mtl-2.2.1: Monad classes, using functional dependencies

Copyright (c) Andy Gill 2001(c) Oregon Graduate Institute of Science and Technology 2001 BSD-style (see the file LICENSE) libraries@haskell.org experimental non-portable (multi-param classes, functional dependencies) Safe Haskell98

Description

Inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html) Advanced School of Functional Programming, 1995.

Synopsis

class (Monoid w, Monad m) => MonadWriter w m | m -> w where #

Methods

writer :: (a, w) -> m a #

writer (a,w) embeds a simple writer action.

tell :: w -> m () #

tell w is an action that produces the output w.

listen :: m a -> m (a, w) #

listen m is an action that executes the action m and adds its output to the value of the computation.

pass :: m (a, w -> w) -> m a #

pass m is an action that executes the action m, which returns a value and a function, and returns the value, applying the function to the output.

Instances

listens :: MonadWriter w m => (w -> b) -> m a -> m (a, b) #

listens f m is an action that executes the action m and adds the result of applying f to the output to the value of the computation.

• listens f m = liftM (id *** f) (listen m)

censor :: MonadWriter w m => (w -> w) -> m a -> m a #

censor f m is an action that executes the action m and applies the function f to its output, leaving the return value unchanged.

• censor f m = pass (liftM (\x -> (x,f)) m)

# The Writer monad

type Writer w = WriterT w Identity #

A writer monad parameterized by the type w of output to accumulate.

The return function produces the output mempty, while >>= combines the outputs of the subcomputations using mappend.

runWriter :: Writer w a -> (a, w) #

Unwrap a writer computation as a (result, output) pair. (The inverse of writer.)

execWriter :: Writer w a -> w #

Extract the output from a writer computation.

• execWriter m = snd (runWriter m)

mapWriter :: ((a, w) -> (b, w')) -> Writer w a -> Writer w' b #

Map both the return value and output of a computation using the given function.

• runWriter (mapWriter f m) = f (runWriter m)

# The WriterT monad transformer

newtype WriterT w m a :: * -> (* -> *) -> * -> * #

A writer monad parameterized by:

• w - the output to accumulate.
• m - The inner monad.

The return function produces the output mempty, while >>= combines the outputs of the subcomputations using mappend.

Constructors

 WriterT FieldsrunWriterT :: m (a, w)

Instances

• execWriterT m = liftM snd (runWriterT m)
• runWriterT (mapWriterT f m) = f (runWriterT m)