[Haskell-cafe] What monad am I in?
philip.weaver at gmail.com
Tue Sep 2 21:49:44 EDT 2008
On Tue, Sep 2, 2008 at 5:33 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:
> ghci has some crazy defaulting rules for expressions at the top level.
> In particular, it tries to unify those expressions with a few
> different types, including IO.
> On the other hand, the let-expression is typed like regular Haskell
> and you run into the monomorphism restriction.
Right. Just to make it clear for the original poster, this monomorphism
restriction is not about GHCi specifically, just GHC in general. With the
-fno-monomorphism-restriction, you will not get this error.
> -- ryan
> On Tue, Sep 2, 2008 at 1:25 PM, Henry Laxen <nadine.and.henry at pobox.com>
> > Dear Group,
> > When I fire up ghci and define:
> > increment x = return (x+1)
> > I can say:
> > Main> increment 1
> > and ghci dutifully replies 2. Also as expected, the type signature of
> > increment is: (Num a, Monad m) => a -> m a
> > However, if I say:
> > Main> let a = increment 1
> > I get:
> > <interactive>:1:8:
> > Ambiguous type variable `m' in the constraint:
> > `Monad m' arising from a use of `increment' at <interactive>:1:8-18
> > Probable fix: add a type signature that fixes these type variable(s)
> > Have I, like Monsier Jourdain, been running in the IO monad all my
> > life, and didn't even know it?
> > Thanks,
> > Henry Laxen
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Haskell-Cafe