(dynamic-wind BEFORE THUNK AFTER)
- Calls BEFORE; runs THUNK; calls AFTER; returns THUNK's value.
- If THUNK raises, AFTER still runs before the raise propagates.
- Implementation: outcome-sentinel pattern (same trick as guard
and with-exception-handler) — catch THUNK's raise inside a
host guard, run AFTER unconditionally, then either return the
value or re-raise outside the catch.
Not implemented: call/cc-escape tracking. R7RS specifies that
dynamic-wind's BEFORE and AFTER thunks should re-run when control
re-enters or exits the dynamic extent via continuations. That
requires explicit dynamic-extent stack tracking, deferred until
a consumer needs it (probably never needed for pure-eval Scheme
programs; matters for first-class-continuation-heavy code).
5 tests: success ordering, return value, after-on-raise,
raise propagation, nested wind.
237 total Scheme tests now (62 + 23 + 49 + 78 + 25).
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).
lib/scheme/runtime.sx — full R7RS-base surface:
- Arithmetic: variadic +/-/*//, abs, min, max, modulo, quotient,
remainder. Predicates zero?/positive?/negative?.
- Comparison: chained =/</>/<=/>=.
- Type predicates: number?/boolean?/symbol?/string?/char?/vector?/
null?/pair?/procedure?/not.
- List: cons/car/cdr/list/length/reverse/append.
- Higher-order: map/filter/fold-left/fold-right/for-each/apply.
These re-enter scheme-apply to invoke user-supplied procs.
- String: string-length/string=?/string-append/substring.
- Char: char=?.
- Vector: vector/vector-length/vector-ref/vector->list/list->vector/
make-vector.
- Equality: eqv?/equal?/eq? (all = under the hood for now).
Built via small adapters: scm-unary, scm-binary, scm-fold (variadic
left-fold with identity + one-arity special), scm-chain (n-ary
chained comparison).
**Bugfix in eval.sx set! handler.** The :else branch had two
expressions `(dict-set! ...) val` — SX cond branches don't run
multiple expressions, they return nil silently (or evaluate only
the first, depending on shape). Wrapped in (begin ...) to force
sequential execution. This fix also unblocks 4 set!-dependent
tests in lib/scheme/tests/syntax.sx that were silently raising
during load (and thus not counted) — syntax test count jumps
from 45 → 49.
Classic programs verified:
- factorial 10 → 3628800
- fib 10 → 55
- recursive list reverse → working
- sum of squares via fold-left + map → 55
212 total Scheme tests: parse 62 + eval 23 + syntax 49 + runtime 78.
All green.
The env-as-value section in runtime tests demonstrates
scheme-standard-env IS a refl-env? — kit primitives operate on it
directly, confirming the third-consumer adoption with zero adapter.
