Implement ergonomic API for multi-output folds by beojan · Pull Request #1006 · Framework-R-D/phlex

beojan · 2026-10-21T22:35:39Z

A function that takes multiple non-const lvalue references as it's first N parameters is transformed into one that takes a non-const lvalue reference to a tuple when passed into a fold call.

A possible implementation of #998.

Code
- Added make_fold_like.hpp to adapt multi-output fold callables.
- Updated glue::fold to apply make_fold_like before constructing the fold algorithm.
- Callables with fewer than two accumulator references remain unchanged.
Tests
- Updated add_and_count to use separate non-const atomic references for sum and count.
- Preserved the existing accumulation behavior.

A function that takes multiple non-const lvalue references as it's first N parameters is transformed into one that takes a non-const lvalue reference to a tuple when passed into a `fold` call.

coderabbitai · 2026-10-21T22:35:52Z

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📝 Walkthrough

glue::fold now adapts callables with multiple accumulator references. The new adapter separates accumulator parameters from input parameters and binds the accumulators to tuple elements. The fold test now passes its accumulators as separate references.

Changes: Fold callable adaptation

Layer / File(s)	Summary
Adapter contract, implementation, and fold integration `phlex/metaprogramming/make_fold_like.hpp`, `phlex/core/glue.hpp`, `test/fold_test.cpp`	The adapter identifies leading non-const lvalue-reference parameters as accumulators, binds tuple elements by reference, and forwards input arguments. `glue::fold` uses the adapter. The test callable accepts separate `sum` and `count` references.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Feature

Possibly related issues

Issue 998 describes the separate-accumulator callable shape supported by this change. It also requests compatibility and additional behavior beyond the changes summarized here.

Suggested reviewers: knoepfel

Merge Risk: 🟡 Moderate · up to bb9b7

The new multi-output fold adaptation does not work for member-function folds, because the bound object is dropped. Free functions and lambdas are unaffected. Fix or explicitly scope out member folds before merging.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 20.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: adding an ergonomic API for folds with multiple output accumulators.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Inline comments:
Review comments at @phlex/core/glue.hpp:
- Line 81: Update the algorithm_bits call for the make_fold_like(f) adapter so
multi-output member-function pointers invoke through bound_obj_ rather than the
generic delegate path. Preserve bound_obj_ as the target object while the
adapter binds accumulator references before forwarding the call.

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phlex/core/glue.hpp
phlex/metaprogramming/make_fold_like.hpp
test/fold_test.cpp

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Files:

test/fold_test.cpp
phlex/core/glue.hpp
phlex/metaprogramming/make_fold_like.hpp

codecov · 2026-10-21T22:43:16Z

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main    #1006      +/-   ##
==========================================
+ Coverage   88.56%   88.66%   +0.09%     
==========================================
  Files         187      188       +1     
  Lines        7776     7788      +12     
  Branches     1012     1012              
==========================================
+ Hits         6887     6905      +18     
+ Misses        643      639       -4     
+ Partials      246      244       -2

Flag	Coverage Δ
amd64	`89.28% <100.00%> (?)`
arm64	`88.66% <100.00%> (?)`
scripts	`?`
unittests	`?`

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Files with missing lines	Coverage Δ
phlex/core/glue.hpp	`100.00% <100.00%> (ø)`
phlex/metaprogramming/make_fold_like.hpp	`100.00% <100.00%> (ø)`

... and 3 files with indirect coverage changes

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Implement ergonomic API for multi-output folds

bb9b7b7

A function that takes multiple non-const lvalue references as it's first N parameters is transformed into one that takes a non-const lvalue reference to a tuple when passed into a `fold` call.

coderabbitai Bot reviewed Oct 21, 2026

View reviewed changes

Comment thread phlex/core/glue.hpp

beojan added 2 commits October 21, 2026 15:55

Member function pointers don't get wrapped by make_fold_like

0978c90

Fix PMF detection in make_fold_like

426ce64

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Implement ergonomic API for multi-output folds - #1006

Implement ergonomic API for multi-output folds#1006
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beojan:multi-output-fold-ergo

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