fp-ts
Typed functional programming library for TypeScript providing algebraic data types (Option, Either, Task, IO, Reader, Writer, State) and type classes (Functor, Monad, Applicative). fp-ts enables total functions, explicit error handling via Either (Right/Left), and composable async operations via TaskEither — patterns from Haskell/Scala applied in TypeScript.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
Pure functional library. No side effects by design. Total functions reduce null reference and exception-based security bugs.
⚡ Reliability
Best When
Your team has functional programming expertise and TypeScript experience, and you want algebraic data types and total functions with compile-time error handling guarantees.
Avoid When
Your team is not deeply familiar with functional programming — Effect-TS (simpler API) or plain async/await with Result types is more approachable.
Use Cases
- • Model agent operation results as Either<Error, Success> to enforce explicit error handling in TypeScript without exceptions
- • Chain async operations with TaskEither for agent API calls that may fail — compose with pipe() for readable sequential pipelines with typed errors
- • Use Option<T> instead of null/undefined in agent data models to eliminate null reference errors at the type system level
- • Build parser/validator pipelines for agent input processing using fp-ts Decoder (io-ts) for validated, typed deserialization
- • Compose complex agent workflows from pure functions using fp-ts pipe() and flow() without mutation or intermediate variables
Not For
- • Teams unfamiliar with functional programming and category theory — fp-ts requires deep FP knowledge; the learning curve is steep
- • Rapid prototyping or simple scripts — the verbosity of fp-ts patterns is overkill for small utilities or one-off scripts
- • Mixed JS/TS codebases — fp-ts is TypeScript-first; without strict TypeScript the type guarantees that justify the complexity are lost
Interface
Authentication
No authentication — local TypeScript library.
Pricing
fp-ts is open source and free.
Agent Metadata
Known Gotchas
- ⚠ fp-ts v3 is in development with significant API changes — many tutorials and examples are for v2; verify version compatibility before following examples
- ⚠ HKT (Higher Kinded Types) type parameters in fp-ts generate complex TypeScript error messages that require FP expertise to understand
- ⚠ pipe() vs flow() distinction matters — pipe() takes a value as first arg and applies functions; flow() creates a new function; mixing them causes type errors
- ⚠ TaskEither sequencing with sequenceArray runs tasks in parallel by default — use pipe(TE.sequenceArray, TE.map) carefully when ordering matters
- ⚠ fp-ts Option fold() and match() have swapped argument order vs Haskell — None case comes first, Some case second in fp-ts (opposite of many other FP libraries)
- ⚠ io-ts (companion runtime type library) is separate from fp-ts core — install both for runtime validation with fp-ts Either integration
Alternatives
Full Evaluation Report
Detailed scoring breakdown, competitive positioning, security analysis, and improvement recommendations for fp-ts.
Scores are editorial opinions as of 2026-03-06.