How to forward pipe a value to the left side parameter of a function?

Question

For example,

let fn x y = printfn "%i %i" x y
1 |> fn 2 // this is the same as fn 2 1 instead of fn 1 2

How to make it fn 1 2?

This is just a simplified example for test. I have a complex function return a value and I want to forward pipe it to the left side (in stead of right side) of a function.

Answer 1

I assume that you have at least two pipes. Otherwise, fn 1 2 does the job; why should we make it more complicated?

For commutative functions (which satisfy f x y = f y x) such as (+), (*), etc. you can just use 1 |> fn 2.

For other functions, I can see a few alternatives:

1. Use backward pipes

   arg1
   |> fn1
   |> fn2 <| arg2
   

   I tend to use this with care since operators' precedence might cause some subtle bugs.

2. Use flip function

   let inline flip f x y = f y x
   
   Array.map2 (fun y d -> (y - d) ** 2.0) y d
   |> Array.sum
   |> flip (/) 2.0
   

   It's not pretty but it's clear that order of arguments in (/) is treated differently.

3. Use anonymous functions with fun and function keywords
   This is handy if you need pattern matching in place.

   input
   |> createOption
   |> function None -> 0 | Some _ -> 1
   

IMO, you don't have to pipe all the way. Create one more let binding, then your intention is clear and you can avoid bugs due to unusual styles.

let sum =
   Array.map2 (fun y d -> (y - d) ** 2.0) y d
   |> Array.sum
 sum / 2.0    

Answer 2

Here is a solution that looks pretty. I don't know if it is too practical though

1 |> fn <| 2