CUPID as a Code Review and Refactoring Lens

Overview

CUPID is a set of five properties — Composable, Unix philosophy, Predictable, Idiomatic, Domain-based — described by Daniel Terhorst-North as things good code tends toward, not rules it must comply with. That makes them ideal review lenses: you are asking "how strongly does this code exhibit this property?" and "what would moving it further in this direction look like?"

Use them as questions, not verdicts.

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The Five Lenses

C — Composable: plays well with others

Code is composable when it can be combined with other code in many contexts without modification.

Review questions:

• Can I use this function/module independently, without pulling in unrelated state?
• Is the API surface the minimum needed — no accidental exposure?
• Are dependencies explicit (passed in) rather than implicit (global/ambient)?
• Does it work at multiple call sites, or does it secretly assume one?

Refactoring signals:

• Many parameters that describe context rather than the operation itself
• Functions that only make sense when called in a specific order
• Hidden global state or singleton dependencies
• "God object" arguments that carry everything

Move toward: Smaller surface area. Dependencies injected. Works in isolation.

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U — Unix philosophy: does one thing well

Not just single responsibility — the function does its one thing completely and well, with a clear, nameable purpose.

Review questions:

• Can I describe what this does in one short sentence without using "and"?
• Does the name match what it actually does?
• Are there multiple abstraction levels mixed together (orchestration alongside detail)?
• Would splitting it make each part simpler or just create indirection?

Refactoring signals:

• "and" or "or" in function/method names
• Functions that both compute and persist and notify
• Comments that describe sections within a function (each section wants to be a function)
• Length as a proxy: long functions often do several things

Move toward: One clear responsibility. Name that earns no qualification. Flat abstraction level.

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P — Predictable: does what you expect

Given its name and context, the code behaves consistently, without surprises. Deterministic where possible. Fails loudly rather than silently.

Review questions:

• Given only the name, would a reader expect the observed behaviour?
• Are there hidden side effects beyond what the signature suggests?
• Does it behave the same way every time with the same inputs?
• Does it fail loudly (exception/error) or silently (returns null/zero/empty)?

Refactoring signals:

• Boolean parameters that change behaviour significantly (boolean trap)
• Functions that mutate their arguments
• Swallowed exceptions or silent fallbacks
• Different return types depending on a flag
• Names that understate what the function does ("get" that also writes)

Move toward: Behaviour matches name. Side effects visible in signature. Loud failure.

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I — Idiomatic: feels natural

Code that fits its language, ecosystem, and team conventions. A reader familiar with the context should not be surprised by the patterns used.

Review questions:

• Would a developer new to this codebase (but experienced in this language) find this familiar?
• Are language features used as intended, not fought against?
• Does it follow the team/project conventions established elsewhere?
• Is anything reimplementing something the standard library provides?

Refactoring signals:

• Manual iteration where a higher-order function is conventional
• Reimplementing collection operations, parsing, or formatting from scratch
• Patterns that need a comment to explain why they exist
• Inconsistent style compared to surrounding code

Move toward: Follows the grain of the language. Uses established patterns. No surprises for the familiar reader.

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D — Domain-based: the solution domain models the problem domain

The code uses the language of the business or problem domain. Names come from the domain, not from technical implementation details.

Review questions:

• Do the names in this code appear in conversations with domain experts?
• Could a domain expert (non-programmer) read the high-level logic and recognise what it describes?
• Are there technical names where domain names exist (e.g. dataList vs invoices)?
• Does the structure of the code mirror the structure of the domain?

Refactoring signals:

• Generic names: data, item, manager, handler, util, helper
• Technical names for business concepts: UserRecord instead of Customer
• Domain logic buried inside infrastructure or UI code
• No shared vocabulary between code and domain conversations (missing ubiquitous language)

Move toward: Names from the domain. Structure that reflects domain relationships. Code a domain expert could navigate.

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Quick Reference

Property	Core question	Primary smell
Composable	Can I use this anywhere without baggage?	Hidden dependencies, large surface area
Unix	Does it do one thing, completely?	"and" in name, mixed abstraction levels
Predictable	Does it behave as its name suggests?	Hidden side effects, silent failure
Idiomatic	Does it feel natural here?	Reinventing the wheel, inconsistent patterns
Domain-based	Does it speak the domain's language?	Generic or technical names for business concepts

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Using CUPID as a Review Lens

Work through each property in turn for the code under review. For each one:

1. Ask the review questions above
2. Note which properties are weakly exhibited
3. Describe a concrete change that would move toward the property

Avoid framing as pass/fail. The goal is: "this code is more Predictable than Composable — what would make it more Composable?"

Using CUPID as a Refactoring Guide

When deciding where to start refactoring:

1. Score each property loosely (strong / weak / absent) for the target code
2. Prioritise the weakest property that, if improved, would make the others easier
3. Domain-based often unlocks everything else: when names are right, the right structure becomes obvious
4. Composable refactoring tends to have the highest leverage — isolated code is testable, movable, reusable

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Common Mistakes

• Treating CUPID as a checklist — these are properties to aspire toward, not boxes to tick. A function can be partially Predictable.
• Conflating Unix with small — a function can be small and still do two things. The question is clarity of purpose, not line count.
• Skipping Domain-based — it feels soft but it is often the most impactful lens. Code that speaks the domain language is self-documenting and easier to reason about.
• Applying all five at once — pick the one or two properties that are most weakly exhibited and focus there. Trying to improve all five simultaneously leads to over-engineering.

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Optional: README Status Addendum

After completing a CUPID assessment, offer to add or update a CUPID Status section in the project's README. Do not add it automatically — offer it and let the user decide.

When to offer

• After completing a full five-property assessment
• After a significant refactoring pass guided by CUPID lenses
• When the user asks to document the code quality state

Badge

Add a shields.io static badge to the project's badge row (alongside CI and language badges) that links to the #cupid-status section. Update the date whenever the assessment is refreshed:

[![CUPID Status](https://img.shields.io/badge/CUPID-assessed_YYYY--MM--DD-DAA520?style=flat-square)](README.md#cupid-status)

The goldenrod colour (DAA520) is intentional — it matches the CUPID five-star rating aesthetic and is visually distinct from the green CI badges. The badge links directly to the status table in the same README.

GitHub label

If the project has a GitHub repository, ensure a CUPID assessed label exists for tagging PRs that include a CUPID review. Create it once with:

gh label create "CUPID assessed" --color "DAA520" --description "PR includes a CUPID code quality assessment" --repo <owner>/<repo>

Apply the label when opening a PR that includes a full CUPID review or a targeted CUPID-guided refactoring.

Standard format

Add a ## CUPID Status section (or update it if already present) using this template:

## CUPID Status

Last assessed: YYYY-MM-DD

| Property | Strength | Top opportunity |
| --------- | -------- | --------------- |
| Composable | ★★★★☆ | Brief description |
| Unix philosophy | ★★★☆☆ | Brief description |
| Predictable | ★★★★★ | — |
| Idiomatic | ★★★☆☆ | Brief description |
| Domain-based | ★★★★☆ | Brief description |

Strength ratings: use ★ symbols on a 1–5 scale. A — in the opportunity column means no meaningful opportunity was identified.

Top opportunity: one sentence describing the single most impactful change for that property. Keep it specific enough to be actionable, not a restatement of the property definition.

Placement in the README

Insert the section after any existing badges, overview, or quick-start content, and before detailed technical documentation. It should be easy to find at a glance — near the top, not buried at the bottom.

The badge belongs in the badge row at the very top of the README, after language/CI badges.

Workflow

If the project uses a branch/PR workflow (e.g. a CLAUDE.md that requires working on branches), follow that workflow to add the addendum: create a branch, make the change, open a PR. Do not commit directly to main if the project convention says otherwise.