Proposal: While Loop Expression

Status: Approved Approved: 2026-03-05 Author: Eric (with Claude) Created: 2026-03-05

Summary

Add while condition do body loop syntax as sugar over loop { if !condition then break; body }.

while self.pos < self.buf.len() do {
    self.pos += 1
}

Motivation

The Problem

Ori’s only general-purpose loop is loop { } with explicit break. Conditional loops require a guard-and-break pattern:

// Current: verbose conditional loop
loop {
    if self.pos >= self.buf.len() then break;
    if !self.buf[self.pos].is_whitespace() then break;
    self.pos += 1
}

This is the most common loop pattern in systems code — lexers, parsers, protocol handlers, stream processors — and Ori makes it unnecessarily verbose.

Comparison

Every language in the reference set has a while loop or equivalent:

Ori’s closest relatives (Rust, Swift, Zig) all have while loops. Gleam omits it but also has tail-call optimization making recursion zero-cost — Ori does not guarantee TCO.

Real-World Impact

A lexer in Ori without while:

// Skip whitespace
loop {
    if self.pos >= self.buf.len() then break;
    match self.buf[self.pos] {
        ' ' | '\t' | '\n' | '\r' -> self.pos += 1,
        _ -> break,
    }
}

// Read identifier
let start = self.pos;
loop {
    if self.pos >= self.buf.len() then break;
    if !self.buf[self.pos].is_alphanumeric() then break;
    self.pos += 1
}

With while:

// Skip whitespace
while self.pos < self.buf.len() do
    match self.buf[self.pos] {
        ' ' | '\t' | '\n' | '\r' -> self.pos += 1,
        _ -> break,
    }

// Read identifier
let start = self.pos;
while self.pos < self.buf.len() && self.buf[self.pos].is_alphanumeric() do
    self.pos += 1

Design

Syntax

while_expr = "while" [ label ] expression "do" expression .

The while keyword introduces the condition. do separates condition from body (consistent with for...do).

while condition do body

while condition do {
    statement1;
    statement2;
}

Semantics

while condition do body desugars to:

loop {
    if !condition then break;
    body
}

The desugaring is purely syntactic — no new runtime semantics.

Type

while...do has type void (same as for...do). It does not produce a value.

break value inside a while loop is a compile-time error (E0860), just as in for...do. Use loop { } for value-producing loops.

continue value inside a while loop is a compile-time error (E0861), just as in loop. While loops do not accumulate values.

Break and Continue

break and continue (without values) work inside while just as they do in loop:

while self.pos < self.buf.len() do {
    if self.buf[self.pos] == '#' then break;   // exit while
    if self.buf[self.pos] == ' ' then {
        self.pos += 1;
        continue                                // next iteration
    };
    process(self.buf[self.pos]);
    self.pos += 1
}

Labels

Labels work on while loops just as on loop and for:

while:outer scanning do {
    while self.pos < self.buf.len() do {
        if done then break:outer
    }
}

No while...yield

There is no while...yield form. A while loop has no inherent source to map from, making yield semantics unclear. Use loop with explicit accumulation or for with an iterator for collection building.

Grammar Changes

// Add to expression productions:
while_expr = "while" [ label ] expression "do" expression .

New reserved keyword: while — must be added to the reserved keyword list (7.3.1).

Interaction with Other Features

Mutable Self (companion proposal)

while + mutable self makes imperative loops natural:

impl Lexer {
    @skip_whitespace (self) -> void = {
        while self.pos < self.buf.len() && self.buf[self.pos].is_whitespace() do
            self.pos += 1
    }
}

Loop Expressions

while does not replace loop. Use loop when:

• You need an infinite loop
• You want to produce a value with break value
• The termination condition is complex or mid-body

Error Messages

Break With Value in While

error[E0860]: `break` with value in `while...do`
  --> src/main.ori:5:20
   |
 5 |     if done then break 42,
   |                  ^^^^^^^^ `while...do` does not produce a value
   |
   = help: use `loop { }` to produce a value with `break`
   = help: or remove the value: `break`

Continue With Value in While

error[E0861]: `continue` with value in `while`
  --> src/main.ori:5:20
   |
 5 |     if skip then continue 42,
   |                  ^^^^^^^^^^^ `while` does not collect values
   |
   = help: use `break` to exit with a value
   = help: or remove the value: `continue`

Migration / Compatibility

• New keyword: while must become reserved. Any identifier named while in existing code becomes invalid.
• No breaking changes to existing constructs. loop and for are unchanged.
• Gradual adoption: Existing loop { if !cond then break; ... } can be migrated to while cond do ... incrementally.

References

• Spec 16 — Control Flow
• Loop Expression Proposal
• For-Do Expression Proposal

Changelog

• 2026-03-05: Initial draft
• 2026-03-05: Approved — resolved open questions (no yield, do-only syntax, always void type), fixed grammar to use label production, added error messages for break/continue with value