Undo

Description

This code has been taken from http://haskell.org A Monad transformer UndoT on a state supporting undo , redo and hput to push the last state on history. Redo stack is blanked on hput

Synopsis

data History s = History {

current :: s

undos :: [s]

redos :: [s]

}

type HStateT s m = StateT (History s) m

class (Monad m, MonadState (History s) (HStateT s m)) => HCtx m s

newtype Monad m => UndoT s m a = UndoT (HStateT s m a)

undo :: HCtx m s => UndoT s m Bool

redo :: HCtx m s => UndoT s m Bool

hput :: HCtx m s => s -> UndoT s m ()

blank :: s -> History s

evalUndoT :: Monad m => UndoT s m a -> s -> m a

execUndoT :: Monad m => UndoT s m a -> s -> m s

Documentation

data History s

Source

State stacks wrapping states in time

Constructors

History

current :: s	last state putted
undos :: [s]	the history of putted states (reversed) without the redos
redos :: [s]	history of the undo

Instances

Show s => Show (History s)

type HStateT s m = StateT (History s) m

Source

a state monad transformer with the state history

class (Monad m, MonadState (History s) (HStateT s m)) => HCtx m s

Source

facility to write signatures context

newtype Monad m => UndoT s m a

Source

a wrapper around HStateT to derive his classes and add an instance

Constructors

UndoT (HStateT s m a)

Instances

Monad m => MonadState s (UndoT s m)

MonadTrans (UndoT s)

Monad m => Monad (UndoT s m)

Monad m => Functor (UndoT s m)

MonadIO m => MonadIO (UndoT s m)

undo

Source

:: HCtx m s
=> UndoT s m Bool	False if the undo stack was empty
tries to get back one step the state

redo

Source

:: HCtx m s
=> UndoT s m Bool	False if the redo stack was empty
tries to get back the undo operation

hput

Source

:: HCtx m s
=> s	the new state to put
-> UndoT s m ()	monading
push the old state in the undo stack and set the new state (alternative to put)

blank :: s -> History s

Source

an History of one state

evalUndoT

Source

:: Monad m
=> UndoT s m a	a UndoT action
-> s	the initial state
-> m a	the result
run the UndoT monad transformer spitting out the computation result in the inner monad

execUndoT

Source

:: Monad m
=> UndoT s m a	a UndoT action
-> s	the initial state
-> m s	the final state
run the UndoT monad transformer spitting out the final state in the inner monad

Produced by Haddock version 2.3.0