Position based tuple pattern match in Haskell

Question

Is it possible to pattern match tuples in Haskell, but without knowing the dimension of tuple? I want to create a function which mach against any tuple, which first element is A, like:

data A = A Int
test args@(A a,..) = a

I know there is Data.Tuple.Select module and I can use it like this:

test args = case sel1 args of
    A a -> a
    ...

But is this the only way of doing this or has Haskell got some default mechanisms to match against any dimension tuple?

Answer 1

You could use the ViewPatterns extension to pattern match the result of a function applied to an argument:

{-# LANGUAGE ViewPatterns #-}

data A = A Int
test (fst -> A a) = a

You could use lenses to project out arbitrary fields:

{-# LANGUAGE ViewPatterns #-}

import Control.Lens
import Control.Arrow ((&&&))

data A = A Int
test (fields _1 _3 -> (A x, A y)) = x + y

fields f1 f2 = (^.f1) &&& (^.f2)

-- > test (A 1, A 2, A 3)
-- > 4

Answer 2

If you don't want to use type classes, you could also use nested tuples. So instead of having a tuple of type (A, B, C, D) you have a tuple of (A, (B, (C, D))).

You could then easily match against the first element of a however deeply nested tuple, like this:

test :: (A, b) -> Int
test (A a, _) = a