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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.
Passed 20 of 22 test cases with the skill, and 5 of 22 without it.
and 17 more in the full report
17 fixed3 passed either way0 still failing2 broke
Tokens, turns and time describe resource use, not a dollar price.
One of 17 test cases the skill fixed. Same model, same prompt, with and without the skill.
I'm calling standard library function os.ReadFile which returns ([]byte, error). Show how to immediately wrap this call's return values into a unified result monad in Go.
| Checked | Without skill | With skill |
|---|---|---|
| Uses mo.TupleToResult to wrap the os.ReadFile call | Missed | Passed |
The instructions your model reads when the skill loads.
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.
Go 1.18+ library providing type-safe monadic types with zero dependencies. Inspired by Scala, Rust, and fp-ts.
Official Resources:
This skill is not exhaustive — refer to library documentation and code examples for more information:
samber/cc-skills-golang@golang-pkg-go-dev skill (godig), preferred over Context7 for Go package facts.samber/cc-skills-golang@golang-gopls skill (gopls).go get github.com/samber/mo
For an introduction to functional programming concepts and why monads are valuable in Go, see Monads Guide.
| 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 |
test cases passed, vs 5 / 22 without
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