mtl
MTL is a collection of monad classes, extending the transformers
package, using functional dependencies for generic lifting of monadic
actions.
Structure
Transformers in MTL are divided into classes and data types. Classes
define the monadic operations of transformers. Data types, generally
from the transformers
package, implement transformers, and MTL
provides instances for all the transformer type classes.
MTL and transformers
use a common module, data type, and function
naming scheme. As an example, let’s imagine we have a transformer
Foo
.
In the Control.Monad.Foo
module, we’d find:
 A type class
MonadFoo
with the transformer operations.
 A data type
FooT
with instances for all monad transformer classes.
 Functions to run the transformed computation, e.g.
runFooT
. For
the actual transformers, there are usually a number of useful runner
functions.
Lifting
When using monad transformers, you often need to “lift” a monadic
action into your transformed monadic action. This is done using the
lift
function from MonadTrans
in the Control.Monad.Trans.Class
module:
lift :: (Monad m, MonadTrans t) => m a > t m a
The action m a
is lifted into the transformer action t m a
.
As an example, here we lift an action of type IO a
into an action of
type ExceptT MyError IO a
:
data MyError = EmptyLine
mightFail :: ExceptT MyError IO ()
mightFail = do
l < lift getLine
when (null l) (throwError EmptyLine)
Transformers
The following outlines the available monad classes and transformers in
MTL and transformers
. For more details, and the corresponding
documentation of the mtl
version you are using, see the
documentation on Hackage.

Control.Monad.Cont
The Continuation monad transformer adds the ability to use
continuationpassing style
(CPS)
in a monadic computation. Continuations can be used to manipulate
the control flow of a program, e.g. early exit, error handling, or
suspending a computation.
 Class:
Control.Monad.Cont.Class.MonadCont
 Transformer:
Control.Monad.Cont.ContT

Control.Monad.Error
(deprecated!)
The Error monad transformer has been deprecated in favor of
Control.Monad.Except
.

Control.Monad.Except
The Except monad transformer adds the ability to fail with an
error in a monadic computation.
 Class:
Control.Monad.Except.Class.MonadError
 Transformer:
Control.Monad.Except.ExceptT

Control.Monad.Identity
The Identity monad transformer does not add any abilities to a
monad. It simply applies the bound function to its inner monad
without any modification.
 Transformer:
Control.Monad.Trans.Identity.IdentityT
(in the transformers
package)
 Identity functor and monad:
Data.Functor.Identity.Identity
(in the base
package)

Control.Monad.RWS
A convenient transformer that combines the Reader, Writer, and
State monad transformers.
 Lazy transformer:
Control.Monad.RWS.Lazy.RWST
(which is the default, exported by Control.Monad.RWS
)
 Strict transformer:
Control.Monad.RWS.Strict.RWST

Control.Monad.Reader
The Reader monad transformer represents a computation which can
read values from an environment.
 Class:
Control.Monad.Reader.Class.MonadReader
 Transformer:
Control.Monad.Reader.ReaderT

Control.Monad.State
The State monad transformer represents a computation which can
read and write internal state values. If you only need to read
values, you might want to use
Reader
instead.
 Class:
Control.Monad.State.Class.MonadState
 Lazy transformer:
Control.Monad.State.Lazy.StateT
(the default, exported by Control.Monad.State
)
 Strict transformer:
Control.Monad.State.Strict.StateT

Control.Monad.Writer
The Writer monad transformer represents a computation that can
produce a stream of data in addition to the computed values. This
can be used to collect values in some data structure with a
Monoid
instance. This can be used for things like logging and
accumulating values throughout a computation.
 Class:
Control.Monad.Writer.Class.MonadWriter
 Lazy transformers:
Control.Monad.Writer.Lazy.WriterT
 Strict transformers:
Control.Monad.Writer.Strict.WriterT

Control.Monad.Accum
The Accum
monad transformer represents a computation which
manages appendonly state, or a writer that can read all
previous inputs. It binds a function to a monadic value by
lazily accumulating subcomputations via (<>)
. For more general
access, use State instead.
 Class:
Control.Monad.Accum
 Transformer:
Control.Monad.Trans.Accum.AccumT

Control.Monad.Select
The Select
monad transformer represents a computation which
can do backtracking search using a ‘ranked’ evaluation strategy.
Binding a function to a monad value chains together evaluation
strategies in the sense that the results of previous strategies
may influence subsequent rank and evaluation strategies in
subcomputations.
 Class:
Control.Monad.Select
 Transformer:
Control.Monad.Trans.Select.SelectT
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