How deeply does Learn Haskell by Example cover StateT?

Defines the type parameters and three essential operations, then demonstrates recursive state updates while deferring less-used functions.

StateT

As explained in Learn Haskell by Example

StateT is a Haskell monad transformer that threads and updates state through computations in an underlying monad.

StateT is a Haskell monad transformer that adds readable and updatable state to computations in an underlying monad. StateT s m a records the state type, underlying monad, and computation result. section 15.1.2

Use the core operations to manage state during a computation:

get retrieves the current state. section 15.1.2
put replaces the current state with a supplied value. section 15.1.2
modify applies a function to the existing state. section 15.1.2
liftIO performs IO inside StateT while preserving the threaded state. section 15.1.2

Counting directories during recursive traversal

Define type App a = StateT Int IO a, perform filesystem operations with liftIO, increment the counter with modify, and run the computation using execStateT (example "foo") 0. A richer application state can track recursion depth and transferred files with a Stats record, helper modification functions, and a Monoid identity for an empty state. section 15.1.2

How Learn Haskell by Example covers it

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Also explained in

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Functional Design and Architecture: Examples in HaskellStateT is a Haskell monad transformer that adds a mutable-state abstraction to an underlying monadic computation.

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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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