One line F# code is hard to understand

Question

What is the meaning of this one line F# snippet

let x x = x + 2 in x 2;;

It is valid and just return 4.

But what is it?? is x a function? is it also a parameter? also x seems to be calling itself (x 2) but its not marked with "rec".

Can anyone explain?

Answer 1

So lets try to understand what happens.

let x x

defines a function called x which takes an argument of x.

`let x x = x + 2`

means you have a function x which takes an argument also called x and returns x+2

The final part in x 2 calls the function with an argument of 2.

So the function can be written as

let f x = x + 2
f 2

which obviously returns 4.

Answer 2

The 'in' means that it's not using light syntax. Basically, the in means that the prior binding is defined for the following expression. Since the in keyword means that the binding of x is valid for the following expression, another way of writing it would be to replace x with the value of the binding like so:

(fun x -> x + 2) 2

I think it's important to be aware of the fact that all bindings work in this way, i.e. bindings are a way of writing expressions in a more understandable way.

If you look here, you can learn more about the more OCaml like syntax F# has available if you have #light off specified (on is the default).

Answer 3

let x x = x + 2 in x 2;;

Disambiguate the variables by renaming the outer x to f:

let f x = x + 2 in f 2;;

Indent:

let f x =
  x + 2
f 2

So it defined a function that adds two and applies it to two to get four.