StateT
StateT is a Haskell monad transformer that threads and updates state through computations in an underlying monad.
Functional Design and Architecture: Examples in Haskell
StateT is a Haskell monad transformer that adds a mutable-state abstraction to an underlying monadic computation.
- How it is used
- Structures stateful game and ship-control scripts while retaining IO or domain-specific monadic actions.
- How deep it goes
- Explains types, get, put, lift, nested stacks, runners, and tests; defers transformer foundations to appendixes.
- Examples
- Uses aging scripts, a Minesweeper board reveal, layered score-and-board state, tests, and spaceship rotation control.
- What you can do after
- Build stateful scripts over IO or a custom monad, lift inner actions, run stack layers, and test final state.
Read how Functional Design and Architecture: Examples in Haskell explains StateT →
Learn Haskell by Example
StateT is a Haskell monad transformer that threads and updates state through computations in an underlying monad.
- How it is used
- Threads mutable application data through computations that also perform effects such as filesystem IO.
- How deep it goes
- Defines the type parameters and three essential operations, then demonstrates recursive state updates while deferring less-used functions.
- Examples
- Uses recursive directory traversal, counters, a Stats record, modify helpers, execStateT, and GHCi execution.
- What you can do after
- Apply it via Control.Monad.Trans.State.Strict to build effectful computations that read, replace, and update threaded state.
Related concepts
- monad transformerkind of · A monad transformer is a parameterized construction that adds one monad’s computational effects to another while preserving monadic composition.
- liftingused by · Lifting transforms a value, function, or operation so it can participate in computations within a computational context.
- ReaderT monadalternative to · ReaderT is a monad transformer that adds access to a read-only environment to an underlying monad.
- 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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