Piping, Composition, and Currying

Question

It seems to me that all of these are related. What is the difference?

Answer 1

In addition to what Daniel wrote, there is a very close correspondence between piping (the |> and <| operators) and function composition (the >> and << operators).

When you use piping to pass some data to a seqence of functions:

nums |> Seq.filter isOdd
     |> Seq.map square
     |> Seq.sum

... then this is equivalent to passing the input to a function obtained using function composition:

let composed = 
     Seq.filter isOdd
  >> Seq.map square
  >> Seq.sum

composed nums

In practice, this often means that you can replace function declaration that uses piping on the argument with a composition of functions (and use the fact that functions can be used as values). Here is an example:

// Explicit function declaration
foo (fun x -> x |> bar |> goo)

// Equivalent using function composition
foo (bar >> goo)