Define a function start that returns all elements of the given list except the last one:
start list = start' list []
where
start' (x:[]) acc = acc
start' (x:xs) acc = start' xs (acc:x)
When trying to compile, an error is being thrown on the last expression (acc:x) in line 4. Why is this happening? Regardless of efficiency, why is this code incorrect?
The error is occurring because (acc:x) is attempting to concatenate a list acc with an element x. However, acc is a list and x is an element, so the types do not match.
To fix this issue, you need to wrap x in a list in order to concatenate it with acc. The corrected code is shown below:
start list = start' list []
where
start' (x:[]) acc = acc
start' (x:xs) acc = start' xs (acc ++ [x])
This code is incorrect because it has a syntax error. The ++ operator is used for list concatenation in Haskell, and it requires both operands to be lists. In the original code, acc is a list but x is an element, so the code fails to compile.
The corrected code uses the ++ operator to concatenate acc with a list containing only x: (acc ++ [x]). This ensures that both operands of ++ are lists, allowing the code to compile correctly.