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Desugaring of do-syntax intros unification var with -XRebindableSyntax

    • Closed Issue created by Gabor Greif @ggreif

    When compiling below snippet with GHC 7.8.x (and I believe v7.10.x)

    {-# LANGUAGE RankNTypes, RebindableSyntax #-}

import qualified Prelude as P

(>>=) :: a -> ((forall b . b) -> c) -> c
a >>= f = f P.undefined
return a = a
fail s = P.undefined

t1 = 'd' >>= (\_ -> 'k')

t2 = do _ <- 'd'
        'k'

main = P.putStrLn [t1, t2]

    we get this error:

    Ztest.hs:12:9:
    Cannot instantiate unification variable `t0`
    with a type involving foralls: forall b. b
      Perhaps you want ImpredicativeTypes
    In a stmt of a 'do' block: _ <- 'd'
    In the expression:
      do { _ <- 'd';
           'k' }
    In an equation for `t2`:
        t2
          = do { _ <- 'd';
                 'k' }
Failed, modules loaded: none.

    In GHC HEAD (and v7.6.x) the error does not appear. Nevertheless I'll file this bug for addition of a regression test.

    Discussion here: https://mail.haskell.org/pipermail/ghc-devs/2015-February/008383.html

    Trac metadata
    Trac field Value
    Version 7.8.4
    Type Bug
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component Compiler
    Test case
    Differential revisions
    BlockedBy
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    Operating system
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