feat: support yielding value blocks

README.md

@@ -178,9 +178,9 @@ optionals use `?T`; values of `T` and `null` initialize optional storage, parame
 
 fallible values use explicit `T ! E` channels, where `E` is an enum, record or tagged union. success and error values initialize storage, arguments and returns; fallible values also work in arrays, records and callbacks. `void ! E` functions may fall through successfully. native unbacked enums and tagged unions compose with `A | B`, including grouped/imported error sets; compatible subsets widen without changing variant identities. immutable match subjects narrow within explicit/grouped arms and final `else` arms, allowing residual errors to propagate into smaller channels.
 
-`try expression` evaluates once, extracts success or returns a compatible error from the enclosing fallible function. `expression catch [|err|] fallback` evaluates only the selected path; fallbacks may be expressions, terminal statement blocks or, for void success, blocks that fall through. inferred `T!` signatures and yielding catch blocks are not implemented. write `builtin!(...)` without a gap before `!`; in general expressions a spaced `T ! (E)` denotes a fallible type.
+`try expression` evaluates once, extracts success or returns a compatible error from the enclosing fallible function. `expression catch [|err|] fallback` evaluates only the selected path; fallbacks may be expressions, yielding or terminal statement blocks or, for void success, blocks that fall through. inferred `T!` signatures are not implemented. write `builtin!(...)` without a gap before `!`; in general expressions a spaced `T ! (E)` denotes a fallible type.
 
-`defer statement` and `errdefer [|err|] statement` register reached cleanup in the current block; braces introduce explicit nested scopes. cleanup runs LIFO when its scope exits, including return, break and continue; return values are evaluated and copied before cleanup. `errdefer` requires a fallible function and runs only on a language-error return while its scope remains active. cleanup cannot return, propagate with `try` or transfer control out of itself. source errors remain ordinary values: catch does not intercept diagnostic traps or compiler allocation/IO failures, and traps do not unwind source cleanup. a fallible `main` prints its success value normally; an unhandled error prints nothing and exits with status `1`, without creating a diagnostic.
+`defer statement` and `errdefer [|err|] statement` register reached cleanup in the current block; braces introduce explicit nested scopes. cleanup runs LIFO when its scope exits, including return, yield, break and continue; returned and yielded values are evaluated and copied before cleanup. `errdefer` requires a fallible function and runs only on a language-error return while its scope remains active, not on a successful yield. cleanup cannot return, propagate with `try` or transfer control out of itself; a nested value block may yield locally. source errors remain ordinary values: catch does not intercept diagnostic traps or compiler allocation/IO failures, and traps do not unwind source cleanup. a fallible `main` prints its success value normally; an unhandled error prints nothing and exits with status `1`, without creating a diagnostic.
 
 `sizeof!(T)` and `alignof!(T)` report the supported target's native layout, including aggregate padding, slice descriptors and nullable pointers. `ptrcast!(value)` takes its full result pointer type from context; `ptrcast!(T, value)` states it explicitly. casts preserve the storage location and optional presence, permit byte reinterpretation, and cannot remove readonly qualification. `constcast!` changes pointer or slice mutability without changing the pointee type; it does not make genuinely readonly storage writable or extend its lifetime. `anyopaque` is a pointer-only pointee, not a storable or dereferenceable value.
 
@@ -200,9 +200,20 @@ enums use `Kind :: enum { number, empty }`, qualified members such as `Kind.numb
 
 untagged `Bits :: union { integer u64, real f64 }` overlays its members in shared storage. keyed construction selects exactly one member; reading another member reinterprets the same bytes. tagged unions use `union(enum)` or `union(Kind)`; an explicit enum must contain every union variant. construct them with `Choice { number = 42 }`, contextual `.number{42}`, `.point{x = 40, y = 2}` for record payloads, or `.empty` for `void` payloads. field reads/writes validate the active tag. nested aggregates copy by value, including across calls. CLI union results print `{payload}` (or `{}` for a void payload); untagged results print the first member.
 
-`if (guard) then_value else else_value` is a boolean value expression. `if value |payload| then_value else else_value` instead tests optional presence and binds the exact payload as an immutable by-value capture in the then expression; nested optionals preserve present-null payloads. the guard runs once and only the selected expression executes; `else` is required and binds to the nearest conditional. both runtime branches must fit the result context, including optional/fallible channels and aggregate copies. constant-input procedures, direct `$()` evaluation, generic branch specialization, explicit compile-time subject selection and generated type positions reuse checked match execution. boolean guards require parentheses unless they are calls or explicit `$()` expressions; optional captures also permit bare guards. parenthesize a conditional used as a match-arm value to distinguish it from an `if` statement. yielding value blocks remain unsupported.
+`if (guard) then_value else else_value` is a boolean value expression. `if value |payload| then_value else else_value` instead tests optional presence and binds the exact payload as an immutable by-value capture in the then expression; nested optionals preserve present-null payloads. the guard runs once and only the selected expression executes; `else` is required and binds to the nearest conditional. both runtime branches must fit the result context, including optional/fallible channels and aggregate copies. constant-input procedures, direct `$()` evaluation, generic branch specialization, explicit compile-time subject selection and generated type positions reuse checked match execution. boolean guards require parentheses unless they are calls or explicit `$()` expressions; optional captures also permit bare guards. parenthesize a conditional used as a match-arm value to distinguish it from an `if` statement.
 
-`match value { .number |n|: n + 2, .empty: 0 }` selects one arm after evaluating its subject once. matches support enums, tagged unions, composed error sets, numeric/boolean scalars, comma-separated patterns, numeric ranges, final `else` arms, arm-local payload captures, statement bodies and expression results. checking rejects missing coverage, duplicate/overlapping patterns, unreachable arms and incompatible captures/results. block arms can return or transfer loop control; yielding value blocks and pointer captures belong to later gates.
+value blocks contain ordinary statements and produce a result with `yield expression`:
+
+```hon
+answer i16 :: {
+    value i16 := 40
+    yield value + 2
+}
+```
+
+`yield` exits the nearest enclosing value block, match arm or catch handler with a copied result. ordinary nested statement blocks and loops do not change its target; return, break and continue retain their usual targets. conditional branches also accept yielding blocks. result context reaches yielded expressions, including narrow integers, aggregates and optional/fallible channels. nonvoid fallthrough, incompatible yields and yields outside a value source diagnose; only reached invalid paths trap. constant evaluation and type-producing procedures support yielding blocks, generated storage/defaults and owned snapshots. empty, singleton and keyed record/tuple literals retain their existing syntax; statement-bearing braces select a value block. labeled yields and value-producing loops remain unsupported.
+
+`match value { .number |n|: n + 2, .empty: 0 }` selects one arm after evaluating its subject once. matches support enums, tagged unions, composed error sets, numeric/boolean scalars, comma-separated patterns, numeric ranges, final `else` arms, arm-local payload captures, statement bodies and expression results. checking rejects missing coverage, duplicate/overlapping patterns, unreachable arms and incompatible captures/results. block arms can yield results, return or transfer loop control; pointer captures belong to a later gate.
 
 a final `inline |tag|:` arm on an enum, or `inline |payload[, tag]|:` on a tagged union, expands into one ordinary arm for each variant not covered earlier. dispatch remains runtime and evaluates the subject once. enum captures and optional union tags are immutable compile-time enum values; payloads are immutable by-value captures with each variant's exact type, including a `void` value for empty variants. either capture may be discarded with `_`. static tag matches/conditions select variant-specific operations before checking, including generic calls, nested inline arms and type-producing procedure bodies. ordinary payloads remain runtime values, and return/break/continue/deferred cleanup retain ordinary match semantics. redundant/nonfinal inline arms, invalid captures and scalar/untagged subjects are diagnosed. expansion shares the 4096-step type-control bound.