Question

Consider the two examples below:

let myList = [0..10]
List.map (fun x -> x + 5)
    (List.filter (fun x -> x % 3 = 0) myList)

and

let myList = [0..10]
List.map (fun x -> x + 5) (List.filter (fun x -> x % 3 = 0) myList)

Both examples produce the same result:

val myList : int list = [0; 1; 2; 3; 4; 5; 6; 7; 8; 9; 10]
val it : int list = [5; 8; 11; 14]

F# being a white-space sensitive language, is there technically a difference between these two examples?

Answer 1

I think F# Code Formatting Guidelines is very helpful for you, especially the first section about General rules for indentation.

Here is a relevant excerpt from the page:

When you break long lines, the continuation of the line must be indented farther than the enclosing construct. For example, function arguments must be indented farther than the first character of the function name, as shown in the following code.

let someFunction param1 param2 =
    let result = myFunction1 param1
                     param2
    result * 100

So indentation in the first example helps F# compiler parse your example correctly.

F# being a white-space sensitive language, is there technically a difference between these two examples?

No. Two code fragments have the same meaning here; the only difference is readability. I prefer the first one since my eyes don’t have to go to far right to read the whole function.

Sometimes when a line is too long, you could reorder arguments using pipes and break that line to multiple ones:

myList
|> List.filter (fun x -> x % 3 = 0)
|> List.map (fun x -> x + 5)

UPDATE:

I took some time to aggregate information from different sources and wrote up a comprehensive guide to F# Formatting Conventions; you might want to look into it.