State monad
A state monad represents stateful computation as a function from an input state to a result paired with an output state, enabling composable state transitions.
Functional Design and Architecture: Examples in Haskell
A state monad is a monadic abstraction that threads changeable state through computations while presenting state transitions as composable values.
- How it is used
- Organizes immutable application state as monadic context, avoiding repetitive explicit model threading in stateful scripts.
- How deep it goes
- Explains get, put, bind-based state threading, runners, and StateT stacks, while deliberately deferring category theory.
- Examples
- Uses Age updates, a Minesweeper board, state-stack tests, and spaceship rotation control scripts.
- What you can do after
- Apply it via State and StateT to structure, run, and test pure stateful computations with layered effects.
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Functional Programming in Scala, Second Edition
A state monad represents a stateful computation as a function from an input state to a result paired with an output state.
- How it is used
- Represents stateful computations as composable values instead of directly mutating shared state.
- How deep it goes
- Defines unit, flatMap, get, and set, then demonstrates state threading; exercises defer laws and combinator behavior.
- Examples
- Uses zipWithIndex to number list elements by incrementing an Int state through a for-comprehension.
- What you can do after
- Build it from scratch by implementing State, unit, flatMap, get, set, and state-threading programs.
Read how Functional Programming in Scala, Second Edition explains state monad →
Related concepts
- monadkind of · A monad is a functional abstraction that encapsulates computational context and provides operations for composing computations while preserving that context.
- state-passingimplements · State-passing is a technique in which state is supplied as a function argument and the resulting computation returns updated state.
- runStateused by · runState is a Haskell state-monad function that executes a stateful computation from an initial state and returns its result together with the final state.
- monad stackpart of · A monad stack is a nested composition of monads or monad transformers that combines multiple computational effects behind one monadic interface.
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