Is it possible to write tacit functions in F#

Question

Tacit or point-free style programming allows one to create functions without regard to their arguments. Can this be done in F#?

Answer 1

Just to go with Chuck's answer & Chris Smiths' comment, you could write

let digits = string_of_int >> String.length
digits 9000;; // 4
[1; 10; 100] |> List.map digits;; // [1;2;3]

When you combine those composition & pipeline operators with higher-order functions, you can do complicated stuff very succinctly:

let prodSqrtAbs = Seq.map (abs>>sqrt) >> Seq.reduce (*)
prodSqrtAbs [| -9.0; 4.0 |];;  // 6.0

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EDIT: I just read about J and its implicit fork operator. That is very powerful. You can build equivalent higher-order operators in F#, but they won't be applied implicitly. So, for example, first define lift (using explicit arguments)

let lift op a b x = op (a x) (b x)

and then apply it explicitly

let avg = lift (/) List.sum List.length

to get something resembling the J example on the Wikipedia page you linked to. But its not quite "tacit."

Answer 2

Sure. All you need is function composition and currying, and both of these are possible in F#.

let compose f1 f2 = fun x -> f1 (f2 x);;
let digits = compose String.length string_of_int;;
digits 9000;; // 4