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Can anyone explain to me why my Haskell function gives rise to a type-definition error? Originally, I wrote the following function to subtract one from the first n elements in a list: dec_first :...
#2: Nominated for promotion
by
Alexei
·
2023-07-26T15:19:00Z (over 1 year ago)
#1: Initial revision
by
mr Tsjolder
·
2023-07-13T15:17:26Z (over 1 year ago)
How to use function composition for applying a function to first elements of a list?
Can anyone explain to me why my Haskell function gives rise to a type-definition error? Originally, I wrote the following function to subtract one from the first `n` elements in a list: ```haskell dec_first :: Int -> [Int] -> [Int] dec_first 0 l = l dec_first n (x:xs) = (x-1):dec_first (n-1) xs ``` However, as a challenge, I wanted to write this function using available standard functions, using function compositions. This is how I ended up writing the function (for the subtraction part) ```haskell dec_first :: Int -> [Int] -> [Int] dec_first = map (subtract 1) . take ``` but this function does not compile due to a type mismatch. In my attempt to fix the error, I also tried the following variant: ```haskell dec_first :: Int -> [Int] -> [Int] dec_first n = map (subtract 1) . take n ``` This function actually does work as expected. However, if I specify all parameters again, things breaks again: ```haskell dec_first :: Int -> [Int] -> [Int] dec_first n l = map (subtract 1) . take n l ``` Can anyone help me understand why this function only works with one variable, but not in the other settings?