Gleam is a type-safe and extensible programming language available for the Erlang virtual machine and JavaScript runtime.

The recently released version 0.25 introduces a long-awaited new feature:useexpression(useexpressions).

According to the official introduction,useexpression is a syntactic sugar that turns all subsequent expressions into an anonymous function that is passed as an additional argument to the function call.

For example, suppose there is a function to be calledwith_filewhich opens a file, passes the opened file to the given function so it can read or write to it, and closes the file.

// Define the function
pub fn with_file(path, handler) {
  let file = open(path)
  handler(file)
  close(file)
}

// Use it
pub fn main() {
  with_file("pokemon.txt", fn(file) {
    write(file, "Oddish\n")
    write(file, "Farfetch'd\n")
  })
}

by usinguse, this function can be called without additional indentation.Use belowuseThe example compiles to the exact same code as above.

pub fn main() {
  use file <- with_file("pokemon.txt")
  write(file, "Oddish\n")
  write(file, "Farfetch'd\n")
}

And it’s not limited to a single argument, it can take functions of any argument, including functions that don’t take any arguments.

Release Notes | Release Notes