[Haskell-cafe] Missing common function for Maybe types
wiggly at wiggly.org
Tue Jul 31 12:07:21 UTC 2018
Your `foobar` function cannot work, it doesn't handle the case when the
URL is neither http nor https.
I would use catMaybes and headMay to work this out...untested code, and
you'll need the `safe` library for headMay too...
parse :: ByteString -> Maybe (Url Http, Option scheme)
parse input = headMay $ catMaybes $ fmap input [parseUrlHttp, parseUrlHttps]
And everything is lazy so if the input gets parsed correctly by the
first call you never actually perform the second one....
On 31/07/18 08:07, Marc Busqué wrote:
> I have two functions:
> foo :: a -> Maybe b
> bar :: a -> Maybe c
> From which I want to build a higher order function:
> foobar :: a -> (a -> Maybe b) -> (a -> Maybe c) -> Either b c
> The implementation I need is:
> foobar x f g =
> case (f x) of
> Nothing -> g x
> Just y -> Just y
> I'm a bit surprised that looking at hoogle I don't find a built-in
> solution for this quite common need for `Maybe` types (or perhaps for
> any monad).
> Am I looking in the wrong way? Does it exist a similar abstraction but
> with a different shape?
> Thanks in advance,
> Marc Busqué
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