Fix ghc-paths for Bazel 0.27

The amount of shell indirection seems to have changed in Bazel 0.27.
Meaning that we would need twice as many `\` for the `sed` call on
Windows. The `tr` command is insensitive to single backslashes, so it's
an easier alternative.

hazel/ghc_paths.bzl

@@ -5,23 +5,23 @@ def _ghc_paths_module_impl(ctx):
         inputs = [ghc],
         outputs = [ctx.outputs.out],
         command = """
-      {ghc} --print-libdir
-      {ghc} --print-libdir | sed 's:\\\\:/:g'
-
       cat > {out} << EOM
 module GHC.Paths (
         ghc, ghc_pkg, libdir, docdir
   ) where
 
-libdir, docdir, ghc, ghc_pkg :: FilePath
-
-poulet = "$({ghc} --print-libdir)"
-libdir  = "$({ghc} --print-libdir | sed 's:\\\\:/:g')"
-docdir  = "DOCDIR_IS_NOT_SET"
+ghc, ghc_pkg, docdir, libdir :: FilePath
 
 ghc     = "{ghc}"
 ghc_pkg = "{ghc_pkg}"
-EOM""".format(
+
+docdir  = "DOCDIR_IS_NOT_SET"
+EOM
+
+      echo -n 'libdir  = "' >> {out}
+      {ghc} --print-libdir | tr '\\' '/' | tr -d '[:space:]' >> {out}
+      echo '"' >> {out}
+""".format(
             ghc = ghc.path,
             ghc_pkg = tools.ghc_pkg.path,
             out = ctx.outputs.out.path,

hazel/test/ghc-paths-test.hs

@@ -1,15 +1,15 @@
 module Main where
 
 import Control.Monad (when)
-import GHC.Paths (ghc, ghc_pkg, libdir, docdir, poulet)
+import GHC.Paths (ghc, ghc_pkg, libdir, docdir)
 import System.Exit (exitFailure)
 import System.IO (hPutStrLn, stderr)
 
 main :: IO ()
 main = do
   let require name value =
         when (null value) $ do
-          hPutStrLn stderr (name ++ " should not be empty" ++ poulet)
+          hPutStrLn stderr (name ++ " should not be empty")
           exitFailure
   require "ghc" ghc
   require "ghc_pkg" ghc_pkg