[Haskell-cafe] Inferred typing?

[Daniel Fischer]

Am Dienstag 12 Mai 2009 07:59:01 schrieb michael rice:
> In the code below, is the type returned by the return functions inferred
> from the result type in the function type signature, i.e., just change the
> result type to Maybe Int and the code will return a Maybe monad, (Just 4),
> instead of a List monad?

You can find out such things yourself, just remove the type signature and ask ghci/hugs 
what they think:

*MType> :t fn
fn :: (MonadPlus m) => [a] -> m a

>
> Michael
>
> =========
>
> import Monad
>
> fn :: [Int] -> [Int]
> fn l = mzero `mplus` (return (head l)) `mplus` (return (last l))
>
> ================
>
> *Main> :l test5
> [1 of 1] Compiling Main             ( test5.hs, interpreted )
> Ok, modules loaded: Main.
> *Main> fn [4,5,6,7,8]
> [4,8]
> *Main>