Haskell Patterns not matched: (_:_) _

Question

I'm writing a function that takes an input a list, creates sublists, and retrieves n elements that it outputs into a new list. I'm writing guards depending on the value of the inputs, but I keep getting the error "Patterns not matched: (:) _".

Anyone identifies the issue?

nKsets :: [Int] -> Int -> [[Int]]
nKsets [] _ = error "Empty list should not be given as input"
nKsets l  n
    | n <= 0 = []
    | n > 0 = ...

The error I'm getting is:

Pattern match(es) are non-exhaustive
In an equation for ‘nKsets’: Patterns not matched: (_:_) _

score 5 · Accepted Answer · answered Sep 23 '22 at 18:47

The exhaustivity matcher doesn't know that all values of n are either greater than 0 or not greater than 0, so it assumes that your second definition is not exhaustive. Use otherwise instead of n > 0 to guarantee that a guard succeeds.

nKsets :: [Int] -> Int -> [[Int]]
nKsets [] _ = error "Empty list should not be given as input"
nKsets l  n
    | n <= 0 = []
    | otherwise = ...

You can also use a third definition instead of two guards on the second:

nKsets :: [Int] -> Int -> [[Int]]
nKsets [] _ = error "Empty list should not be given as input"
nKsets l n | n <= 0 = []
nKsets l n = ...

Haskell Patterns not matched: (_:_) _

1 Answers1