let is the F# keyword used to bind any value to a name, it's used to bind the so called primitive types such as a string or an integer, to bind to a function or more complex structures such as arrays or records.

Here's how you can bind a string to an identifier named x:

let x = "some text"

The above snippet would be consider a constant in some other languages such as JavaScript. In F# there's no var or const there's only let and since in F# every value is immutable by default, that snippet is the equivalent of a constant.

Note that let in F# is different than let in JavaScript and this we'll be mentioned later on this page.

We're going to see more on functions later but here's how you can bind a function to an identifier named add:

let add x y =
    x + y

In the above snippet a function that adds two integers is being bound to an identifier named add and them two values are being bound to the identifiers x and y.

Shadowing

In F#, bindings are immutable by default which means that normally one can't reassign a value to a named binding, rather, if you try to do so using let what will happen is that you'll shadow an existing binding.

For instance, in the following example:

let x = "the answer"
let x = 42

In this case x is not being redefined nor its type is being changed from a string to an int, instead there are two different bindings with the same name x to different values. Of course, this example is not very useful because one binding is happening right after the other, but consider the following:

let printName name =
    let stripLastName name =
        if (String.exists (fun c -> c = ' ') name) then
            name.Split([|' '|]).[0]
        else
            name
    printfn "%s" name // Will print "John Doe"
    printfn "%s" (stripLastName name)

printName "John Doe" // Will print "John"

Don't worry too much if you don't fully grasp the above code, the main goal of that snippet is to demonstrate that the function printName expects an argument named name and in its body it defines another function stripLastName that also expects an argument name. Inside the scope of stripLastName the name argument is creating a new binding and thus shadowing the name argument received on the printName function. And that can be asserted by the two prints at the end of the printName function.

Comparing with JavaScript

The main differences that the let keyword in F# has from the same keyword in JavaScript are:

  • In JavaScript one would use let to define a named variable, an its value can be reassigned which is not the case in F#.
  • In JavaScript let is scope bound, so one can declare a new variable with an already used name as long as it is in a different scope, in F# that can be done within the same scope.

References