Writing for both State and StateT

[Derek Elkins]

On Tue, 22 Apr 2003 23:27:33 -0500
Jon Cast <jcast@ou.edu> wrote:

> 
> Derek Elkins <ddarius@hotpop.com> wrote:
> > On Tue, 22 Apr 2003 23:05:09 +0100
> > Alastair Reid <alastair@reid-consulting-uk.ltd.uk> wrote:
> > 
> > > 
> > > > Unfortunately, in this case, with that function in isolation I get,
> > > 
> > > > 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.

Yes, I've tried it. I didn't mean GHCi doesn't EVER validate the constraints on input, just that it doesn't when you use :t.  As :t isn't part of the language, H98 conformance doesn't matter, and as the example uses multiparameter typeclasses, it's beyond H98 anyways.

For example, see SimulatingDependentTypes on the Haskell Wiki.  If you type :t ex4 it will spit out the constraints in GHCi.  If you type simply ex4 it will fail to typecheck.