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