fix(sparql): bind boolean expression results as xsd:boolean

[Anai-Guo]

Problem

bool is a subclass of int in Python, so in _eval_extend (trustgraph-flow/trustgraph/query/sparql/algebra.py) the (int, float) branch catches Python bools first and the bool branch written just below it is dead code:

elif isinstance(val, (int, float)):
    new_sol[var_name] = Term(type=LITERAL, value=str(val))
elif isinstance(val, str):
    new_sol[var_name] = Term(type=LITERAL, value=val)
elif isinstance(val, bool):                     # unreachable
    new_sol[var_name] = Term(
        type=LITERAL, value=str(val).lower(),
        datatype="http://www.w3.org/2001/XMLSchema#boolean"
    )

evaluate_expression() returns a Python bool for a large part of the expression language — its own docstring says so ("The result value (Term, bool, number, string, or None)") — including every relational operator, IN, EXISTS / NOT EXISTS, REGEX, isIRI / isLITERAL / isBLANK, BOUND and LANGMATCHES.

So BIND(?a > ?b AS ?ok) binds Term(LITERAL, value="True") with no datatype, instead of Term(LITERAL, value="true", datatype=xsd:boolean).

Why it produces wrong results, not just a wrong lexical form

_effective_boolean() in expressions.py only reads a literal as a boolean when its datatype is xsd:boolean. An untyped literal falls through to the string rule:

if val.datatype == "http://www.w3.org/2001/XMLSchema#boolean":
    return v.lower() in ("true", "1")
...
return len(v) > 0

"False" is five characters long, so its EBV is True. A query like

BIND(?a > ?b AS ?ok) . FILTER(?ok)

keeps every solution, including the ones where the comparison was false.

Fix

Move the bool branch ahead of (int, float). The branch bodies are untouched.

Verification

_eval_extend was extracted from algebra.py and _effective_boolean from expressions.py with ast.get_source_segment (no hand-copying), the graph-walking neighbours stubbed, and the BIND(...) . FILTER(?ok) chain run before and after the patch:

BIND value	unpatched bound term	FILTER	patched bound term	FILTER
True	value='True' datatype=None	True	value='true' datatype=xsd:boolean	True
False	value='False' datatype=None	True ❌	value='false' datatype=xsd:boolean	False ✅
1	value='1' datatype=None	True	value='1' datatype=None	True
0	value='0' datatype=None	True	value='0' datatype=None	True
42	value='42' datatype=None	True	value='42' datatype=None	True
2.5	value='2.5' datatype=None	True	value='2.5' datatype=None	True
'text'	value='text' datatype=None	True	value='text' datatype=None	True

Only bool results change; int, float, str and Term results bind exactly as before.

🤖 Generated with Claude Code

[github-actions]

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