data Maybe a = Nothing | Just a vordefinierte Funktionen
import Data.Maybe
fromJust :: Maybe a -> a
> fromJust (Just 42)
42
-- es gibt auch
maybeToList :: Maybe a -> [a]
--
aber:
> maybeToList Nothing []
[] > maybe "Fehler" (show) 42
"42"
> maybe "Fehler" (show) Nothing
"Fehler"