The dreaded M-R

Malcolm Wallace Malcolm.Wallace at
Tue Jan 31 06:19:59 EST 2006

"Simon Marlow" <simonmar at> writes:

> Given the new evidence that it's actually rather hard to demonstrate any
> performance loss in the absence of the M-R with GHC, I'm attracted to
> the option of removing it in favour of a warning.

As another data point, today for the first time I received an error
(not a warning) from ghc about the M-R:

    Ambiguous type variable `a' in the constraint:
      `Ord a' arising from use of `Data.Set.insert' at Pretty.hs:28:11-20
    Possible cause: the monomorphism restriction applied to the following:
      addToSet :: a -> Data.Set.Set a -> Data.Set.Set a (bound at Pretty.hs:28:0)
    Probable fix: give these definition(s) an explicit type signature
                  or use -fno-monomorphism-restriction

So, without the M-R or a type signature, my code is OK.  The proposal
to accept this code but produce an optional warning is (I think)
better than the current error.


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