Question

I can do this:

let foo = bar
    |> baz

but I get a syntax error when I do this:

let foo = bar
    <| baz

why?

I also get the syntax error when ever I define my own infix operators and try to use them in this manner.

Answer 1

Unless I’m missing something, the two operators (as well as any other infix operator) behave the same. When I define bar and baz and use your example, I get an error in both of the cases. The off-side rule in F# allows you to indent the operator less far from the left, but the column of the arguments has to match:

let bar = 1
let baz x = x + 1

let foo1 = bar 
        |> baz

let foo2 = baz 
        <| bar

Both of these give an error message:

let foo1 = bar 
  |> baz

let foo2 = baz 
  <| bar

(Although you may see only the first error, because the F# compiler stops reporting additional errors after syntax error – probably because it cannot guarantee that they will be sensible)

The rules are quite well described in Code Formatting Guidelines on MSDN.