/golang-samber-mo

name: golang-samber-mo
description: "Monadic types for Golang using samber/mo — Option, Result, Either, Future, IO, Task, and State types for type-safe nullable values, error handling, and functional composition with pipeline sub-packages. Apply when using or adopting samber/mo, when the codebase imports `github.com/samber/mo`, or when considering functional programming patterns as a safety design for Golang. Not for nil-safety and zero-value design without this library (→ See `samber/cc-skills-golang@golang-safety` skill), nor for native error wrapping with fmt.Errorf, errors.Is and errors.As (→ See `samber/cc-skills-golang@golang-error-handling` skill)."
user-invocable: true
license: MIT
compatibility: Designed for Claude Code, Codex or similar harness, and for projects using Golang.
metadata:
  author: samber
  version: "1.1.2"
  openclaw:
    emoji: "🎭"
    homepage: https://github.com/samber/cc-skills-golang
    requires:
      bins:
        - go
    install: []
    skill-library-version: "1.16.0"
allowed-tools: Read Edit Write Glob Grep Bash(go:*) Bash(golangci-lint:*) Bash(git:*) Agent WebFetch mcp__context7__resolve-library-id mcp__context7__query-docs AskUserQuestion Bash(godig:*) Bash(gopls:*) LSP mcp__gopls__*
paths:
  - "**/*.go"

**Persona:** You are a Go engineer bringing functional programming safety to Go. You use monads to make impossible states unrepresentable — nil checks become type constraints, error handling becomes composable pipelines.

**Thinking mode:** Reason as thoroughly as possible when designing multi-step Option/Result/Either pipelines — wrong type choice creates unnecessary wrapping/unwrapping that defeats the purpose of monads. On Claude Code, use `ultrathink` to trigger extended thinking explicitly.

samber/mo — Monads and Functional Abstractions for Go

Go 1.18+ library providing type-safe monadic types with zero dependencies. Inspired by Scala, Rust, and fp-ts.

**Official Resources:**

• [pkg.go.dev/github.com/samber/mo](https://pkg.go.dev/github.com/samber/mo)
• [github.com/samber/mo](https://github.com/samber/mo)

This skill is not exhaustive — refer to library documentation and code examples for more information:

• For Go package docs, symbols, versions, importers, and known vulnerabilities, → See `samber/cc-skills-golang@golang-pkg-go-dev` skill (`godig`), preferred over Context7 for Go package facts.
• To navigate this library's usage in your own code (definitions, call sites, diagnostics), → See `samber/cc-skills-golang@golang-gopls` skill (`gopls`).
• Context7 remains a fallback for docs not indexed on pkg.go.dev.

go get github.com/samber/mo

For an introduction to functional programming concepts and why monads are valuable in Go, see [Monads Guide](./references/monads-guide.md).

Core Types at a Glance

| Type | Purpose | Think of it as... | | --- | --- | --- | | `Option[T]` | Value that may be absent | Rust's `Option`, Java's `Optional` | | `Result[T]` | Operation that may fail | Rust's `Result<T, E>`, replaces `(T, error)` | | `Either[L, R]` | Value of one of two types | Scala's `Either`, TypeScript discriminated union | | `EitherX[L, R]` | Value of one of X types | Scala's `Either`, TypeScript discriminated union | | `Future[T]` | Async value not yet available | JavaScript `Promise` | | `IO[T]` | Lazy synchronous side effect | Haskell's `IO` | | `Task[T]` | Lazy async computation | fp-ts `Task` | | `State[S, A]` | Stateful computation | Haskell's `State` monad |

Option[T] — Nullable Values Without nil

Represents a value that is either present (`Some`) or absent (`None`). Eliminates nil pointer risks at the type level.

import "github.com/samber/mo"

name := mo.Some("Alice")          // Option[string] with value
empty := mo.None[string]()        // Option[string] without value
fromPtr := mo.PointerToOption(ptr) // nil pointer -> None

// Safe extraction
name.OrElse("Anonymous")  // "Alice"
empty.OrElse("Anonymous")  // "Anonymous"

// Transform if present, skip if absent
upper := name.Map(func(s string) (string, bool) {
    return strings.ToUpper(s), true
})

**Key methods:** `Some`, `None`, `Get`, `MustGet`, `OrElse`, `OrEmpty`, `Map`, `FlatMap`, `Match`, `ForEach`, `ToPointer`, `IsPresent`, `IsAbsent`.

Option implements `json.Marshaler/Unmarshaler`, `sql.Scanner`, `driver.Valuer` — use it directly in JSON structs and database models.

For full API reference, see [Option Reference](./references/option.md).

Result[T] — Error Handling as Values

Represents success (`Ok`) or failure (`Err`). Equivalent to `Either[error, T]` but specialized for Go's error pattern.

// Wrap Go's (value, error) pattern
result := mo.TupleToResult(os.ReadFile("config.yaml"))

// Same-type transform — errors short-circuit automatically
upper := mo.Ok("hello").Map(func(s string) (string, error) {
    return strings.ToUpper(s), nil
})
// Ok("HELLO")

// Extract with fallback
val := upper.OrElse("default")

**Go limitation:** Direct methods (`.Map`, `.FlatMap`) cannot change the type parameter — `Result[T].Map` returns `Result[T]`, not `Result[U]`. Go methods cannot introduce new type parameters. For type-changing transforms (e.g. `Result[[]byte]` to `Result[Config]`), use sub-package functions or `mo.Do`:

import "github.com/samber/mo/result"

// Type-changing pipeline: []byte -> Config -> ValidConfig
parsed := result.Pipe2(
    mo.TupleToResult(os.ReadFile("config.yaml")),
    result.Map(func(data []byte) Config { return parseConfig(data) }),
    result.FlatMap(func(cfg Config) mo.Result[ValidConfig] { return validate(cfg) }),
)

**Key methods:** `Ok`, `Err`, `Errf`, `TupleToResult`, `Try`, `Get`, `MustGet`, `OrElse`, `Map`, `FlatMap`, `MapErr`, `Match`, `ForEach`, `ToEither`, `IsOk`, `IsError`.

For full API reference, see [Result Reference](./references/result.md).

Either[L, R] — Discriminated Union of Two Types

Represents a value that is one of two possible types. Unlike Result, neither side