ReaderT monad
ReaderT is a monad transformer that adds access to a read-only environment to an underlying monad.
Functional Design and Architecture: Examples in Haskell
ReaderT is the Haskell monad transformer that adds an immutable environment to an underlying monadic computation.
- How it is used
- Uses an immutable Reader environment to inject service handles into dependency-inverted business logic.
- How deep it goes
- Explains environment definition, layered execution, ask, lift, and boundary unwrapping, then stops short of reducing boilerplate.
- Examples
- Demonstrates AppEnv, meteor reporting, ask, lift, runReaderT, and runtime injection through a reporting application.
- What you can do after
- Apply it by defining an environment, retrieving dependencies with ask, lifting underlying actions, and running the scenario with runReaderT.
Read how Functional Design and Architecture: Examples in Haskell explains ReaderT monad →
Learn Haskell by Example
ReaderT is a Haskell monad transformer that adds access to a read-only environment to an underlying monad.
- How it is used
- Builds applications that share read-only configuration while retaining effects from an underlying monad such as IO.
- How deep it goes
- Explains the type parameters, ask, runReaderT, and lifting, but stops short of broader transformer composition details.
- Examples
- Uses a file-synchronization Config containing source and destination paths, then prints a configured path mapping.
- What you can do after
- Build it from scratch by defining a ReaderT application, querying configuration, lifting IO, and running the action with an environment.
Related concepts
- IOuses · IO is an abstraction representing input/output or other side-effecting computations and operations, often as composable values whose execution is separately controlled.
- monad transformerkind of · A monad transformer is a parameterized construction that adds one monad’s computational effects to another while preserving monadic composition.
- monadkind of · A monad is a functional abstraction that encapsulates computational context and provides operations for composing computations while preserving that context.
- service handle patternuses · The Service Handle pattern represents access to a service through an explicit handle that encapsulates the service interaction boundary.
- monad stackpart of · A monad stack is a nested composition of monads or monad transformers that combines multiple computational effects behind one monadic interface.
- free monadalternative to · A free monad represents computations as composable syntax over an operation set, with interpretation and execution supplied separately.
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