Use

Gleam lacks exceptions, macros, type classes, early returns, and a variety of other features, instead going all-in with just first-class-functions and pattern matching. This makes Gleam code easier to understand, but it can sometimes result in excessive indentation.

Gleam's use expression is for calling functions that take a callback as an argument without increasing the indentation of the code.

All the code below the use becomes an anonymous function that is passed as a final argument to the function on the right hand side of the <-, and the assigned variables become the arguments to the anonymous function.

This code:

pub fn main() -> Nil {
  use a, b <- my_function
  next(a)
  next(b)
}

Expands into this code:

pub fn main() -> Nil {
  my_function(fn(a, b) {
    next(a)
    next(b)
  })
}

To ensure that your use code works and is as understandable as possible, the right-hand-side ideally should be a regular function call rather than a more complex expression, which would be more difficult to read.

This is a very capable and useful feature, but excessive application of use may result in unclear code, especially to beginners. Usually the regular function call syntax results in more approachable code!