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
– ephemient, Commented Jul 28, 2017 at 2:00

Community · Accepted Answer · 2020-06-20 09:12:55Z

3

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.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jul 28, 2017 at 1:56

Alex

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