Learning OCaml: Numerical Type Conversions

Today I’m going to cover a very basic topic - conversions between OCaml’s primary numeric types int and float. I guess most of you are wondering if such a basic topic deserves a special treatment, but if you read on I promise that it will be worth it.

So, let’s start with the basics that probably everyone knows:

• you can convert integers to floats with float_of_int
• you can convert floats to integers with int_of_float

int_of_float 10.5;;
- : int = 10
float_of_int 9;;
- : float = 9

Both functions live in Stdlib module, which is opened by default in OCaml. Here it gets a bit more interesting. For whatever reasons there’s also a float function, that’s a synonym to float_of_int. There’s no int function, however. Go figure why…

Here’s a bit of trivia for you - int_of_float does truncation to produce an integer. And there’s also a truncate function that’s another alias for int_of_float. Again, for whatever reasons it seems there are no functions that allow you to produce an integer by rounding up or down. (although such functions exist for floats - e.g. Float.round, Float.floor and Float.ceil)

int_of_float 5.5;;
- : int = 5
truncate 5.5;;
- : int = 5
5.5 |> Float.round |> int_of_float;;
- : int = 6

More interestingly, OCaml 4.08 introduced the modules Int and Float that bring together common functions for operating on integers and floats.¹ And there are plenty of type conversion functions in those modules as well:

• Int.to_float and Int.of_float
• Float.of_int and Float.to_int
• Int.to_string and Float.to_string

The introduction of the Int and Float modules was part of (ongoing) effort to make OCaml’s library more modular and more useful. I think that’s great and I hope you’ll agree that most of the time it’s a better idea to use the new modules instead of reaching to the “historical” functions in the Stdlib module. Sadly, most OCaml tutorials out there make no mention of the new modules, so I’m hoping that my article (and tools like ChatGPT) will steer more people in the right direction.

If you’re familiar with Jane Street’s Base, you’ll probably notice that it also employs similar structure when it comes to integer and float functionality.

Which type conversion functions do you prefer? Why?

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

1. Technically speaking, Float existed before 4.08, but it was extended then. 4.08 also introduced the modules Bool, Fun, Option and Result. Good stuff! ↩