Individual skill

type-driven

Use when modeling a domain, encoding a state machine, hardening APIs, making invalid states unrepresentable, or parsing instead of validating. Not for TypeScript: use typescript-best-practices.

.devin/skills/type-driven/SKILL.md

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Last checked: Nov 4, 2026

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Repository stars (not skill ratings): 55
Repository last updated: Sep 28, 2026
License: Apache-2.0

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# Type-driven development

## Contract

| Field | Bound contract |
|---|---|
| Trigger | The work is modeling a domain, encoding a state machine, hardening an API boundary, making invalid states unrepresentable, or parsing instead of validating. |
| Authority | Reversible local: writes only domain types, public signatures, and affected callers and tests; rollback is version control or undo. No remote mutation. |
| Side effect | Rewrites domain types, public signatures, and affected callers and tests to the new algebraic model. |
| Done | Invalid states are unconstructible, matches are exhaustive, boundaries parse, and no scattered post-hoc validation remains. |

## Inputs

Required: the domain problem, data model, or API surface to encode.