Doesn't build with Cabal HEAD (2.1)

[tfausak]

I am working on a project that requires features from the latest HEAD version of Cabal (see haskell/cabal#4799). Something in my dependency tree pulled in cabal-doctest which caused my build to fail with this error:

Error: While constructing the build plan, the following exceptions were encountered:

In the dependencies for cabal-doctest-1.0.2:
    Cabal-2.1.0.0 must match >=1.10 && <2.1 (latest applicable is 2.0.0.2)
needed due to example-0.0.0 -> cabal-doctest-1.0.2

Plan construction failed.

I initially thought this was a problem with distributive (see ekmett/distributive#37) but was sent here instead.

I tried to fix this by bumping the bounds (--allow-newer), but the build failed:

    /private/tmp/cabal-doctest/src/Distribution/Extra/Doctest.hs:227:7: error:
        • Couldn't match expected type ‘IO ()’
                      with actual type ‘String -> IO ()’
        • In a stmt of a 'do' block:
            rewriteFile (testAutogenDir </> "Build_doctests.hs")
            $ unlines
                ["module Build_doctests where", "", "import Prelude", "", ....]
          In the second argument of ‘($)’, namely
            ‘do { let testBI = testBuildInfo suite;
                  let additionalFlags
                        = maybe ... words
                          $ lookup "x-doctest-options" $ customFieldsBI testBI;
                  let additionalModules
                        = maybe ... words
                          $ lookup "x-doctest-modules" $ customFieldsBI testBI;
                  let additionalDirs'
                        = maybe ... words
                          $ lookup "x-doctest-source-dirs" $ customFieldsBI testBI;
                  .... }’
          In the expression:
            when (testName suite == fromString testSuiteName)
            $ do { let testBI = testBuildInfo suite;
                   let additionalFlags
                         = maybe ... words
                           $ lookup "x-doctest-options" $ customFieldsBI testBI;
                   let additionalModules
                         = maybe ... words
                           $ lookup "x-doctest-modules" $ customFieldsBI testBI;
                   let additionalDirs'
                         = maybe ... words
                           $ lookup "x-doctest-source-dirs" $ customFieldsBI testBI;
                   .... }
    
    /private/tmp/cabal-doctest/src/Distribution/Extra/Doctest.hs:227:20: error:
        • Couldn't match type ‘[Char]’
                         with ‘Distribution.Verbosity.Verbosity’
          Expected type: Distribution.Verbosity.Verbosity
            Actual type: FilePath
        • In the first argument of ‘rewriteFile’, namely
            ‘(testAutogenDir </> "Build_doctests.hs")’
          In the expression:
            rewriteFile (testAutogenDir </> "Build_doctests.hs")
          In a stmt of a 'do' block:
            rewriteFile (testAutogenDir </> "Build_doctests.hs")
            $ unlines
                ["module Build_doctests where", "", "import Prelude", "", ....]

[RyanGlScott]

It looks like the type of rewriteFile changed in Cabal HEAD: haskell/cabal@0aa73b7#diff-fe949fe985d4f115d0eee5b8268ef014R1385

Perhaps this patch would do the trick? (Disclaimer: I haven't tried compiling this.)

diff --git a/src/Distribution/Extra/Doctest.hs b/src/Distribution/Extra/Doctest.hs
index b87b811..239eb70 100644
--- a/src/Distribution/Extra/Doctest.hs
+++ b/src/Distribution/Extra/Doctest.hs
@@ -224,7 +224,12 @@ generateBuildModule testSuiteName flags pkg lbi = do
       createDirectoryIfMissingVerbose verbosity True testAutogenDir
 
       -- write autogen'd file
-      rewriteFile (testAutogenDir </> "Build_doctests.hs") $ unlines
+#if MIN_VERSION_Cabal(2,1,0)
+      rewriteFile verbosity
+#else
+      rewriteFile
+#endif
+                  (testAutogenDir </> "Build_doctests.hs") $ unlines
         [ "module Build_doctests where"
         , ""
         , "import Prelude"

[RyanGlScott]

Actually, this change to rewriteFile's type signature was reverted in haskell/cabal@5797013, so as of GHC 8.4.1-alpha2 (which comes shipped with Cabal-2.1.0.0), cabal-doctest-1.0.4 builds without any changes! In light of this, I'll close this issue.