Felipe Lessa wrote:
On Wed, Jul 22, 2009 at 06:00:38PM +0200, Matthias Görgens wrote:
] I need to take some elements from the front of a list. However the
] criteria are somewhat complex.
]
] > walk f [] = []
] > walk f (x:xs) = case f x
] > of Just g -> x : walk g xs
] > Nothing -> []
]
] For each item the `predicate' f either returns Nothing, when it thinks
] we should not take any more elements, or return Just another
] `predicate' to apply to the next element.
]
] However the type system does not like my function. How can I mollify it?
It's actually simple! The problem is the type of 'f' would be
type TypeOfF a = a -> Maybe (TypeOfF a)
However, this type synonym isn't valid for the obvious reason (it
is "infinite", I don't remember the correct name). Data types,
on the other hand, may be recursive.
type TypeOfF a = a -> Maybe (TypeOfF' a)
newtype TypeOfF' a = F {unF :: TypeOfF a}
Another way to do it is to use an alternative to Maybe, defined like this:
> data MaybeFun a = NoFun | Fun (a -> MaybeFun a)
...which gives this definition for walk:
> walk :: (a -> MaybeFun a) -> [a] -> [a]
> walk f [] = []
> walk f (x:xs) = case f x
> of Fun g -> x : walk g xs
> NoFun -> []
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