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.

Answer 2

A point-free function is a value.
As other answers say, F# does not allow generic values. However, it perfectly allows generic functions. Let's convert sum into a function by adding a fake unit parameter:

let sum_attempt1() = Seq.reduce (+)
let v1 = [1.0; 2.0]     |> sum()    // float
// inferred by first usage:
// val sum_attempt1: unit -> (seq<float> -> float)

This works, although it is not yet generic. Marking the function inline does the trick:

let inline sum() = Seq.reduce (+)
// val sum: unit -> (seq<'a> -> 'a)

// Use
let v1 = [1; 2]         |> sum()    // int
let v2 = [1.0; 2.0]     |> sum()    // float
let v3 = ["foo"; "bar"] |> sum()    // string

Answer 3

You can do it in point free style, but you need to add a (monomorphic) type annotation:

let sum : int seq -> int = Seq.reduce (+)