fp-either-ref
Quick reference for Either type. Use when user needs error handling, validation, or operations that can fail with typed errors.
What this skill does
# Either Quick Reference
Either = success or failure. `Right(value)` or `Left(error)`.
## When to Use
- You need a quick fp-ts reference for typed synchronous error handling.
- The task involves validation, fallible operations, or converting throwing code to `Either`.
- You want a compact cheat sheet rather than a long tutorial.
## 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: '[email protected]', 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)
)
```
Use Either when **error type matters** and you want to chain operations.
## Limitations
- Use this skill only when the task clearly matches the scope described above.
- Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
- Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.
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