# Referential Transparency

> A design rule that an expression can be replaced by its value without changing the program's behavior.

Record: `architecture:referential-transparency`
Kind: principle
Layer: [Computation Core](https://banes-lab.com/records/layer/computation-core.md)
Severity: contextual
Scope: function, expression
Canonical: https://banes-lab.com/ontology#architecture-referential-transparency

Listed in [Architecture principles](https://banes-lab.com/api/records/architecture.md), after [Predictability](https://banes-lab.com/records/architecture/predictability.md) and before [Pure Functions](https://banes-lab.com/records/architecture/pure-functions.md).

## Repair

- Refactored by: Extract Pure Function, Inject Dependency
- Detected by: hidden dependency on time/random/global state
- Violated by: same input producing different output
- Measured by: pure function coverage
- Enforced by: [code review](https://banes-lab.com/records/architecture/code-review.md), functional boundaries

## Requires

- [Pure Functions](https://banes-lab.com/records/architecture/pure-functions.md)
- [Immutability](https://banes-lab.com/records/architecture/immutability.md)

## Reinforces

- [Determinism](https://banes-lab.com/records/architecture/determinism.md)
- [Testability](https://banes-lab.com/records/architecture/testability.md)

## Enables

- [Safe Substitution](https://banes-lab.com/records/lexicon/safe-substitution.md)

## Conflicts with

- [Side Effects](https://banes-lab.com/records/lexicon/side-effects.md)

## In tension with

- [Stateful IO](https://banes-lab.com/records/lexicon/stateful-io.md)

## Tensions

- [Referential Transparency / Stateful IO](https://banes-lab.com/records/tension/referential-transparency-stateful-io.md)

## Severity

- [contextual](https://banes-lab.com/records/vocabulary/severity-contextual.md)

## Category

- [Correctness / Determinism / Verification](https://banes-lab.com/records/architecture-category/correctness-determinism-verification.md)

## Linked from

- [Computation and resource](https://banes-lab.com/software-architecture/principles/computation-and-resource.md)
- [Correctness / Determinism / Verification](https://banes-lab.com/ontology/principles/architecture-category-correctness-determinism-verification.md)
- [Core Vocabulary](https://banes-lab.com/ontology/lexicon/lexicon-category-core-vocabulary.md)
- [Correctness / Determinism / Verification](https://banes-lab.com/ontology/lexicon/lexicon-category-correctness-determinism-verification.md)
- [Severity levels](https://banes-lab.com/ontology/schema/the-vocabulary-severity.md)
- [The resolutions](https://banes-lab.com/ontology/schema/the-resolutions.md)