@haskell mtl2.3.1
Monad classes for transformers, using functional dependencies
Categories
License
BSD3Clause
Maintainer
chessai <chessai1996@gmail.com>, Emily Pillmore <emilypi@cohomolo.gy>, Koz Ross <koz.ross@retrofreedom.nz>
Links
Versions
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
 Class:

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
 Class:

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 thetransformers
package)  Identity functor and monad:
Data.Functor.Identity.Identity
(in thebase
package)
 Transformer:

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 byControl.Monad.RWS
)  Strict transformer:
Control.Monad.RWS.Strict.RWST
 Lazy transformer:

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
 Class:

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 byControl.Monad.State
)  Strict transformer:
Control.Monad.State.Strict.StateT
 Class:

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
 Class:

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
 Class:

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
 Class:
Resources
mtl
on Hackage The Monad Transformers chapter in "What I Wish I Knew When Learning Haskell".
 References:
 This package is inspired by the paper Functional Programming with Overloading and HigherOrder Polymorphism, by Mark P Jones, in Advanced School of Functional Programming, 1995 (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html).
Installation
Tested Compilers
Dependencies (2)
 base >=4.12 && <5
 transformers >=0.5.6 && <0.7 Show all…
Dependents (1985)
@hackage/amqpworker, @hackage/lifxlan, @hackage/morpheusgraphqlclient, @hackage/clashlibhedgehog, @hackage/extensibleskeleton, @hackage/melf, Show all…