eval.sx adds the `guard` syntactic operator with R7RS-compliant
clause dispatch: var binds to raised value in a fresh child env;
clauses tried in order; `else` is catch-all; no match re-raises.
Implementation uses a "catch-once-then-handle-outside" pattern to
avoid the handler self-raise loop:
outcome = host-guard {body} ;; tag raise vs success
if outcome was raise:
try clauses → either result or sentinel
if sentinel: re-raise OUTSIDE the host-guard scope
runtime.sx binds R7RS exception primitives:
- raise V
- error MSG IRRITANT... → {:scm-error MSG :irritants LIST}
- error-object?, error-object-message, error-object-irritants
- with-exception-handler HANDLER THUNK
(same outcome-sentinel pattern — handler's own raises propagate
outward instead of re-entering)
12 tests cover: catch on raise, predicate dispatch, else catch-all,
no-error pass-through, first-clause-wins, re-raise-on-no-match,
error-object construction and accessors.
232 total Scheme tests now (62 + 23 + 49 + 78 + 20).
scheme-standard-env binds:
- call/cc — primary
- call-with-current-continuation — alias
Implementation wraps SX's host call/cc, presenting the captured
continuation k as a Scheme procedure that accepts a single value
(or a list of values for multi-arg invocation). Single-shot
escape semantics: when k is invoked, control jumps out of the
surrounding call/cc form. Multi-shot re-entry isn't safely
testable without delimited-continuation infrastructure (the
captured continuation re-enters indefinitely if invoked after
the call/cc returns) — deferred to a follow-up commit if needed.
Tests cover:
- No-escape return value
- Escape past arithmetic frames
- Detect/early-exit idiom over for-each
- Procedure? on the captured k
220 total Scheme tests now (62 + 23 + 49 + 78 + 8).
