How to write an infix function

Question

Is there a way to write an infix function not using symbols? Something like this:

let mod x y = x % y
x mod y

Maybe a keyword before "mod" or something.

Answer 1

The existing answer is correct - you cannot define an infix function in F# (just a custom infix operator). Aside from the trick with pipe operators, you can also use extension members:

// Define an extension member 'modulo' that 
// can be called on any Int32 value
type System.Int32 with
  member x.modulo n = x % n

// To use it, you can write something like this:
10 .modulo 3

Note that the space before . is needed, because otherwise the compiler tries to interpret 10.m as a numeric literal (like 10.0f).

I find this a bit more elegant than using pipeline trick, because F# supports both functional style and object-oriented style and extension methods are - in some sense - close equivalent to implicit operators from functional style. The pipeline trick looks like a slight misuse of the operators (and it may look confusing at first - perhaps more confusing than a method invocation).

That said, I have seen people using other operators instead of pipeline - perhaps the most interesting version is this one (which also uses the fact that you can omit spaces around operators):

// Define custom operators to make the syntax prettier
let (</) a b = a |> b
let (/>) a b = a <| b    
let modulo a b = a % b 

// Then you can turn any function into infix using:
10 </modulo/> 3

But even this is not really an established idiom in the F# world, so I would probably still prefer extension members.