[Haskell-cafe] What monad am I in?

[Jonathan Cast]

On Tue, 2008-09-02 at 20:25 +0000, Henry Laxen 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:
> 
> <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?

Yes.  This is a peculiarity of GHCi (and ghc -e) --- IO actions at
top-level are executed by default, while non-IO values are simply
printed out.

jcc