F# value restriction in empty list

Question

I have a F# function:

let removeEven (listToGoUnder : _ list) =
    let rec listRec list x =
        match list with
        | [] -> []
        | head::ta         


        

Answer 1

The empty list ([]) is quite special; it can be a list of any type. Therefore, the compiler complains that you don't have a specific type for []. Adding type annotation on the argument helps to solve the problem:

let results = removeEven ([]: int list)

or more idiomatic type annotation as suggested by @kvb:

let results: int list = removeEven []

This is probably beyond the question, but your function should be named as removeOdd since indices often start from 0 and your function removes all elements with odd indices. Moreover, things are much more clear if you use pattern matching on first two elements of the list rather than keep a counter x for checking indices:

let rec removeOdd = function
    | [] -> []
    | [x] -> [x]
    | x::_::xs -> x::removeOdd xs