PureScript - Pattern match arrays of unknown length

Question

Is there a way to pattern match arrays of an unknown length in PureScript? As an example, here's how I would do it with a List in Haskell:

addFirstTwo :: (Num a) => [a] -> a
addFirstTwo (x:y:_) = x + y

I tried something similar (using Array a instead of [a]) in PureScript, but got the following error:

Operator Data.Array.(:) cannot be used in a pattern as it is an alias for function Data.Array.cons. Only aliases for data constructors may be used in patterns.

I know I can use Lists in PureScript instead of Arrays, but I'd like to pattern match with arrays. I didn't see how to do this after reading the Array pattern matching section of PureScript by Example.

Answer 1

Array cons patterns have been removed from the language long ago. You can use uncons from Data.Array instead, like so:

case uncons xs of
  Just {head: x, tail} ->
    case uncons tail of
      Just {head: y, tail: _} -> x + y
      Nothing -> 0
  Nothing -> 0

But beware that uncons ∈ Θ(n).

Alternatively you can use take from Data.Array:

case take 2 xs of
  [a, b] -> a + b
  _ -> 0

take ∈ Θ(min(n, m)) (in this case m = 2).

Or even (!!) from Data.Array:

case xs !! 0, xs !! 1 of
  Just x, Just y -> x + y
  _, _ -> 0

In general lists are more pleasant to work with than arrays are.