Week 9 Studio: Getting something back from a match

Wiring conditional and immediate assignment, @X, and the enumerate-all-matches idiom into last week's matcher

19 April 2027

Before the session#

Bring last week’s matcher with all seven primitives from Movement III’s vocabulary working correctly under the three-condition cursor contract. This session doesn’t add new matching primitives; it adds a way to get information back out of a match that already succeeds.

In the session#

• Add conditional assignment, and make sure it actually undoes. Extend your pattern type with a .-style capture node, wired so that a capture made during a match attempt that later gets backtracked out of is discarded along with it — not merely overwritten by whatever the next attempt produces, which would happen to look the same in many cases but is the wrong semantics. Construct a test where a pattern captures something, then fails further along and backtracks past the capture point into a different alternative that captures something else instead; confirm the final bound value belongs to whichever attempt actually succeeded, not to a stale first attempt.
• Add immediate assignment as a genuine side effect. An $-style capture should fire the moment its subpattern matches, independent of whether the match attempt it’s nested in ultimately succeeds or fails. Build a case that makes this visible: a pattern with an immediate assignment inside a branch that goes on to fail overall, and confirm the variable still shows the (now technically “wrong,” in the sense of belonging to a failed attempt) value the immediate assignment set.
• Add @X as a cursor-only capture. This should reuse whatever machinery immediate assignment needed — @X is a null match with a position-valued, unconditional bind — rather than requiring its own separate capture mechanism.
• Build the enumerate-all-matches program. Using only what your interpreter already has from Movements I through III — a loop construct (or the goto field, driving a labeled statement back to itself), conditional assignment, and FAIL — write a small program that finds every occurrence of a pattern in a subject and reports each one. Confirm it terminates (the loop must stop when the subject is exhausted, not retry forever) and that it reports true non-overlapping occurrences, not the same one repeatedly.

Sample transcript#

Genuine captured output from this course’s own checkpoint-3 reference solution — the fullest transcript of the three checkpoints, since it is the first one exercising conditional assignment and backtracking together:

$ dune exec bin/main.exe -- examples/bind_replace.sno
HELLO SNOBOL4
HELLO, ALICE!
CORRECTLY FAILED, LINE2 UNCHANGED = NAME: BOB

$ dune exec bin/main.exe -- examples/primitives.sno
LEN(5) MATCHED ALL OF HELLO
LEN(6) CORRECTLY FAILED (ONLY 5 CHARS)
SPAN CAPTURED LEADING WHITESPACE: [   ]
NOTANY CORRECTLY FAILED ON AN ALL-DIGIT SUBJECT
LETTERS=ABC DIGITS=123
ARB CAPTURED VAL=42

bind_replace.sno’s third line is the one to check by hand before the session: a conditional assignment inside a match that goes on to fail overall must leave LINE2 exactly as it was, not holding a stale capture from the attempt that failed — the same undo discipline this session’s first studio task asks you to build and test directly.

Afterwards#

Movement III is complete: patterns as values, a correctly-backtracking matcher over seven primitives, and both ways of getting information back out of a match. Checkpoint 3, due at the end of this week, asks you to bring all three weeks together against a fuller sample transcript than any single session has exercised alone.