Re: Writing for both State and StateT

Derek Elkins wrote:

On Tue, 22 Apr 2003 23:05:09 +0100 Alastair Reid wrote:

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Unfortunately, in this case, with that function in isolation I get,

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Ambiguous type variable(s) `s', `m' in the constraint `MonadState s m' arising from use of `get' ....

An easy way to get round this is to give a completely bogus type signature like:

foo :: a

The compiler will report a mismatch and the error message will tell you what it thinks the type is.

Or the straightforward and simple answer: in GHCi or Hugs, just do :t f. E.g. :t get ==> (MonadState s m) => m s. GHCi (and I imagine Hugs too) doesn't try to validate the constraints.

Have you tried this? GHCi does indeed typecheck the input (``validate the constraints'' as you put it). It has to, to be a Haskell 98-conforming interpreter. Jon Cast