Expandable switch

[ipapadop]

Closes #23

[qodo-code-review]

PR Summary by Qodo

Expandable deferred switch expressions with append() and unified result mapping

✨ Enhancement 🧪 Tests 📝 Documentation 🕐 20-40 Minutes

AI Description

• Add switch_expression::append() to extend existing deferred switches with new cases.
• Replace conditional/switch-specific result mappers with a shared detail::map_result helper.
• Add unit/integration coverage for append semantics, variants, and move-only case bodies.

Diagram

graph TD
  U["User code"] --> S["switch_expression"] --> C["choose_case()"] --> M["detail::map_result"] --> R["result_type"]
  S --> A["append()"] --> S2["expanded switch"]
  T["Tests"] --> A

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High-Level Assessment

The following are alternative approaches to this PR:

1. Provide `extend(other_switch)` / switch composition

• ➕ Allows combining two pre-built switches without unpacking cases
• ➕ Potentially clearer for builder-style construction
• ➖ More API surface area (composition rules, default precedence, type merging)
• ➖ Harder to keep simple value-category (lvalue/rvalue) semantics correct

2. Implement append via `std::tuple_cat` + constructor forwarding

• ➕ Can be more direct than std::apply for tuple manipulation
• ➕ Might reduce lambda complexity
• ➖ Still needs careful forwarding/copy semantics for owned vs referenced expressions
• ➖ May increase template instantiation noise without clear readability gains

Recommendation: The chosen append() design (two overloads for const& and &&) is a good fit: it preserves reference semantics when copying and enables moving owned expressions when the source switch is an rvalue. Consider adding switch-to-switch composition only if users commonly need to merge pre-assembled switches; otherwise append() keeps the API minimal.

Files changed (8) +146 / -73

Enhancement (2) +92 / -33

map_result.hppAdd shared 'detail::map_result' utility +40/-0

Add shared 'detail::map_result' utility

• Introduces a reusable result-mapping helper that normalizes void/non-void evaluated values into the target result type (including 'std::monostate' for void branches).

include/deferred/detail/map_result.hpp

switch.hppAdd 'switch_expression::append()' and unify result mapping +52/-33

Add 'switch_expression::append()' and unify result mapping

• Adds 'append()' overloads for 'const&' and '&&' to create a new switch expression with additional cases, preserving evaluation order (existing cases first). Replaces the switch-specific result mapper with 'detail::map_result' and performs minor template cleanup ('tuple_size_v').

include/deferred/switch.hpp

Refactor (2) +6 / -40

conditional.hppRefactor conditional result mapping to shared helper +3/-28

Refactor conditional result mapping to shared helper

• Removes the conditional-specific 'map_conditional_result' and switches conditional evaluation to use 'detail::map_result' from a shared header.

include/deferred/conditional.hpp

while.hppSimplify while_expression call operators +3/-12

Simplify while_expression call operators

• Consolidates the call operator overload set by replacing the previous 'const&'/'&'/'&&' trio with a simpler 'const' and non-const overload pair while keeping behavior consistent.

include/deferred/while.hpp

Tests (2) +46 / -0

switch.cppAdd integration test for append() behavior +22/-0

Add integration test for append() behavior

• Adds an integration test validating that appended cases are considered after existing cases, and that the original switch remains unchanged when appending from a const reference.

test/integration/switch.cpp

switch.cppAdd unit tests for append() with variants and move-only bodies +24/-0

Add unit tests for append() with variants and move-only bodies

• Adds unit tests verifying that append supports heterogeneous result types (variant expansion) and that moving a switch into append works with move-only case bodies.

test/unit/switch.cpp

Documentation (2) +2 / -0

AGENTS.mdDocument expandable switch expressions +1/-0

Document expandable switch expressions

• Updates the project overview to explicitly mention that switch expressions can be expanded via 'append()'.

AGENTS.md

README.mdAdvertise expandable deferred switch expressions +1/-0

Advertise expandable deferred switch expressions

• Adds a README bullet highlighting expandable deferred switch expressions as a supported feature.

README.md

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[qodo-code-review]

Code Review by Qodo

🐞 Bugs (0) 📘 Rule violations (0) 📎 Requirement gaps (0) 🎨 UX issues (0) 🔗 Cross-repo conflicts (0) 📜 Skill insights (0)

1. map_result.hpp guard mismatch ✓ Resolved 📘 Rule violation ⚙ Maintainability

Description

The new header guard macro is DEFERRED_DETAIL_MAP_RESULT_HPP, which does not follow the required
#ifndef DEFERRED_FILENAME_HPP naming format for the header name. This can break the repository’s
documented consistency requirements for include guards.

Code

include/deferred/detail/map_result.hpp[R4-5]

+#ifndef DEFERRED_DETAIL_MAP_RESULT_HPP
+#define DEFERRED_DETAIL_MAP_RESULT_HPP

Evidence

PR Compliance ID 8 requires header guards to use the DEFERRED_FILENAME_HPP naming format. The
added file include/deferred/detail/map_result.hpp defines #ifndef DEFERRED_DETAIL_MAP_RESULT_HPP
/ #define DEFERRED_DETAIL_MAP_RESULT_HPP, which does not match that required pattern for a
map_result.hpp header.

AGENTS.md: Headers Must Use the Required Header Guard Naming Format
include/deferred/detail/map_result.hpp[4-6]

Agent prompt

The issue below was found during a code review. Follow the provided context and guidance below and implement a solution

## Issue description
`include/deferred/detail/map_result.hpp` uses the header guard `DEFERRED_DETAIL_MAP_RESULT_HPP`, but the required format is `DEFERRED_FILENAME_HPP` (where `FILENAME` is the header’s filename uppercased).

## Issue Context
This repository enforces a specific header guard naming scheme for consistency.

## Fix Focus Areas
- include/deferred/detail/map_result.hpp[4-6]

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