[Haskell-cafe] What monad am I in?
ryani.spam at gmail.com
Tue Sep 2 20:33:02 EDT 2008
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.
On Tue, Sep 2, 2008 at 1:25 PM, Henry Laxen <nadine.and.henry at pobox.com> wrote:
> 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:
> 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?
> Henry Laxen
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> Haskell-Cafe at haskell.org
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