From antigravity-awesome-skills
Quick reference for Either type. Use when user needs error handling, validation, or operations that can fail with typed errors.
npx claudepluginhub absjaded/antigravity-awesome-skillsThis skill uses the workspace's default tool permissions.
Either = success or failure. `Right(value)` or `Left(error)`.
Verifies tests pass on completed feature branch, presents options to merge locally, create GitHub PR, keep as-is or discard; executes choice and cleans up worktree.
Guides root cause investigation for bugs, test failures, unexpected behavior, performance issues, and build failures before proposing fixes.
Writes implementation plans from specs for multi-step tasks, mapping files and breaking into TDD bite-sized steps before coding.
Either = success or failure. Right(value) or Left(error).
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
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
E.getOrElse(err => default) // Get Right or default
E.match(onLeft, onRight) // Pattern match
E.toUnion(either) // E | A (loses type info)
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}`
)
// ❌ 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.