Haskell pattern matching on vectors

Question

Is it possible to use list style pattern matching on vectors?

ie

import qualified Data.Vector as V

f :: V.Vector a -> a
f (x:xs) = x 

gives an error

Answer 1

-XViewPatterns can let you do this:

{-# LANGUAGE ViewPatterns #-}
module VecViewPats where

import Data.Vector (Vector)
import qualified Data.Vector as V

uncons :: Vector a -> Maybe (a, Vector a)
uncons v = if V.null v
  then Nothing
  else Just (V.unsafeHead v, V.unsafeTail v)

vsum :: Num a => Vector a -> a
vsum (uncons -> Just (a,av)) = a + vsum av
vsum (uncons -> Nothing) = 0

Or -XLambdaCase

import Control.Category ((>>>))
-- ...
vsum :: Num a => Vector a -> a
vsum = uncons >>> \case
  Just (a,av) -> a + vsum av
  Nothing     -> 0

But honestly, that's seems like a bit of a code smell as you're using one data structure (Vector) as another ([]) which suggests that maybe your choice of data structure is off.

If you really just want to treat it like a list for the purposes of some algorithm, why not use toList?