acme-realworld (empty) → 0.1

raw patch · 4 files changed

+141/−0 lines, 4 filesdep +basesetup-changed

Dependencies added: base

Files

• Acme/RealWorld.hs +69/−0
• LICENSE +30/−0
• Setup.hs +2/−0
• acme-realworld.cabal +40/−0

@@ -0,0 +1,69 @@+-- |+-- Module:      Acme.RealWorld+-- Copyright:   (c) Joseph Adams 2011+-- Maintainer:  joeyadams3.14159@gmail.com+-- Portability: GHC-only+--+-- Primitives for manipulating the state of the universe.++{-# LANGUAGE MagicHash, UnboxedTuples #-}+module Acme.RealWorld (+    -- * The RealWorld type+    RealWorld,++    -- * Universe manipulation primitives+    getWorld,+    putWorld,+    execIO,++    -- * Derived combinators+    hypothetically,+) where++import Control.Exception (bracket)+import GHC.IO+import GHC.Exts++fromState :: State# RealWorld -> RealWorld+fromState = unsafeCoerce#++toState :: RealWorld -> State# RealWorld+toState = unsafeCoerce#++-- | Retrieve the current state of the universe.+getWorld :: IO RealWorld+getWorld = IO (\s -> (# s, fromState s #))++-- | Set the current state of the universe.  Program values are not affected by+-- this operation, but the rest of the universe is.+--+-- 'putWorld' may not be called on the same state twice (this is enforced by+-- the runtime system).  Otherwise, it would be possible to trap the universe+-- in a temporal loop:+--+-- >getWorld >>= forever . putWorld+putWorld :: RealWorld -> IO ()+putWorld s' = IO (\_ -> (# toState s', () #))++-- | Given an action, construct a function that, given a state of the universe,+-- returns the state of the universe after the action has occurred.+--+-- Example:+--+-- >main = do+-- >    let f = execIO $ putStrLn "Second"+-- >        g = execIO $ putStrLn "First"+-- >        getWorld >>= putWorld . f . g+execIO :: IO a -> RealWorld -> RealWorld+execIO (IO k) = \w -> case k (toState w) of (# s', _ #) -> fromState s'++-- | Perform an action and return its value, but undo any side effects to the+-- universe.  Thus, it appears to return instantly, regardless of how long the+-- action would take to run.+--+-- The caller must ensure that the program /would/ have enough time to perform+-- the computation.  Otherwise, either an exception will be thrown, or the+-- operation will block because it never gets a chance to restore the original+-- state of the universe.+hypothetically :: IO a -> IO a+hypothetically action = bracket getWorld putWorld (\_ -> action)

@@ -0,0 +1,30 @@+Copyright (c) 2011, Joseph Adams++All rights reserved.++Redistribution and use in source and binary forms, with or without+modification, are permitted provided that the following conditions are met:++    * Redistributions of source code must retain the above copyright+      notice, this list of conditions and the following disclaimer.++    * Redistributions in binary form must reproduce the above+      copyright notice, this list of conditions and the following+      disclaimer in the documentation and/or other materials provided+      with the distribution.++    * Neither the name of Joseph Adams nor the names of other+      contributors may be used to endorse or promote products derived+      from this software without specific prior written permission.++THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

@@ -0,0 +1,2 @@+import Distribution.Simple+main = defaultMain

@@ -0,0 +1,40 @@+name:                acme-realworld+version:             0.1+synopsis:            Primitives for manipulating the state of the universe+description:+    GHC represents an IO action internally as a function from one state of the+    world to the next:+    .+    >State# RealWorld -> (# State# RealWorld, a #)+    .+    This module provides a very useful capability stemming directly from this+    representation: the ability to save the current state of the universe and+    restore it later.  This provides a way to \"undo\" certain types of side+    effects in the IO monad:+    .+    >import Acme.Missiles+    >import Acme.RealWorld+    >+    >main :: IO ()+    >main = do+    >    -- Save the current state of the universe+    >    world_as_we_know_it <- getWorld+    >+    >    -- Cause serious international side effects+    >    launchMissiles+    >+    >    -- After realizing that was a terrible, terrible mistake, restore the+    >    -- pre-war state of the universe.+    >    putWorld world_as_we_know_it+license:             BSD3+license-file:        LICENSE+author:              Joey Adams+maintainer:          joeyadams3.14159@gmail.com+copyright:           Copyright (c) Joseph Adams 2011+category:            Acme+build-type:          Simple+cabal-version:       >=1.8++library+    exposed-modules:    Acme.RealWorld+    build-depends:      base == 4.*