghcjs

We need to set up EM_CACHE correctly so that it works, of course.

Note that the solution relies on the assumption that this should only
happen when we cross compile, i.e. the extra logic to account for the
cc-less stdenv in pkgsCross.ghcjs is only present in crossCabalFlags.

Co-authored-by: sternenseemann <sternenseemann@systemli.org>

pkgs/development/haskell-modules/generic-builder.nix

@@ -1,13 +1,24 @@
 { lib, stdenv, buildPackages, buildHaskellPackages, ghc
-, jailbreak-cabal, hscolour, cpphs, nodejs
+, jailbreak-cabal, hscolour, cpphs
 , ghcWithHoogle, ghcWithPackages
+, nodejs
 }:
 
 let
   isCross = stdenv.buildPlatform != stdenv.hostPlatform;
+
+  # Pass the "wrong" C compiler rather than none at all so packages that just
+  # use the C preproccessor still work, see
+  # https://github.com/haskell/cabal/issues/6466 for details.
+  cc =
+    if stdenv.hasCC then "$CC"
+    else if stdenv.hostPlatform.isGhcjs then "${emscripten}/bin/emcc"
+    else "$CC_FOR_BUILD";
+
   inherit (buildPackages)
     fetchurl removeReferencesTo
-    pkg-config coreutils gnugrep glibcLocales;
+    pkg-config coreutils gnugrep glibcLocales
+    emscripten;
 in
 
 { pname
@@ -40,7 +51,8 @@ in
 , enableSharedExecutables ? false
 , enableSharedLibraries ? !stdenv.hostPlatform.isStatic && (ghc.enableShared or false)
 , enableDeadCodeElimination ? (!stdenv.isDarwin)  # TODO: use -dead_strip for darwin
-, enableStaticLibraries ? !(stdenv.hostPlatform.isWindows || stdenv.hostPlatform.isWasm)
+# Disabling this for ghcjs prevents this crash: https://gitlab.haskell.org/ghc/ghc/-/issues/23235
+, enableStaticLibraries ? !(stdenv.hostPlatform.isWindows || stdenv.hostPlatform.isWasm || stdenv.hostPlatform.isGhcjs)
 , enableHsc2hsViaAsm ? stdenv.hostPlatform.isWindows
 , extraLibraries ? [], librarySystemDepends ? [], executableSystemDepends ? []
 # On macOS, statically linking against system frameworks is not supported;
@@ -188,10 +200,7 @@ let
   crossCabalFlags = [
     "--with-ghc=${ghcCommand}"
     "--with-ghc-pkg=${ghc.targetPrefix}ghc-pkg"
-    # Pass the "wrong" C compiler rather than none at all so packages that just
+    "--with-gcc=${cc}"
-    # use the C preproccessor still work, see
-    # https://github.com/haskell/cabal/issues/6466 for details.
-    "--with-gcc=${if stdenv.hasCC then "$CC" else "$CC_FOR_BUILD"}"
   ] ++ optionals stdenv.hasCC [
     "--with-ld=${stdenv.cc.bintools.targetPrefix}ld"
     "--with-ar=${stdenv.cc.bintools.targetPrefix}ar"
@@ -315,7 +324,7 @@ let
     optionals doBenchmark benchmarkToolDepends;
   nativeBuildInputs =
     [ ghc removeReferencesTo ] ++ optional (allPkgconfigDepends != []) (assert pkg-config != null; pkg-config) ++
-    setupHaskellDepends ++ collectedToolDepends;
+    setupHaskellDepends ++ collectedToolDepends ++ optional stdenv.hostPlatform.isGhcjs nodejs;
   propagatedBuildInputs = buildDepends ++ libraryHaskellDepends ++ executableHaskellDepends ++ libraryFrameworkDepends;
   otherBuildInputsHaskell =
     optionals doCheck (testDepends ++ testHaskellDepends) ++
@@ -442,6 +451,11 @@ stdenv.mkDerivation ({
   '' + ''
     done
   ''
+  + (optionalString stdenv.hostPlatform.isGhcjs ''
+    export EM_CACHE="$(realpath "$(mktemp -d emcache.XXXXXXXXXX)")"
+    cp -Lr ${emscripten}/share/emscripten/cache/* "$EM_CACHE/"
+    chmod u+rwX -R "$EM_CACHE"
+  '')
   # only use the links hack if we're actually building dylibs. otherwise, the
   # "dynamic-library-dirs" point to nonexistent paths, and the ln command becomes
   # "ln -s $out/lib/links", which tries to recreate the links dir and fails

pkgs/top-level/release-haskell.nix

@@ -473,6 +473,15 @@ let
               inherit (packagePlatforms pkgs.pkgsCross.ghcjs.haskellPackages)
                 ghc
                 hello
+                microlens
+              ;
+            };
+
+            haskell.packages.ghc98 = {
+              inherit (packagePlatforms pkgs.pkgsCross.ghcjs.haskell.packages.ghc98)
+                ghc
+                hello
+                microlens
               ;
             };
 
@@ -480,6 +489,7 @@ let
               inherit (packagePlatforms pkgs.pkgsCross.ghcjs.haskell.packages.ghcHEAD)
                 ghc
                 hello
+                microlens
               ;
             };
           };