You need to use a view pattern with an explicitly bidirectional pattern
synonym:
pattern P :: (Int,Int) -> Foo
pattern P xy <- ((\(Foo x y) -> (x, y)) -> xy)
where
P (x, y) = Foo x y
If the Foo type had more than one constructor, then you'd need to do
something a bit trickier, like
pattern P xy <- ((\case
Foo x y -> Just (x, y)
_ -> Nothing) -> Just xy)
where
P (x, y) = Foo x y
On Sun, May 17, 2020, 11:57 PM Kazu Yamamoto
Hi,
I have a question about PatternSynonyms. Suppose we have:
data Foo = Foo Int Int
I would like to define a pattern as follows:
pattern P :: (Int,Int) -> Foo pattern P (x,y) = Foo x y
But this results in "parse error on input ‘(’". Are there any ways to use tuple in the left side hand of patterns?
This is important to maintain backward compatibility for the "network" library.
If there is no way, I will define Foo as:
data Foo = Foo (Int,Int) -- awkward
and define P as:
pattern P :: (Int,Int) -> Foo pattern P xy = Foo xy
Regards,
--Kazu _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. lol