Can I capture values from a pattern synonym?

Question

Assuming a pattern:

pattern P :: [Int]
pattern P <- a:_

Can I somehow use a in the function f?

f :: [Int] -> Int
f P = a

The code above generates an error of Not in scope: 'a'.

Answer 1

Ok, this is slightly embarrasing, but I found that just doing this works:

{-# LANGUAGE PatternSynonyms #-}

pattern P :: Int -> [Int]
pattern P a <- a:_

f :: [Int] -> Int
f (P b) = b

main = print $ f [42]

The key point here is that the pattern parameter becomes explicit, but then it's also passed as an b pattern¹ which will be matched. I was missing this one piece of the puzzle.

The drawback is that obviously you need to enumerate all parts of the pattern you want to use.

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¹Of course this can still be called a, I just named it differently for illustration.