feat: preserve inline object type in template resolution (fixes #31)

[spinje]

Summary

Fixes the double-serialization bug where templates in nested structures like {"key": "${dict_var}"} would produce {"key": "{\"nested\": \"value\"}"} instead of the correct {"key": {"nested": "value"}}.

This enables the intuitive pattern of passing multiple data sources to shell commands:

{
  "stdin": {"config": "${config}", "data": "${data}"},
  "command": "jq '.config + .data'"
}

Changes

• New helper methods in TemplateResolver:

  • is_simple_template() - detects if string is exactly ${var}
  • extract_simple_template_var() - extracts variable name from simple template

• Renamed resolve_string() → resolve_template() with type preservation:

  • Simple templates (${var}) preserve original type (dict, list, int, bool, None)
  • Complex templates ("Hello ${name}") return interpolated strings

• Updated callers:

  • workflow_executor.py - nested workflow param mapping
  • node_wrapper.py - refactored to use shared helper (DRY)

Test Coverage

• test_template_type_preservation.py - Core type preservation behavior (12 tests)
• test_shell_stdin_type_preservation.py - Integration tests with shell node (4 tests)
• Updated existing tests to use resolve_template() (36 occurrences across 5 files)

Created Docs

• .taskmaster/tasks/task_103/implementation/implementation-plan.md
• .taskmaster/tasks/task_103/implementation/progress-log.md

File Stats

 13 files changed, 1620 insertions(+), 98 deletions(-)

Testing

make test  # 3236 passed, 9 skipped

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Task-ID: 103

[claude]

Code Review: Template Type Preservation (PR #32)

Summary

This PR fixes a double-serialization bug where simple templates in nested structures (e.g., {"key": "${dict_var}"}) were incorrectly JSON-serialized to strings instead of preserving their original types. The implementation is clean, well-tested, and follows the project's established patterns.

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✅ Critical — No blocking issues found

The implementation is production-ready with no critical issues.

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⚠️ Warnings — Should be addressed

1. SIMPLE_TEMPLATE_PATTERN is too permissive (Medium Priority)

Location: src/pflow/runtime/template_resolver.py:32

Issue:

SIMPLE_TEMPLATE_PATTERN = re.compile(r"^\$\{([^}]+)\}$")

This pattern uses [^}]+ which matches any character except }, including:

• Invalid variable names: ${123}, ${-invalid}, ${$weird}
• Special characters: ${@#$%}
• Spaces: ${ var } (space-padded)

Why it matters:

1. Inconsistency: TEMPLATE_PATTERN on line 26-28 is strict and only matches valid identifiers
2. False positives: Invalid templates will be treated as simple templates and attempt resolution
3. Debugging confusion: ${123} would be treated as simple template but fail to resolve

Recommended fix:
Reuse the variable name pattern from TEMPLATE_PATTERN:

# Current strict pattern for valid variable names
TEMPLATE_PATTERN = re.compile(
    r"(?<!\$)\$\{([a-zA-Z_][\w-]*(?:(?:\[\d+\])?(?:\.[a-zA-Z_][\w-]*(?:\[\d+\])?)*)?)\}"
)

# Make SIMPLE_TEMPLATE_PATTERN match the same valid names
SIMPLE_TEMPLATE_PATTERN = re.compile(
    r"^\$\{([a-zA-Z_][\w-]*(?:(?:\[\d+\])?(?:\.[a-zA-Z_][\w-]*(?:\[\d+\])?)*)?)\}$"
)

Why not critical: In practice, the resolve_value() call will fail gracefully for invalid variable names, so this won't cause runtime crashes. But it's sloppy and violates the principle of "fail fast with clear errors."

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2. Missing edge case test: whitespace-padded templates (Low Priority)

Location: tests/test_runtime/test_template_type_preservation.py

Issue:
The current SIMPLE_TEMPLATE_PATTERN would match "${ var }" (with spaces), but this is likely not intended behavior.

Recommended test addition:

def test_whitespace_padded_template_not_simple(self):
    """Templates with whitespace padding should not be treated as simple."""
    # These should be rejected as simple templates
    assert TemplateResolver.is_simple_template("${ var }") is False
    assert TemplateResolver.is_simple_template("${var }") is False
    assert TemplateResolver.is_simple_template("${ var}") is False

If the stricter regex from Warning #1 is adopted, this test will pass automatically.

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3. Documentation: Type preservation behavior not documented in CLAUDE.md (Low Priority)

Location: architecture/reference/template-variables.md or src/pflow/runtime/CLAUDE.md

Issue:
This is a significant behavior change that affects all nodes accepting structured parameters. The architecture docs should document:

1. When types are preserved vs. stringified
2. The simple vs. complex template distinction
3. Examples showing the difference

Recommended addition to architecture/reference/template-variables.md:

## Type Preservation (v0.6.0+)

### Simple Templates
Templates that consist of **only** a variable reference preserve the original type:

\`\`\`json
{
  "data": "${user_object}"  // ← Dict stays dict
}
\`\`\`

### Complex Templates
Templates with text around variables always return strings:

\`\`\`json
{
  "message": "Hello ${name}!"  // ← Always string
}
\`\`\`

### Examples

**Before (v0.5.x):**
\`\`\`python
{"config": "${settings}"}  # → {"config": "{\"key\": \"val\"}"}  ❌
\`\`\`

**After (v0.6.0+):**
\`\`\`python
{"config": "${settings}"}  # → {"config": {"key": "val"}}  ✅
\`\`\`

---

💡 Suggestions — Optional improvements

1. Consider extracting SIMPLE_VAR_PATTERN as shared constant (Code organization)

Location: src/pflow/runtime/template_resolver.py

Current state:
Both TEMPLATE_PATTERN and SIMPLE_TEMPLATE_PATTERN duplicate the variable name regex.

Suggestion:

# Shared pattern for valid variable names with paths and array indices
_VAR_NAME_PATTERN = r"[a-zA-Z_][\w-]*(?:(?:\[\d+\])?(?:\.[a-zA-Z_][\w-]*(?:\[\d+\])?)*)?"

TEMPLATE_PATTERN = re.compile(rf"(?<!\$)\$\{({_VAR_NAME_PATTERN})\}")
SIMPLE_TEMPLATE_PATTERN = re.compile(rf"^\$\{({_VAR_NAME_PATTERN})\}$")

Benefits:

• Single source of truth for "what is a valid variable name"
• Easier to maintain if pattern needs updates
• Self-documenting

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2. Test coverage: Verify behavior with escaped templates (Test quality)

Location: tests/test_runtime/test_template_type_preservation.py

Missing test case:
What happens with $$$\{var} (escaped template)?

Suggested test:

def test_escaped_template_not_treated_as_simple(self):
    """Escaped templates should not be detected as simple templates."""
    assert TemplateResolver.is_simple_template("$$$\{var}") is False

    context = {"var": {"data": "value"}}
    result = TemplateResolver.resolve_template("$$$\{var}", context)
    # Should return literal string "$$\{var}", not the resolved dict
    assert result == "$$\{var}"

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3. Minor: Logging could include context size for debugging (Observability)

Location: src/pflow/runtime/template_resolver.py:369-372

Current:

logger.debug(
    f"Resolved simple template '$\{{var_name}}' -> {resolved!r} (type: {type(resolved).__name__})",
    extra={"var_name": var_name, "value_type": type(resolved).__name__},
)

Suggestion:

logger.debug(
    f"Resolved simple template '$\{{var_name}}' -> {resolved!r} (type: {type(resolved).__name__})",
    extra={
        "var_name": var_name,
        "value_type": type(resolved).__name__,
        "context_keys": list(context.keys())  # ← Helpful for debugging missing vars
    },
)

This helps when debugging "why isn't my variable resolving?" issues.

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🎯 Overall Assessment

Code Quality: 8.5/10

• ✅ Clean separation of concerns (helpers extracted properly)
• ✅ Excellent test coverage (16 test files updated, 2 new test files)
• ✅ Clear naming (is_simple_template, extract_simple_template_var)
• ✅ Good documentation in docstrings
• ⚠️ Regex pattern could be stricter (Warning docs: Set up Mintlify documentation (Task 93) #1)

Architecture: 9/10

• ✅ Follows established patterns (TemplateResolver already had this structure)
• ✅ DRY: Eliminated duplication with node_wrapper._resolve_simple_template
• ✅ Backward compatible: All existing complex templates still work
• ✅ Single responsibility: Each method does one thing well

Test Quality: 9/10

• ✅ Tests document the why (bug fix scenario) not just what
• ✅ Integration tests verify real-world use case (shell + jq)
• ✅ Edge cases covered (None, bool, int, deeply nested)
• ⚠️ Missing whitespace and escaped template tests (minor)

Security: 10/10

• ✅ No credential exposure risk
• ✅ No injection vulnerabilities
• ✅ Input validation handled by existing resolve_value()

Performance: 10/10

• ✅ No performance regression (just reordered logic)
• ✅ Regex compiled once at module load (good)
• ✅ Early return for simple templates (optimization)

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📋 Recommendation

Approve with minor fixes ✅

Before merge:

1. Fix Warning docs: Set up Mintlify documentation (Task 93) #1 (stricter SIMPLE_TEMPLATE_PATTERN) - Recommended
2. Add whitespace edge case test (Warning feat: unify LLM usage via Simon Willison's llm library (Task 95) #2) - Optional
3. Update architecture docs (Warning feat: Smart output display for registry run #3) - Can be separate PR

After merge:

• Consider extracting shared regex pattern (Suggestion docs: Set up Mintlify documentation (Task 93) #1)
• Add escaped template test (Suggestion feat: unify LLM usage via Simon Willison's llm library (Task 95) #2)

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🏆 Highlights

1. Excellent problem analysis: The implementation plan is thorough and shows deep understanding
2. Comprehensive testing: 1620 insertions across 13 files, all tests pass
3. Clear documentation: Progress log and implementation plan are exemplary
4. Follows project standards: Uses make test, make check, proper typing, modern Python

Great work! This is exactly the kind of careful, well-tested change that builds a solid codebase.

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Review completed by Claude Code - pflow code reviewer
Task-ID: 103