FA-4621 / Iterator consumption / Open access

Filter lazily consumes through first match · case 01

The operation returns a result or retained state that violates this contract: Take the first value equal to sentinel from a filtered iterator; return [match or missing,unconsumed source].

Verified by executionVariant 1 · 4 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Collecting every match consumes values beyond the first requested match.

VERIFIED REPAIR

Take the first value equal to sentinel from a filtered iterator; return [match or missing,unconsumed source].

Unsuccessful approach: Reading only the first source value does not scan through rejected values.

Case contract

Take the first value equal to sentinel from a filtered iterator; return [match or missing,unconsumed source]. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.

Why this case matters

A controlled local-runtime regression for collection APIs, language semantics, or ownership wrappers. Fixtures include boundary and interaction cases.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import itertools as it
from collections import deque
N = 1
observations = []
def solve(x, y=None):
    g=iter(x); a=list(filter(lambda v:v==y,g)); return [a[0] if a else "missing",list(g)]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('scan through match', solve(['a', 's', 'b'], 's'), ['s', ['b']])
check('first matches', solve(['s', 'a'], 's'), ['s', ['a']])
check('no match exhausts', solve(['a', 'b'], 's'), ['missing', []])
check('empty', solve([], 's'), ['missing', []])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
scan through match	`['s', []]`	`['s', ['b']]`	Failed
first matches	`['s', []]`	`['s', ['a']]`	Failed
no match exhausts	`['missing', []]`	`['missing', []]`	Passed
empty	`['missing', []]`	`['missing', []]`	Passed

SHA-256 / ac71fd5032c519497b45cd57f765548372c23238bd75ac4a318cbf3da2d1b245

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import itertools as it
from collections import deque
N = 1
observations = []
def solve(x, y=None):
    g=iter(x); a=next(g,"missing"); return [a if a==y else "missing",list(g)]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('scan through match', solve(['a', 's', 'b'], 's'), ['s', ['b']])
check('first matches', solve(['s', 'a'], 's'), ['s', ['a']])
check('no match exhausts', solve(['a', 'b'], 's'), ['missing', []])
check('empty', solve([], 's'), ['missing', []])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
scan through match	`['missing', ['s', 'b']]`	`['s', ['b']]`	Failed
first matches	`['s', ['a']]`	`['s', ['a']]`	Passed
no match exhausts	`['missing', ['b']]`	`['missing', []]`	Failed
empty	`['missing', []]`	`['missing', []]`	Passed

SHA-256 / 0ccf9c46a8e671b6bc0e10a8334adc6d9cd4b4fc12b1032706299fcf6d790403

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
import itertools as it
from collections import deque
N = 1
observations = []
def solve(x, y=None):
    g=iter(x); a=next(filter(lambda v:v==y,g),"missing"); return [a,list(g)]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('scan through match', solve(['a', 's', 'b'], 's'), ['s', ['b']])
check('first matches', solve(['s', 'a'], 's'), ['s', ['a']])
check('no match exhausts', solve(['a', 'b'], 's'), ['missing', []])
check('empty', solve([], 's'), ['missing', []])
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
scan through match	`['s', ['b']]`	`['s', ['b']]`	Passed
first matches	`['s', ['a']]`	`['s', ['a']]`	Passed
no match exhausts	`['missing', []]`	`['missing', []]`	Passed
empty	`['missing', []]`	`['missing', []]`	Passed

SHA-256 / 92d94ed0ab3c8f8bf88b7c9757168f9dae7a5e1230e693eb379bcc00ee4d1e08

Verification & scope

This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:37:41.238371+00:00.

Case digest / af556c198a8178475fe9c37c9cf93f38956fd0f65123956952d6ab2e27e4822a