Monad transformer
A monad transformer is a parameterized construction that adds one monad’s computational effects to another while preserving monadic composition.
Functional Design and Architecture: Examples in Haskell
A monad transformer is a type-level abstraction that adds one monad's effects to another monad.
- How it is used
- Combines pure state handling with IO or other monadic capabilities in stateful application scripts.
- How deep it goes
- Works through StateT stack construction, lift operations, nested contexts, runners, and tests; defers broader transformer mechanics to appendixes.
- Examples
- Uses Minesweeper board and score stacks, reveal-cell console output, stack tests, and spaceship-control scripting.
- What you can do after
- Apply it via StateT to combine state with IO, lift underlying actions, run layered computations, and test resulting state.
Read how Functional Design and Architecture: Examples in Haskell explains monad transformer →
Learn Haskell by Example
A monad transformer is a type constructor that augments an underlying monad with additional computational effects while preserving monadic composition.
- How it is used
- Structures a directory-synchronization application around configuration access, state, accumulated output, and IO.
- How deep it goes
- Explains transformer roles, ReaderT types, environment access, and lifting, while stopping short of detailed transformer stacking mechanics.
- Examples
- Uses a file-synchronization Config, a ReaderT Config IO application, ask, liftIO, and runReaderT.
- What you can do after
- Build effectful application actions that read configuration and perform IO without manually threading environment values.
Read how Learn Haskell by Example explains monad transformer →
Related concepts
- StateTincludes · StateT is a Haskell monad transformer that threads and updates state through computations in an underlying monad.
- ReaderT monadincludes · ReaderT is a monad transformer that adds access to a read-only environment to an underlying monad.
- ExceptTincludes · ExceptT is a Haskell monad transformer that adds computations which can terminate with an error value to an underlying monad.
- WriterTincludes · WriterT is a Haskell monad transformer that accumulates output alongside the result of computations in an underlying monad.
These guides are generated from the text of Manning books by Manning's knowledge graph and link to the sections they draw on. The full books are available on liveBook and at manning.com.