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nyash-codex 192c29efb1 docs: Pattern 4 stub clarification (Phase 195 deferred)

Phase 33-11 Quick Wins - Task 2

Clarified that Pattern 4 (continue statements) is intentionally
not implemented, with explicit error message and migration guide.

Changes:
1. Added clear STUB IMPLEMENTATION header to module docs
2. Updated lower() to return explicit error (not silent stub)
3. Added #[doc(hidden)] and #[allow(dead_code)] annotations
4. Created comprehensive migration guide (phase-195-pattern4.md)

Status:
- Not implemented: Pattern 4 (continue)
- Use instead: Pattern 1-3 (simple/break/if+phi)
- Reason: Continue requires complex PHI analysis, lower priority
- Build: ✅ Success (no errors, no new warnings)

Migration path:
- Pattern 1: Simple while loops (no break/continue)
- Pattern 2: Loops with break
- Pattern 3: Loops with if + PHI (workaround for continue)
- Pattern 4: (FUTURE) Loops with continue statements

2025-12-07 03:13:05 +09:00

2.5 KiB

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Phase 195: Pattern 4 (Loop with Continue) Implementation Plan

Status: Deferred (not yet implemented)

Overview

Pattern 4 handles loops with continue statements that skip to the next iteration. This is the most complex loop pattern due to additional control flow requirements.

Why Deferred?

Continue semantics require additional PHI and control flow analysis
- Continue creates an additional edge to the loop header
- Requires phi nodes for both continue and normal paths
- More complex than break (which exits the loop)
Pattern 3 covers most practical cases
- Pattern 1: Simple while loops
- Pattern 2: Loops with break
- Pattern 3: Loops with if + PHI (most common complex pattern)
- Pattern 4: Loops with continue (less common in practice)
Lower priority than break/if patterns
- Break patterns (Pattern 2) are more common
- If + PHI patterns (Pattern 3) handle complex control flow
- Continue can often be refactored using if statements

Example Use Case

local i = 0
local sum = 0
loop(i < 10) {
  i = i + 1
  if (i % 2 == 0) {
    continue  // Skip even numbers
  }
  sum = sum + i
}
// sum = 25 (1+3+5+7+9)

Implementation Requirements

When implemented, Pattern 4 lowering will need to:

Detect continue statements in the loop body
Generate PHI nodes for continue target (loop header)
Handle carrier variables (i, sum) across continue boundaries
Generate exit PHI nodes for final values after loop

Control Flow Diagram

       header
         |
         v
       body
         |
    /----+----\
   /           \
  v             v
continue      normal
  |             |
  \-----+-------/
        |
        v
      latch
        |
    /---+---\
   /         \
  v           v
loop        exit

Workaround

Until Pattern 4 is implemented, use Pattern 3 (if + PHI) instead:

local i = 0
local sum = 0
loop(i < 10) {
  i = i + 1
  if (not (i % 2 == 0)) {  // Invert condition
    sum = sum + i
  }
}

Migration Path

Pattern 1: Simple while loops (no break/continue)
Pattern 2: Loops with break
Pattern 3: Loops with if + PHI
Pattern 4: (FUTURE) Loops with continue statements

src/mir/builder/control_flow/joinir/patterns/pattern4_with_continue.rs - Stub implementation
src/mir/join_ir/lowering/loop_with_continue_minimal.rs - Lowering logic (TODO)

Timeline

Phase 195+: Implementation planned but deferred
Priority: Lower than Pattern 1-3
Complexity: High (additional control flow edges)

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