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

Documentation

data History s Source

State stacks wrapping states in time

Constructors

History
Fields 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) mSource

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

Instances

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

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)	the MonadState instance for the wrapper
MonadTrans (UndoT s)
Monad m => Monad (UndoT s m)
Monad m => Functor (UndoT s m)
MonadIO m => MonadIO (UndoT s m)

undo Source

Arguments

:: HCtx m s
=> UndoT s m Bool	False if the undo stack was empty

tries to get back one step the state

redo Source

Arguments

:: HCtx m s
=> UndoT s m Bool	False if the redo stack was empty

tries to get back the undo operation

hput Source

Arguments

:: 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 sSource

an History of one state

evalUndoT Source

Arguments

:: 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

Arguments

:: 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