ReasonML Quick Start Guide

Currying

Some of the Belt standard library functions have a U suffix, such as this one:

Belt.List.makeBy

You can see the suffix here:

Belt.List.makeByU

The U suffix stands for uncurried. Before going further, let's define currying.

In Reason, every function accepts exactly one argument. This seems to contradict many of our earlier examples:

let add = (a, b) => a + b;

The preceding add function looks as if it accepts two arguments, but it is actually just syntactic sugar for the following:

let add = a => b => a + b;

The add function accepts a single argument, a, which returns a function that accepts a single argument, b, and then returns the result of a + b.

In Reason, both versions are valid and have the same compiled output. In JavaScript, ...