select.md

SELECT compatibility profile

SELECT evaluates WHEN conditions in order and executes only the first matching branch. Later conditions and bodies are skipped, including their host requests. If nothing matches, OTHERWISE executes; without OTHERWISE, execution simply continues. Empty SELECTs and empty branches are valid in the recorded RexxJS baseline.

SELECT
WHEN mode = 'read' THEN CALL read_data
WHEN mode = 'write' THEN CALL write_data
OTHERWISE SAY 'unknown mode'
END

A multiline WHEN ends at the next WHEN, OTHERWISE or END. Its body can contain nested SELECTs, actual DO loops, IFs and routine calls. A multiline OTHERWISE runs through the matching END and must be the final clause. Inline branch statements must finish in their own clause.

SELECT
WHEN ready THEN
    DO i = 1 TO 3
        CALL TESTHOST i
    END
    SAY 'complete'
OTHERWISE
    SAY 'waiting'
END

The reference also accepts WHEN condition THEN DO with nothing after DO on the header. That DO is a branch marker, not an additional nested block: the next matching END closes SELECT. It does not require a second END. For clarity, use bare THEN for multiline branches and introduce any actual DO block on its own following clause. Inline OTHERWISE DO is outside the supported profile; use a multiline OTHERWISE with a separate DO statement.

Another permissive reference behavior is preserved: inline OTHERWISE clauses can occur before later WHEN clauses, and repeated inline OTHERWISE bodies concatenate in source order. All WHEN conditions still run before any fallback body. A matched WHEN skips every OTHERWISE body. Ordinary scripts should place one OTHERWISE after the WHEN clauses.

RETURN and EXIT propagate out of either selected branch type. LEAVE and ITERATE apply to the enclosing loop; SELECT introduces no loop scope. An error or early return restores loop variables through the existing execution frames. Conditions and body instructions retain their original source lines.

Hosts and execution budgets

Conditions can call registered functions, and selected bodies can suspend for COMMAND or FUNCTION replies. Partial expression results remain cached while waiting, so earlier host calls in a condition are not repeated. A false reply advances to the next WHEN. A true reply prevents later conditions from running. Invalid JSON replies leave the request pending; accepted host errors terminate at the condition or body line.

SELECT compiles into branch and jump instructions. It uses the same public C/Aether event API and needs no selection frame across yields. Branch and jump instructions count toward the existing instruction budget. The complete source still parses before any application effect is dispatched. Malformed selections and excessive nesting therefore fail during creation.

Reference parser limits and verification

The reference condition resolver can misread arithmetic across multiple function calls, such as ABS(2) + ABS(3) > 4, as one function invocation. RexxAE evaluates the arithmetic. This is a pinned known difference with assertions for both implementations; it is not counted as parity. Shared syntax assigns the sum first:

LET magnitude = ABS(2) + ABS(3)
SELECT
WHEN magnitude > 4 THEN SAY 'match'
END

SELECT fixtures compare complete programs and host dispatch order against RexxJS. Native specs cover memoization, errors, parse-before-effect behavior, loop unwinding and suspended teardown. Binary consumers exercise both library variants.