Can this be expressed in point free style?

Question

Given the following expression to sum an IEnumerable of numbers:

let sum l = l |> Seq.reduce(+)  //version a

is it possible to eliminate

Answer 1

"Eta conversion" simply means adding or removing the argument. The problem you are hitting is called value restriction. In ML languages, a value declared as a value, ie. declared without explicit arguments, cannot have a generic type, even if it has a function type. Here is some relevant literature. The idea is to prevent a ref cell from holding values of different types. For example, without value restriction, the following program would be allowed:

let f : 'a -> 'a option =
    let r = ref None
    fun x ->
        let old = !r
        r := Some x
        old

f 3           // r := Some 3; returns None : int option
f "t"         // r := Some "t"; returns Some 3 : string option!!!

As kvb said, if you do not intend the function to be generic, then you can add a type signature and use point-free style.