Writing for both State and StateT

[Jon Cast]

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.

Jon Cast