Fp either ref

Skill sickn33/agentic-awesome-skills/plugins/agentic-awesome-skills-claude/skills/fp-either-ref

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Install

npx -y skills add sickn33/agentic-awesome-skills --skill fp-either-ref

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What its author says it does

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Quick reference for Either type. Use when user needs error handling, validation, or operations that can fail with typed errors.

SKILL.md

2.5 KB, 585 tokens by cl100k_base, as published. Nobody here has run it

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

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

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

E.getOrElse(err => default)  // Get Right or default
E.match(onLeft, onRight)     // Pattern match
E.toUnion(either)            // E | A (loses type info)

Common Patterns

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

// ❌ 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.