BangPatterns bang is silently removed #116

New Issue

2018-01-22T18:12:46+01:00

lspitzner commented

2018-01-22 18:12:46 +01:00

(Migrated from github.com)

I think I saw this in do-notation. This is rather serious, as this kind of error will not be detected, but the code will behave differently.

lspitzner commented

2018-02-13 23:00:56 +01:00

(Migrated from github.com)

I'll document a bit how I approach this issue for anyone interested.

First thing I noticed is that this does not appear in ghc-8.0. But in ghc-8.2.* a failing testcase would be

{-# LANGUAGE BangPatterns #-}
func = do
  let !abc = def
  pure ()

Looking at the parse trees via --dump-ast-full, they are indeed a good bit different. Just for the !abc = def part above, we have:

8.0

PatBind
  A Just (Ann (DP (0,0)) [] [] [((G AnnBang),DP (0,0))] Nothing Nothing)
    BangPat
      A Just (Ann (DP (0,0)) [] [] [((G AnnVal),DP (0,0))] Nothing Nothing)
        VarPat
          A Nothing
            Unqual {OccName: abc}
  GRHSs
    [ A Just (Ann (DP (0,-1)) [] [] [] Nothing Nothing)
        GRHS
          []
          A Just (Ann (DP (0,1)) [] [] [] Nothing Nothing)
            HsVar
              A Just (Ann (DP (0,0)) [] [] [((G AnnVal),DP (0,0))] Nothing Nothing)
                Unqual {OccName: def}
    ]
    A (Nothing) EmptyLocalBinds
  PlaceHolder
  PlaceHolder
  (,) [] []

8.2

FunBind
  A Just (Ann (DP (0,0)) [] [] [((G AnnVal),DP (0,0))] Nothing Nothing)
    Unqual {OccName: abc}
  MG
    A Nothing
      [ A Just (Ann (DP (0,0)) [] [] [((G AnnBang),DP (0,0)),((G AnnEqual),DP (0,1))] Nothing Nothing)
          Match
            FunRhs
              A Just (Ann (DP (0,0)) [] [] [((G AnnVal),DP (0,0))] Nothing Nothing)
                Unqual {OccName: abc}
              Prefix
              SrcStrict
            []
            Nothing
            GRHSs
              [ A Just (Ann (DP (0,-1)) [] [] [] Nothing Nothing)
                  GRHS
                    []
                    A Just (Ann (DP (0,1)) [] [] [] Nothing Nothing)
                      HsVar
                        A Just (Ann (DP (0,0)) [] [] [((G AnnVal),DP (0,0))] Nothing Nothing)
                          Unqual
                            {OccName: def}
              ]
              A (Nothing) EmptyLocalBinds
      ]
    []
    PlaceHolder
    FromSource
  WpHole
  PlaceHolder
  []

As brittany currently ignores the first argument of Match we ignore the only trace of the bang, namely SrcStrict.

I'll write a fix next.

I'll document a bit how I approach this issue for anyone interested. First thing I noticed is that this does not appear in ghc-8.0. But in ghc-8.2.* a failing testcase would be ~~~~.hs {-# LANGUAGE BangPatterns #-} func = do let !abc = def pure () ~~~~ --- Looking at the parse trees via --dump-ast-full, they are indeed a good bit different. Just for the `!abc = def` part above, we have: - 8.0 ~~~~ PatBind A Just (Ann (DP (0,0)) [] [] [((G AnnBang),DP (0,0))] Nothing Nothing) BangPat A Just (Ann (DP (0,0)) [] [] [((G AnnVal),DP (0,0))] Nothing Nothing) VarPat A Nothing Unqual {OccName: abc} GRHSs [ A Just (Ann (DP (0,-1)) [] [] [] Nothing Nothing) GRHS [] A Just (Ann (DP (0,1)) [] [] [] Nothing Nothing) HsVar A Just (Ann (DP (0,0)) [] [] [((G AnnVal),DP (0,0))] Nothing Nothing) Unqual {OccName: def} ] A (Nothing) EmptyLocalBinds PlaceHolder PlaceHolder (,) [] [] ~~~~ - 8.2 ~~~~ FunBind A Just (Ann (DP (0,0)) [] [] [((G AnnVal),DP (0,0))] Nothing Nothing) Unqual {OccName: abc} MG A Nothing [ A Just (Ann (DP (0,0)) [] [] [((G AnnBang),DP (0,0)),((G AnnEqual),DP (0,1))] Nothing Nothing) Match FunRhs A Just (Ann (DP (0,0)) [] [] [((G AnnVal),DP (0,0))] Nothing Nothing) Unqual {OccName: abc} Prefix SrcStrict [] Nothing GRHSs [ A Just (Ann (DP (0,-1)) [] [] [] Nothing Nothing) GRHS [] A Just (Ann (DP (0,1)) [] [] [] Nothing Nothing) HsVar A Just (Ann (DP (0,0)) [] [] [((G AnnVal),DP (0,0))] Nothing Nothing) Unqual {OccName: def} ] A (Nothing) EmptyLocalBinds ] [] PlaceHolder FromSource WpHole PlaceHolder [] ~~~~ As `brittany` currently ignores the first argument of `Match` we ignore the only trace of the bang, namely `SrcStrict`. I'll write a fix next.

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Reference: hexagoxel/brittany#116