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Diffstat (limited to 'docs/users_guide/bugs.rst')
| -rw-r--r-- | docs/users_guide/bugs.rst | 39 |
1 files changed, 39 insertions, 0 deletions
diff --git a/docs/users_guide/bugs.rst b/docs/users_guide/bugs.rst index c0cffa0ee8..4dc49f0328 100644 --- a/docs/users_guide/bugs.rst +++ b/docs/users_guide/bugs.rst @@ -49,6 +49,45 @@ Lexical syntax reserving ``forall`` as a keyword has significance. For instance, GHC will not parse the type signature ``foo :: forall x``. +- The ``(!)`` operator, when written in prefix form (preceded by whitespace + and not followed by whitespace, as in ``f !x = ...``), is interpreted as a + bang pattern, contrary to the Haskell Report, which prescribes to treat ``!`` + as an operator regardless of surrounding whitespace. Note that this does not + imply that GHC always enables :extension:`BangPatterns`. Without the + extension, GHC will issue a parse error on ``f !x``, asking to enable the + extension. + +- Irrefutable patterns must be written in prefix form:: + + f ~a ~b = ... -- accepted by both GHC and the Haskell Report + f ~ a ~ b = ... -- accepted by the Haskell Report but not GHC + + When written in non-prefix form, ``(~)`` is treated by GHC as a regular + infix operator. + + See `GHC Proposal #229 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0229-whitespace-bang-patterns.rst>`__ + for the precise rules. + +- Strictness annotations in data declarations must be written in prefix form:: + + data T = MkT !Int -- accepted by both GHC and the Haskell Report + data T = MkT ! Int -- accepted by the Haskell Report but not GHC + + See `GHC Proposal #229 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0229-whitespace-bang-patterns.rst>`__ + for the precise rules. + +- As-patterns must not be surrounded by whitespace:: + + f p@(x, y, z) = ... -- accepted by both GHC and the Haskell Report + f p @ (x, y, z) = ... -- accepted by the Haskell Report but not GHC + + When surrounded by whitespace, ``(@)`` is treated by GHC as a regular infix + operator. + + See `GHC Proposal #229 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0229-whitespace-bang-patterns.rst>`__ + for the precise rules. + + .. _infelicities-syntax: Context-free syntax |