Crate loop_let

loop_let

This crate provides a macro for a new control flow mechanism in Rust proposal loop let. Which is a immutable loop pattern that is meant for tail call recursive algorithms earning tail call optimization for free.

This crate provides the loop_let! macro that implements the loop let pattern. The syntax is as follows:

loop_let!((pattern) = (initial); {
   // code ...
});
 
loop_let!(fails (pattern) = (initial); {
  // code ...
});

The pattern is the pattern that will be matched against the input. The initial is the initial value of the input. The code block is the code that will be executed in every iteration of loop. It’s important to notice that the code block must return either a Continue or a Break.

Continue is used to set the input value for the next iteration of the loop. Break is used to break the loop and return the value.

The fails keyword is used before the pattern when using fallible patterns. This way, the loop breaks once the pattern fails. By using the fails keyword, the loop_let macro will return () (this means you can only use Break(())). So it can’t be used as an expression.

Why?

The main purpose of this crate is to encourage the utilization of tail call recursive algorithms in Rust rather than the traditional recursion which relies on the compiler to optimize the code.

Some other earnings of using the loop let pattern in comparison to other tail call recursive approaches are:

• Support for pattern matching (even fallible patterns)
• Loops as an expression
• Avoids the need for mut
• Avoids the need of tail call optimization at the compiler level
• Increased readability

This pattern is used in Clojure with the loop/recur pattern where a loop might return a value or call itself with new arguments.

This is a proposal for a new control flow mechanism in Rust as a language construct loop let.

// Proposed syntax (no need for `fails` keyword)
loop let (pattern) = (initial) {
    // ...
    continue (new_input); // New syntax for continue
    // ...
    break (result);
}

Macros

• loop_let
  Implementation of the loop let immutable loop pattern. This implements a loop that is meant for tail call recursive algorithms earning tail call optimization for free.