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ef285b5c97
* fix: stabilize validation and tests on Windows * test: add Windows smoke coverage for skill activation * refactor: make setup_web script CommonJS * fix: repair aegisops-ai frontmatter * docs: add when-to-use guidance to core skills * docs: add when-to-use guidance to Apify skills * docs: add when-to-use guidance to Google and Expo skills * docs: add when-to-use guidance to Makepad skills * docs: add when-to-use guidance to git workflow skills * docs: add when-to-use guidance to fp-ts skills * docs: add when-to-use guidance to Three.js skills * docs: add when-to-use guidance to n8n skills * docs: add when-to-use guidance to health analysis skills * docs: add when-to-use guidance to writing and review skills * meta: sync generated catalog metadata * docs: add when-to-use guidance to Robius skills * docs: add when-to-use guidance to review and workflow skills * docs: add when-to-use guidance to science and data skills * docs: add when-to-use guidance to tooling and automation skills * docs: add when-to-use guidance to remaining skills * fix: gate bundle helper execution in Windows activation * chore: drop generated artifacts from contributor PR * docs(maintenance): Record PR 457 sweep Document the open issue triage, PR supersedence decision, local verification, and source-only cleanup that prepared PR #457 for re-running CI. --------- Co-authored-by: sickn33 <sickn33@users.noreply.github.com>
92 lines
2.2 KiB
Markdown
92 lines
2.2 KiB
Markdown
---
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name: fp-either-ref
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description: Quick reference for Either type. Use when user needs error handling, validation, or operations that can fail with typed errors.
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risk: unknown
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source: community
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version: 1.0.0
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tags: [fp-ts, either, error-handling, validation, quick-reference]
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---
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# Either Quick Reference
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Either = success or failure. `Right(value)` or `Left(error)`.
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## When to Use
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- You need a quick fp-ts reference for typed synchronous error handling.
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- The task involves validation, fallible operations, or converting throwing code to `Either`.
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- You want a compact cheat sheet rather than a long tutorial.
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## Create
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```typescript
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import * as E from 'fp-ts/Either'
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E.right(value) // Success
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E.left(error) // Failure
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E.fromNullable(err)(x) // null → Left(err), else Right(x)
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E.tryCatch(fn, toError) // try/catch → Either
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```
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## Transform
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```typescript
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E.map(fn) // Transform Right value
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E.mapLeft(fn) // Transform Left error
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E.flatMap(fn) // Chain (fn returns Either)
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E.filterOrElse(pred, toErr) // Right → Left if pred fails
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```
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## Extract
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```typescript
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E.getOrElse(err => default) // Get Right or default
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E.match(onLeft, onRight) // Pattern match
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E.toUnion(either) // E | A (loses type info)
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```
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## Common Patterns
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```typescript
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import { pipe } from 'fp-ts/function'
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import * as E from 'fp-ts/Either'
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// Validation
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const validateEmail = (s: string): E.Either<string, string> =>
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s.includes('@') ? E.right(s) : E.left('Invalid email')
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// Chain validations (stops at first error)
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pipe(
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E.right({ email: 'test@example.com', age: 25 }),
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E.flatMap(d => pipe(validateEmail(d.email), E.map(() => d))),
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E.flatMap(d => d.age >= 18 ? E.right(d) : E.left('Must be 18+'))
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)
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// Convert throwing code
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const parseJson = (s: string) => E.tryCatch(
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() => JSON.parse(s),
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(e) => `Parse error: ${e}`
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)
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```
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## vs try/catch
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```typescript
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// ❌ try/catch - errors not in types
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try {
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const data = JSON.parse(input)
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process(data)
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} catch (e) {
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handleError(e)
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}
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// ✅ Either - errors explicit in types
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pipe(
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E.tryCatch(() => JSON.parse(input), String),
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E.map(process),
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E.match(handleError, identity)
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)
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```
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Use Either when **error type matters** and you want to chain operations.
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