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 <kazu@iij.ad.jp> wrote:

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
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