[Haskell] Perhaps a bug in GHC 6.10

Krasimir Angelov kr.angelov at gmail.com
Wed Dec 3 08:45:01 EST 2008


One of my students wrote code like this:

prop_blank s (i,j) li =
  if i==li
       do if something then True else False
       prop_blank (...) (i,j) (li+1)

This code is not accepted from GHC 6.8.3 because Bool is not a monad.
In particular the error message is:

    Couldn't match expected type `Bool' against inferred type `a b'
    In the expression:

Surprisingly the same code is accepted from GHC 6.10. This looks like
a bug for me. The easiest way to reproduce this is to try to evaluate:

> do True

in GHCi with GHC 6.8.3 and with GHC 6.10

Is there any reason to accept this code?


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