In mathematics, functions have no side effects.

They will always return the same result given the same parameters.

f(x) = 2x + 1
f(0) = 1
f(1) = 3

In software development, such functions are called pure.

Yet, not every functions can be pure, like:

  • getting input from the user
  • getting the current time
  • performing a request to an external service

Such impure functions have side effects.

Letlang provides a mechanism to decouple the handling of a side effect from a function.

Using the effect statement, you can declare a new type of side effect:

class log_level(lvl: "debug" | "info");

effect log(log_level, string) -> @ok;

Inside a function, you can then trigger the effect, delegating its handling to the caller.

This is done with the perform keyword followed by a call to the effect:

func greet(name: string) -> @ok {
  perform log("info", "Hello " <> name);

Using a do {} block and one or more intercept clause, the caller can handle the side effect.

An intercept clause uses pattern matching to select which effect to handle.

The last expression of an intercept clause will be used as return value of the perform keyword.

Unhandled effects are then propagated to the builtin runtime Letlang includes during compile time.

If the effect is unknown to the runtime, the process will crash.

module example::main;

import std::io;

pub func main(args: list<string>) -> @ok {
  @ok := do {
  intercept log("debug", _message) {
    # silenced
  intercept log("info", message) {

  perform log("debug", "will crash");