[Haskell-cafe] Inferred typing?
daniel.is.fischer at web.de
Tue May 12 08:50:26 EDT 2009
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
> 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]
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