How do I return Nothing from a function?
How do I return Nothing from a function? When there is no edge in the graph this code fails. *** Exception: Graph.hs:65:9-26: Irrefutable pattern failed for pattern (Data.Maybe.Just w) *Main> :t weight weight :: (Ix t1, Ix t2) => t1 -> t2 -> Array (t1, t2) (Maybe t) -> t weight x y g = w where (Just w) = g!(x,y) Thank you for your time. -- -- Sent from an expensive device which will be obsolete in a few months! :D Casey
You can't return Nothing, because your function doesn't return a maybe. If you make it a maybe, you just need to return the value of g without further processing. Am 21.04.2014 19:53 schrieb "KC" <kc1956@gmail.com>:
How do I return Nothing from a function?
When there is no edge in the graph this code fails. *** Exception: Graph.hs:65:9-26: Irrefutable pattern failed for pattern (Data.Maybe.Just w)
*Main> :t weight weight :: (Ix t1, Ix t2) => t1 -> t2 -> Array (t1, t2) (Maybe t) -> t
weight x y g = w where (Just w) = g!(x,y)
Thank you for your time.
--
--
Sent from an expensive device which will be obsolete in a few months! :D Casey
_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners
If the result of weight is (Maybe t) instead of the then the result of the array lookup can be passed out directly for the caller to check: weight x y g = g!(x, y) Is this what you mean? Cheers, Darren On Apr 21, 2014 10:53 AM, "KC" <kc1956@gmail.com> wrote:
How do I return Nothing from a function?
When there is no edge in the graph this code fails. *** Exception: Graph.hs:65:9-26: Irrefutable pattern failed for pattern (Data.Maybe.Just w)
*Main> :t weight weight :: (Ix t1, Ix t2) => t1 -> t2 -> Array (t1, t2) (Maybe t) -> t
weight x y g = w where (Just w) = g!(x,y)
Thank you for your time.
--
--
Sent from an expensive device which will be obsolete in a few months! :D Casey
_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners
Your problem is that you're explicitly saying that weight can NEVER fail to return a value, so in the case that you run into a Nothing value you need to return some default value instead. Alternatively you could change the signature of weight to instead return a Maybe t type instead. If it's the former, you need something like: weight x y g = case g!(x,y) of Just w -> w Nothing -> 0 -- default weight here or more idiomatically: weight x y g = maybe 0 id (g!(x,y)) Not sure if you need the parens around g!(x,y) in that expression, but I put them in just in case. -R. Kyle Murphy -- Curiosity was framed, Ignorance killed the cat. On Mon, Apr 21, 2014 at 1:53 PM, KC <kc1956@gmail.com> wrote:
How do I return Nothing from a function?
When there is no edge in the graph this code fails. *** Exception: Graph.hs:65:9-26: Irrefutable pattern failed for pattern (Data.Maybe.Just w)
*Main> :t weight weight :: (Ix t1, Ix t2) => t1 -> t2 -> Array (t1, t2) (Maybe t) -> t
weight x y g = w where (Just w) = g!(x,y)
Thank you for your time.
--
--
Sent from an expensive device which will be obsolete in a few months! :D Casey
_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners
participants (4)
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Darren Grant -
KC -
Kyle Murphy -
Norbert Melzer