fp-either-ref
Quick reference for Either type. Use when user needs error handling, validation, or operations that can fail with typed errors.
Best use case
fp-either-ref is best used when you need a repeatable AI agent workflow instead of a one-off prompt.
Quick reference for Either type. Use when user needs error handling, validation, or operations that can fail with typed errors.
Teams using fp-either-ref should expect a more consistent output, faster repeated execution, less prompt rewriting.
When to use this skill
- You want a reusable workflow that can be run more than once with consistent structure.
When not to use this skill
- You only need a quick one-off answer and do not need a reusable workflow.
- You cannot install or maintain the underlying files, dependencies, or repository context.
Installation
Claude Code / Cursor / Codex
Manual Installation
- Download SKILL.md from GitHub
- Place it in
.claude/skills/fp-either-ref/SKILL.mdinside your project - Restart your AI agent — it will auto-discover the skill
How fp-either-ref Compares
| Feature / Agent | fp-either-ref | Standard Approach |
|---|---|---|
| Platform Support | Not specified | Limited / Varies |
| Context Awareness | High | Baseline |
| Installation Complexity | Unknown | N/A |
Frequently Asked Questions
What does this skill do?
Quick reference for Either type. Use when user needs error handling, validation, or operations that can fail with typed errors.
Where can I find the source code?
You can find the source code on GitHub using the link provided at the top of the page.
SKILL.md Source
# Either Quick Reference
Either = success or failure. `Right(value)` or `Left(error)`.
## Create
```typescript
import * as E from 'fp-ts/Either'
E.right(value) // Success
E.left(error) // Failure
E.fromNullable(err)(x) // null → Left(err), else Right(x)
E.tryCatch(fn, toError) // try/catch → Either
```
## Transform
```typescript
E.map(fn) // Transform Right value
E.mapLeft(fn) // Transform Left error
E.flatMap(fn) // Chain (fn returns Either)
E.filterOrElse(pred, toErr) // Right → Left if pred fails
```
## Extract
```typescript
E.getOrElse(err => default) // Get Right or default
E.match(onLeft, onRight) // Pattern match
E.toUnion(either) // E | A (loses type info)
```
## Common Patterns
```typescript
import { pipe } from 'fp-ts/function'
import * as E from 'fp-ts/Either'
// Validation
const validateEmail = (s: string): E.Either<string, string> =>
s.includes('@') ? E.right(s) : E.left('Invalid email')
// Chain validations (stops at first error)
pipe(
E.right({ email: 'test@example.com', age: 25 }),
E.flatMap(d => pipe(validateEmail(d.email), E.map(() => d))),
E.flatMap(d => d.age >= 18 ? E.right(d) : E.left('Must be 18+'))
)
// Convert throwing code
const parseJson = (s: string) => E.tryCatch(
() => JSON.parse(s),
(e) => `Parse error: ${e}`
)
```
## vs try/catch
```typescript
// ❌ try/catch - errors not in types
try {
const data = JSON.parse(input)
process(data)
} catch (e) {
handleError(e)
}
// ✅ Either - errors explicit in types
pipe(
E.tryCatch(() => JSON.parse(input), String),
E.map(process),
E.match(handleError, identity)
)
```
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