Haskell : Error in split function

Question

I am very new at Haskell. I want to implement split function, which splites a list into two parts:

split 2 [1,2,3] → ([1,2], [3]) --means the first part has length 2, the second - length x-2
split 2 [1] → ([1], [])


split :: Int -> [a] -> ([a],[a])
split 0 x = ([], x)
split n x = splitH n x []

splitH :: Int -> [a] -> [a] -> ([a], [a])
splitH n (x:xs) begin | n == 0 = (begin, xs)
                      | otherwise = splitH n-1 xs (x:begin) -- here is the error

    main = print(split 2 [1,2,3] )

But this code does not compile. I get an error

`Couldn't match expected type ‘([a], [a])’
            with actual type ‘[a0] -> [a0] -> ([a0], [a0])’
Relevant bindings include
  begin :: [a] (bound at jdoodle.hs:6:17)
  xs :: [a] (bound at jdoodle.hs:6:13)
  x :: a (bound at jdoodle.hs:6:11)
  splitH :: Int -> [a] -> [a] -> ([a], [a]) (bound at jdoodle.hs:6:1)
Probable cause: ‘splitH’ is applied to too few arguments
In the first argument of ‘(-)’, namely ‘splitH n’
In the expression: splitH n - 1 xs (x : begin)`

What could cause the error ?

For comparison, you can look at the standard implementation of splitAt — ephemient

Alex Alex · Accepted Answer · 2017-07-28T01:56:00

Put parentheses around the expression n-1:

splitH (n-1) xs (x:begin)

Look at section 7, "Function Application Has Precedence over Operators" of this article for an explanation:

So if you see something like this:

f a b c + g d e

you know that you are adding the result of two function calls rather than calling one function with one of the arguments being the sum of two terms.

Haskell : Error in split function

1 Answers