Question

# type foo = Foo of int * int
# let t = (1, 2)
# Foo t
Error: The constructor Foo expects 2 argument(s),
   but is applied here to 1 argument(s)

How is it that I must do Foo (1, 2) to avoid that error even t has the appropriate type?

Answer 1

This is one of the troubling parts of OCaml syntax, in my opinion. Despite the way it looks, the constructor Foo doesn’t require a 2-tuple as its argument. It requires, syntactically, two values in parentheses–but they aren’t a tuple. So it’s simply the case that t has the wrong type. The way to make this work is to say:

let (a, b) = t in Foo (a, b)

The problem really is that parentheses are being used for two different things (or so I claim). Once you get used to this it’s not so difficult to deal with.

Edit: if you want the constructor Foo to take a single tuple, rather than two separate values, you can define it like this:

type foo = Foo of (int * int)

Then the rest of your original code will work.