42 lines
1.6 KiB
Markdown
42 lines
1.6 KiB
Markdown
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# Match Guards — Syntax and Lowering (MVP + Design Notes)
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Status: reference + design additions during freeze (no implementation changes)
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Scope
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- Guarded branches as a readable form of first-match selection.
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- Canonical lowering target: if/else chain + PHI merges.
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Syntax (MVP)
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- Guard chain (first-match wins):
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```nyash
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guard <cond> -> { /* then */ }
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guard <cond> -> { /* then */ }
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else -> { /* else */ }
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```
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- Conditions may combine comparisons, `is/as` type checks, and literals with `&&` / `||`.
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Lowering
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- Always lowers to a linear if/else chain with early exit on first true guard.
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- Merge points use normal PHI formation invariants (see `reference/mir/phi_invariants.md`).
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Design additions (frozen; docs only)
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- Range Pattern (sugar):
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- `guard x in '0'..'9' -> { ... }`
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- Lowers to: `('0' <= x && x <= '9')`.
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- Multiple ranges: `in A..B || C..D` → OR of each bound check.
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- CharClass (predefined sets):
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- `Digit ≡ '0'..'9'`, `AZ ≡ 'A'..'Z'`, `az ≡ 'a'..'z'`, `Alnum ≡ Digit || AZ || az`, `Space ≡ ' '\t\r\n` (MVP set; expandable later).
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- `guard ch in Digit -> { ... }` expands to range checks.
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Errors & Rules (MVP)
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- Default `_` branch does not accept guards.
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- Type guard succeeds inside the then-branch; bindings (e.g., `StringBox(s)`) are introduced at branch head.
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- Short-circuit semantics follow standard branch evaluation (right side is evaluated only if needed).
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Observability (design)
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- `NYASH_FLOW_TRACE=1` may trace how guard chains desugar into if/else.
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Notes
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- This page describes existing guard semantics and adds range/charclass as documentation-only sugar during freeze.
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