fp-go

name

fp-go

description

Use this skill whenever writing, reviewing, or refactoring Go code that uses the fp-go library (github.com/IBM/fp-go/v2). Trigger on any mention of fp-go, functional programming in Go, monads in Go, Option/Either/Result types in Go, IOResult, ReaderIOResult, data-last composition, Pipe/Flow, or do-notation with Bind/ApS in Go. Also trigger when the user wants to convert idiomatic Go error handling into functional pipelines, or asks about optics (lens, prism, traversal) in Go.

fp-go v2 — Functional Programming for Go

Critical Rules for Code Generation

1. Import path is v2: always github.com/IBM/fp-go/v2/..., never github.com/IBM/fp-go/... (that is v1).
2. Data-last: all operations return a function waiting for data. Write option.Map(f)(value), never option.Map(value, f).
3. Non-inferrable type parameters come first: Map[B, A], Ap[B, A], Chain[B, A]. The compiler infers trailing params from arguments; leading params often need explicit annotation.
4. Prefer Result over Either when the error type is Go's error. Result[A] is Either[error, A]. Same for ioresult over ioeither, readerioresult over readerioeither.
5. IO values are lazy: IO[A] is func() A. They describe a computation — you must call () to execute. Don't forget the trailing ().
6. Prefer point-free style: compose with F.Flow and F.Pipe instead of writing inline anonymous functions. If a transformation can be expressed as a composition of named functions, it should be. Point-free pipelines are idiomatic fp-go.

Generation Workflow

Two habits that matter more for fp-go than for idiomatic Go, because the library is low-frequency in training data and easy to misremember:

1. Retrieve before generating. For any pattern not fully covered below — optics beyond simple lenses, traversals, concurrent combinators, the less-common monads — query the fp-go MCP server's search_examples / get_example tools (see the fp-go-mcp skill) for a real signature instead of recalling names like Chain / FlatMap / Bind from memory. Retrieve-then-generate beats generate-then-fix here.
2. Compile before presenting. After writing fp-go code, run go build ./... and go vet ./..., then fix any import, type-parameter, or argument-order error and re-run until clean. The compiler is precise, low-ambiguity feedback, and most fp-go mistakes (wrong leading type param, data-first vs data-last) surface immediately.

Overview

fp-go (import path github.com/IBM/fp-go/v2) brings type-safe functional programming to Go using generics. Every monad follows a consistent interface: once you know the pattern in one monad, it transfers to all others.

All functions use the data-last principle: the data being transformed is always the last argument, enabling partial application and pipeline composition.

Core Types

Type	Package	Represents
Option[A]	option	A value that may or may not be present (replaces nil)
Either[E, A]	either	A value that is either a left error E or a right success A
Result[A]	result	Either[error, A] — recommended default for error handling
IO[A]	io	A lazy computation that produces A (possibly with side effects)
IOResult[A]	ioresult	IO[Result[A]] — lazy computation that can fail
ReaderIOResult[A]	context/readerioresult	func(context.Context) IOResult[A] — context-aware IO with errors
Effect[C, A]	effect	func(C) ReaderIOResult[A] — typed dependency injection + IO + errors; recommended for services

Idiomatic Packages (high-performance, tuple-based)

The idiomatic/ packages use Go-native tuples instead of struct wrappers, offering 2–10× better performance and zero allocations. Use them in hot paths; use standard packages when you need the richer API surface.

• idiomatic/option — (A, bool) tuples
• idiomatic/result — (A, error) tuples
• idiomatic/ioresult — func() (A, error)
• idiomatic/context/readerresult — func(context.Context) (A, error)

Standard Operations

Every monad exports these operations (PascalCase for exported Go names):

fp-go	fp-ts / Haskell	Description
Of	of / pure	Lift a pure value into the monad
Map	map / fmap	Transform the value inside without changing the context
Chain	chain / >>=	Sequence a computation that itself returns a monadic value
Ap	ap / <*>	Apply a wrapped function to a wrapped value
Fold	fold / either	Eliminate the context — handle every case and extract a plain value
GetOrElse	getOrElse / fromMaybe	Extract the value or use a default (Option/Result)
Filter	filter / mfilter	Keep only values satisfying a predicate
Flatten	flatten / join	Remove one level of nesting (M[M[A]] → M[A])
ChainFirst	chainFirst / >>	Sequence for side effects; keeps the original value
Alt	alt / `<	>`
FromPredicate	fromPredicate / guard	Build a monadic value from a predicate
Sequence	sequence	Turn []M[A] into M[[]A]
Traverse	traverse	Map and sequence in one step

Curried (composable) vs. monadic (direct) form:

// Curried — data last, returns a transformer function
option.Map(strings.ToUpper)              // func(Option[string]) Option[string]

// Monadic — data first, immediate execution
option.MonadMap(option.Some("hello"), strings.ToUpper)

Use curried form for pipelines; use Monad* form when you already have all arguments.

Key Type Aliases (defined per monad)

// A Kleisli arrow: a function from A to a monadic B
type Kleisli[A, B any] = func(A) M[B]

// An operator: transforms one monadic value into another
type Operator[A, B any] = func(M[A]) M[B]

Chain takes a Kleisli, Map returns an Operator. The naming is consistent across all monads.

Function Composition with Flow and Pipe (Point-Free Style)

fp-go is designed for point-free programming: compose named functions with Flow and Pipe rather than writing inline anonymous functions. This makes pipelines more readable and eliminates intermediate variable naming.

import (
    F  "github.com/IBM/fp-go/v2/function"
    S  "github.com/IBM/fp-go/v2/string"
    LZ "github.com/IBM/fp-go/v2/lazy"
)

// ✅ GOOD: point-free — compose named functions, no lambda noise
pipeline := F.Flow3(
    R.Eitherize1(strconv.Atoi),
    R.Map(N.Mul(2)),
    R.GetOrElse(F.Constant(0)),
)

// ❌ AVOID: wrapping in unnecessary anonymous functions
pipeline := F.Flow3(
    func(s string) R.Result[int] { return R.Eitherize1(strconv.Atoi)(s) },
    func(r R.Result[int]) R.Result[int] { return R.Map(func(n int) int { return n * 2 })(r) },
    func(r R.Result[int]) int { return R.GetOrElse(func() int { return 0 })(r) },
)

The data-last design means every fp-go operation already returns a function — so you almost never need to wrap them in a lambda. When you do need to adapt arguments, use F.Flow2 to compose:

// Point-free: compose a lens getter with a Kleisli arrow
RIO.Bind(configLens.Set, F.Flow2(userLens.Get, fetchConfigForUser))

// Instead of:
RIO.Bind(configLens.Set, func(s Pipeline) RIO.ReaderIOResult[Config] {
    return fetchConfigForUser(userLens.Get(s))
})

Two forms:

// Flow: compose functions left-to-right, returns a new function
transform := F.Flow3(
    option.Map(strings.TrimSpace),
    option.Filter(S.IsNonEmpty),
    option.GetOrElse(LZ.Of("default")),
)
result := transform(option.Some("  hello  ")) // "hello"

// Pipe: apply a value through a pipeline immediately
result := F.Pipe3(
    option.Some("  hello  "),
    option.Map(strings.TrimSpace),
    option.Filter(S.IsNonEmpty),
    option.GetOrElse(LZ.Of("default")),
)

Pipe1–Pipe20 and Flow1–Flow20 are available (the number = number of transformation steps).

Lifting Go Functions into Monadic Context

Helper	Lifts
Eitherize1..EitherizeN	func(args...) (B, error) → func(args...) Result[B] — primary bridge from Go to fp-go
ChainEitherK	func(A) (B, error) → works inside the monad
ChainOptionK	func(A) Option[B] → works inside the monad
ChainFirstIOK	func(A) IO[B] for side effects, keeps original value
FromPredicate	func(A) bool + error builder → func(A) Result[A]

Examples

Option — nullable values without nil

import (
    O  "github.com/IBM/fp-go/v2/option"
    F  "github.com/IBM/fp-go/v2/function"
    S  "github.com/IBM/fp-go/v2/string"
    P  "github.com/IBM/fp-go/v2/optics/prism"
    "strconv"
)

parseAndDouble := F.Flow3(
    O.FromPredicate(S.IsNonEmpty),
    O.Chain(P.ParseInt().GetOption),
    O.Map(N.Mul(2)),
)

parseAndDouble("21")  // Some(42)
parseAndDouble("")    // None
parseAndDouble("abc") // None

Result — error handling without if-err boilerplate

import (
    R  "github.com/IBM/fp-go/v2/result"
    F  "github.com/IBM/fp-go/v2/function"
    N  "github.com/IBM/fp-go/v2/number"
    P  "github.com/IBM/fp-go/v2/predicate"
    ER "github.com/IBM/fp-go/v2/errors"
    "strconv"
    "errors"
)

parse := R.Eitherize1(strconv.Atoi)  // lifts (int, error) → Result[int]

validate := R.FromPredicate(
    P.Not(N.LessThan(0)),
    ER.OnSome[int]("%d must not be negative"),
)

pipeline := F.Flow2(parse, R.Chain(validate))

pipeline("42")   // Ok(42)
pipeline("-1")   // Error("must be non-negative")
pipeline("abc")  // Error(strconv parse error)

IOResult — lazy IO with error handling

import (
    IOE "github.com/IBM/fp-go/v2/ioresult"
    F   "github.com/IBM/fp-go/v2/function"
    J   "github.com/IBM/fp-go/v2/json"
    "os"
)

readConfig := F.Flow2(
    IOE.Eitherize1(os.ReadFile),           // func(string) IOResult[[]byte]
    IOE.ChainEitherK(J.Unmarshal[Config]), // parse JSON, propagate errors
)

result := readConfig("config.json")() // execute lazily — note the trailing ()

ReaderIOResult — context-aware pipelines (recommended for services)

import (
    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
    F   "github.com/IBM/fp-go/v2/function"
    "context"
)

// type ReaderIOResult[A any] = func(context.Context) func() result.Result[A]

fetchUser := func(id int) RIO.ReaderIOResult[User] {
    return func(ctx context.Context) func() result.Result[User] {
        return func() result.Result[User] {
            // perform IO here
        }
    }
}

pipeline := F.Pipe3(
    fetchUser(42),
    RIO.ChainEitherK(validateUser),    // lift pure (User, error) function
    RIO.Map(enrichUser),               // lift pure User → User function
    RIO.ChainFirstIOK(IO.Logf[User]("Fetched: %v")), // side-effect logging
)

user, err := pipeline(ctx)() // provide context once, execute

Effect — typed dependency injection (recommended for testable services)

Effect[C, A] adds a typed dependency parameter C on top of ReaderIOResult. While context/readerioresult hardcodes context.Context as the environment, Effect lets you define a custom dependencies struct — making dependencies explicit, compile-time checked, and trivially mockable in tests.

Use Effect when your service has dependencies beyond context.Context (database connections, HTTP clients, config, loggers). It is the recommended top-level monad for production service code.

import (
    EF "github.com/IBM/fp-go/v2/effect"
    F  "github.com/IBM/fp-go/v2/function"
    L  "github.com/IBM/fp-go/v2/optics/lens"
)

// 1. Define your dependencies as a struct
type Deps struct {
    DB     DBClient
    Logger Logger
    Config AppConfig
}

// 2. Write effects that declare exactly what they need
// Effect[Deps, User] = func(Deps) ReaderIOResult[User]
fetchUser := func(id int) EF.Effect[Deps, User] {
    return EF.Asks(func(deps Deps) EF.ReaderIOResult[User] {
        // deps.DB is available here — compile-time checked
        return queryUser(deps.DB, id)
    })
}

enrichWithConfig := func(user User) EF.Effect[Deps, EnrichedUser] {
    return EF.Asks(func(deps Deps) EF.ReaderIOResult[EnrichedUser] {
        return RIO.Of(applyConfig(user, deps.Config))
    })
}

// 3. Compose effects — same Map/Chain/Bind/ApS API as every other monad
pipeline := F.Pipe2(
    fetchUser(42),
    EF.Chain(enrichWithConfig),
    EF.Map(func(u EnrichedUser) string { return u.DisplayName }),
)

// 4. Provide dependencies once at the edge, then run
result := EF.Provide(Deps{
    DB:     realDB,
    Logger: zapLogger,
    Config: loadedConfig,
})(pipeline) // returns ReaderIOResult[string]

value := result(ctx)() // provide context, execute

Why Effect over ReaderIOResult: dependencies are typed (compiler catches missing deps), each function's signature declares what it needs (Effect[Deps, A]), testability is trivial (swap Deps{DB: mockDB}), and EF.Local/EF.Provide narrow or eliminate deps for subsystems.

Lifting into Effect: EF.Ask[Deps]() reads the full struct, EF.Asks(f) reads deps and produces a ReaderIOResult, EF.Of(a)/EF.Succeed(a) lift pure values, EF.Fail(err) lifts errors, EF.Eitherize1(f) lifts func(A) (B, error), EF.FromResult(r) lifts a Result, and EF.RunSync(fa) executes synchronously.

Traversal — process slices monadically

import (
    A   "github.com/IBM/fp-go/v2/array"
    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
    F   "github.com/IBM/fp-go/v2/function"
)

// Fetch all users, stop on first error
fetchAll := F.Pipe1(
    A.MakeBy(10, userID),
    RIO.TraverseArray(fetchUser),  // []ReaderIOResult[User] → ReaderIOResult[[]User]
)

When to Use Which Monad

Situation	Use
Value that might be absent	Option[A]
Operation that can fail with custom error type	Either[E, A]
Operation that can fail with error	Result[A]
Lazy IO, side effects	IO[A]
IO that can fail	IOResult[A]
IO + context (cancellation, deadlines)	ReaderIOResult[A] from context/readerioresult
IO + context + typed dependencies (services, DI)	Effect[C, A] — recommended for production services
High-performance services	Idiomatic packages in idiomatic/

Escalation path: Option → Result → IOResult → ReaderIOResult → Effect. Start with the simplest monad that covers your needs. For real-world services with database clients, HTTP clients, or config — go straight to Effect; it provides compile-time dependency safety that ReaderIOResult with raw context.Context cannot.

Do-Notation: Accumulating State with Bind and ApS

When a pipeline needs to carry multiple intermediate results forward, Chain/Map becomes unwieldy because each step only threads one value. Do-notation solves this by accumulating results into a growing struct at each step.

Every monad that supports do-notation exports the same family. Examples below use context/readerioresult (RIO), but the identical API is available in result, option, ioresult, readerioresult, and others.

The Function Family

Function	Kind	What it does
Do(empty S)	—	Lift an empty struct into the monad; starting point
BindTo(setter)	monadic	Convert an existing M[T] into M[S]; alternative start
Bind(setter, f)	monadic	Add a result; f receives the current state and returns M[T]
ApS(setter, fa)	applicative	Add a result; fa is independent of the current state
Let(setter, f)	pure	Add a value computed by a pure function of the state
LetTo(setter, value)	pure	Add a constant value

Lens variants (BindL, ApSL, LetL, LetToL) accept a Lens[S, T] instead of a manual setter.

Bind — Sequential, Dependent Steps

Bind sequences two monadic computations. f receives the full accumulated state so it can read anything gathered so far. Errors short-circuit.

import (
    RIO "github.com/IBM/fp-go/v2/context/readerioresult"
    F   "github.com/IBM/fp-go/v2/function"
    L   "github.com/IBM/fp-go/v2/optics/lens"
    "context"
)

type Pipeline struct {
    User   User
    Config Config
    Posts  []Post
}

var (
    userLens   = L.MakeLens(func(s Pipeline) User   { return s.User },   func(s Pipeline, u User)   Pipeline { s.User = u; return s })
    configLens = L.MakeLens(func(s Pipeline) Config { return s.Config }, func(s Pipeline, c Config) Pipeline { s.Config = c; return s })
    postsLens  = L.MakeLens(func(s Pipeline) []Post { return s.Posts },  func(s Pipeline, p []Post) Pipeline { s.Posts = p; return s })
)

result := F.Pipe3(
    RIO.Do(Pipeline{}),
    RIO.Bind(userLens.Set,   func(_ Pipeline) RIO.ReaderIOResult[User] { return fetchUser(42) }),
    RIO.Bind(configLens.Set, F.Flow2(userLens.Get, fetchConfigForUser)),
    RIO.Bind(postsLens.Set,  F.Flow2(userLens.Get, fetchPostsForUser)),
)

pipeline, err := result(context.Background())()

The setter signature is func(T) func(S1) S2. lens.Set already has this shape. F.Flow2(lens.Get, f) composes the field getter with any Kleisli arrow point-free.

ApS — Independent, Applicative Steps

ApS uses applicative semantics: fa is evaluated without access to state. Use when steps have no dependency on each other.

// Using same lens pattern as Bind — but steps are independent
result := F.Pipe2(
    RIO.Do(Summary{}),
    RIO.ApS(userLens.Set,    fetchUser(42)),     // no access to state
    RIO.ApS(weatherLens.Set, fetchWeather("NYC")), // no access to state
)

Key difference:

	Bind(setter, f)	ApS(setter, fa)
Second argument	func(S1) M[T] — function of state	M[T] — fixed monadic value
Can read prior state?	Yes	No
Semantics	Monadic (sequential)	Applicative (independent)

Let, LetTo, BindTo

• Let(setter, f) — add a value from a pure function of state (no monad, cannot fail)
• LetTo(setter, value) — add a constant
• BindTo(project) — start from an existing M[T] instead of Do(empty)

Lifted Variants for Mixed Monads

Bind*K helpers lift simpler computations into the do-chain: BindResultK/BindEitherK for func(S1) (T, error), BindIOResultK for func(S1) func() (T, error), BindIOK for func(S1) func() T, BindReaderK for func(S1) func(ctx) T.

Do-Notation Decision Guide

Does the new step need to read prior accumulated state?
    YES  →  Bind   (monadic, sequential; f receives current S)
    NO   →  ApS    (applicative, independent; fa is a fixed M[T])

Is the new value derived purely from state, with no monad?
    YES  →  Let    (pure function of S)

Is the new value a compile-time or runtime constant?
    YES  →  LetTo

Starting from an existing M[T] rather than an empty struct?
    YES  →  BindTo

Complete Example — result Monad with Lenses

import (
    R    "github.com/IBM/fp-go/v2/result"
    F    "github.com/IBM/fp-go/v2/function"
    L    "github.com/IBM/fp-go/v2/optics/lens"
    N    "github.com/IBM/fp-go/v2/number"
    "strconv"
)

type Parsed struct {
    Raw    string
    Number int
    Double int
}

var (
    rawLens    = L.MakeLens(
        func(s Parsed) string { return s.Raw },
        func(s Parsed, v string) Parsed { s.Raw = v; return s },
    )
    numberLens = L.MakeLens(
        func(s Parsed) int { return s.Number },
        func(s Parsed, v int) Parsed { s.Number = v; return s },
    )
    doubleLens = L.MakeLens(
        func(s Parsed) int { return s.Double },
        func(s Parsed, v int) Parsed { s.Double = v; return s },
    )
)

var atoi = R.Eitherize1(strconv.Atoi) // func(string) Result[int]

parse := func(input string) R.Result[Parsed] {
    return F.Pipe3(
        R.Do(Parsed{}),
        R.LetTo(rawLens.Set, input),
        R.Bind(numberLens.Set, F.Flow2(rawLens.Get, atoi)),
        R.Let(doubleLens.Set, F.Flow2(numberLens.Get, N.Mul(2))),
    )
}

parse("21")  // Ok(Parsed{Raw:"21", Number:21, Double:42})
parse("abc") // Error(strconv parse error)

Common Mistakes

Mistake	Fix
import "github.com/IBM/fp-go/result"	Use v2: "github.com/IBM/fp-go/v2/result"
option.Map(myOption, f)	Data-last: option.Map(f)(myOption)
either.Map[A, B](f)	Non-inferrable first: either.Map[B](f) or let compiler infer
Using ioeither with error	Use ioresult instead; reserve ioeither for custom error types
readConfig := IOE.Eitherize1(os.ReadFile) then using result directly	IOResult is lazy — call readConfig("path")() with trailing ()
Writing inline setter lambdas for Do-notation	Use L.MakeLens + lens.Set; the signature already matches
Using Bind when steps are independent	Use ApS for independent steps — clearer intent, potentially concurrent
Using context/readerioresult with deps stuffed into context.Context	Use effect.Effect[Deps, A] — typed deps are compile-time checked and testable
Wrapping fp-go operations in anonymous functions	Go point-free: option.Filter(S.IsNonEmpty) not option.Filter(func(s string) bool { return s != "" }), option.GetOrElse(LZ.Of("x")) not option.GetOrElse(func() string { return "x" })
Map with a function that returns Option / Result / another monad	Produces nested M[M[A]]. Use Chain (or Flatten) for A → M[B]; reserve Map for plain A → B.
Closure passed to Map / Chain mutates a captured variable (slice append, counter ++)	Keep it pure — derive and return new values. A mutating closure silently defeats fp-go's guarantees and breaks under Traverse / concurrency.
Mixing idiomatic/result and standard result (or option / ioresult) in one file	Different types — struct wrapper vs (A, error) tuple — that do not interoperate. Pick one representation per file.

Requires Go 1.24+ for generic type alias support.