OCaml Tips: Multiple Let Bindings

One thing that was a bit weird for me in OCaml early on was how to introduce multiple let bindings (e.g. in the body of a function definition). Think something like this in Clojure:

(let [a 1
      b 2
      c 3])

I’ve noticed many people were using the following syntax:

let a = 1 in
let b = 2 in
let c = 3 in
a + b + c

This works, but seemed a bit too verbose to, especially given that in this case the bindings don’t depend on one another (e.g. we don’t compute c from a and b). Turned there’s a slight variation of the let syntax for cases like this one:

let a = 1
and b = 2
and c = 3 in
a + b + c

(* or more compactly *)
let a = 1 and b = 2 and c = 3 in a + b + c

Still a bit too verbose for my taste, but I definitely like it a more over multiple let ... in expressions, as it reads (subjectively) better. As a bonus - this style of introducing bindings clearly shows that all the bindings we introduced are independent of each other, which reduces some of the mental overhead for readers of the code.¹

Note, however, that you can’t do something like this:

let a = 1
and b = 2
and c = a + b in
c

This will result in a compilation error, as you can’t have any of the bindings refer to other bindings within the same let expression. Here you’ll need a let ... in to make the c binding work:

let a = 1 and b = 2 in
let c = a + b in
c

There’s one other option for compact let bindings - pattern matching! Most commonly you’d match against a tuple or some record:

let (a, b, c) = (1, 2, 3) in
a + b + c

(* the parens above are optional and can be omitted *)
let a, b, c = 1, 2, 3 in
a + b + c

let {a; b; c; _} = t in
a + b + c

That’s all I have for you today. Keep hacking!

1. You can read more on the subject here. ↩