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<pre><a name="line-1"></a><span class='hs-comment'>{- |
<a name="line-2"></a>Module : Control.Monad.Reader
<a name="line-3"></a>Copyright : (c) Andy Gill 2001,
<a name="line-4"></a> (c) Oregon Graduate Institute of Science and Technology 2001,
<a name="line-5"></a> (c) Jeff Newbern 2003-2007,
<a name="line-6"></a> (c) Andriy Palamarchuk 2007
<a name="line-7"></a>License : BSD-style (see the file LICENSE)
<a name="line-8"></a>
<a name="line-9"></a>Maintainer : libraries@haskell.org
<a name="line-10"></a>Stability : experimental
<a name="line-11"></a>Portability : non-portable (multi-param classes, functional dependencies)
<a name="line-12"></a>
<a name="line-13"></a>[Computation type:] Computations which read values from a shared environment.
<a name="line-14"></a>
<a name="line-15"></a>[Binding strategy:] Monad values are functions from the environment to a value.
<a name="line-16"></a>The bound function is applied to the bound value, and both have access
<a name="line-17"></a>to the shared environment.
<a name="line-18"></a>
<a name="line-19"></a>[Useful for:] Maintaining variable bindings, or other shared environment.
<a name="line-20"></a>
<a name="line-21"></a>[Zero and plus:] None.
<a name="line-22"></a>
<a name="line-23"></a>[Example type:] @'Reader' [(String,Value)] a@
<a name="line-24"></a>
<a name="line-25"></a>The 'Reader' monad (also called the Environment monad).
<a name="line-26"></a>Represents a computation, which can read values from
<a name="line-27"></a>a shared environment, pass values from function to function,
<a name="line-28"></a>and execute sub-computations in a modified environment.
<a name="line-29"></a>Using 'Reader' monad for such computations is often clearer and easier
<a name="line-30"></a>than using the 'Control.Monad.State.State' monad.
<a name="line-31"></a>
<a name="line-32"></a> Inspired by the paper
<a name="line-33"></a> /Functional Programming with Overloading and Higher-Order Polymorphism/,
<a name="line-34"></a> Mark P Jones (<<a href="http://web.cecs.pdx.edu/~mpj/">http://web.cecs.pdx.edu/~mpj/</a>>)
<a name="line-35"></a> Advanced School of Functional Programming, 1995.
<a name="line-36"></a>-}</span>
<a name="line-37"></a>
<a name="line-38"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Reader</span> <span class='hs-layout'>(</span>
<a name="line-39"></a> <span class='hs-comment'>-- * MonadReader class</span>
<a name="line-40"></a> <span class='hs-conid'>MonadReader</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span>
<a name="line-41"></a> <span class='hs-varid'>asks</span><span class='hs-layout'>,</span>
<a name="line-42"></a> <span class='hs-comment'>-- * The Reader monad</span>
<a name="line-43"></a> <span class='hs-conid'>Reader</span><span class='hs-layout'>,</span>
<a name="line-44"></a> <span class='hs-varid'>reader</span><span class='hs-layout'>,</span>
<a name="line-45"></a> <span class='hs-varid'>runReader</span><span class='hs-layout'>,</span>
<a name="line-46"></a> <span class='hs-varid'>mapReader</span><span class='hs-layout'>,</span>
<a name="line-47"></a> <span class='hs-varid'>withReader</span><span class='hs-layout'>,</span>
<a name="line-48"></a> <span class='hs-comment'>-- * The ReaderT monad transformer</span>
<a name="line-49"></a> <span class='hs-conid'>ReaderT</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span>
<a name="line-50"></a> <span class='hs-varid'>mapReaderT</span><span class='hs-layout'>,</span>
<a name="line-51"></a> <span class='hs-varid'>withReaderT</span><span class='hs-layout'>,</span>
<a name="line-52"></a> <span class='hs-keyword'>module</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-layout'>,</span>
<a name="line-53"></a> <span class='hs-keyword'>module</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Fix</span><span class='hs-layout'>,</span>
<a name="line-54"></a> <span class='hs-keyword'>module</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-layout'>,</span>
<a name="line-55"></a> <span class='hs-comment'>-- * Example 1: Simple Reader Usage</span>
<a name="line-56"></a> <span class='hs-comment'>-- $simpleReaderExample</span>
<a name="line-57"></a>
<a name="line-58"></a> <span class='hs-comment'>-- * Example 2: Modifying Reader Content With @local@</span>
<a name="line-59"></a> <span class='hs-comment'>-- $localExample</span>
<a name="line-60"></a>
<a name="line-61"></a> <span class='hs-comment'>-- * Example 3: @ReaderT@ Monad Transformer</span>
<a name="line-62"></a> <span class='hs-comment'>-- $ReaderTExample</span>
<a name="line-63"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-64"></a>
<a name="line-65"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Reader</span><span class='hs-varop'>.</span><span class='hs-conid'>Class</span>
<a name="line-66"></a>
<a name="line-67"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Reader</span> <span class='hs-layout'>(</span>
<a name="line-68"></a> <span class='hs-conid'>Reader</span><span class='hs-layout'>,</span> <span class='hs-varid'>reader</span><span class='hs-layout'>,</span> <span class='hs-varid'>runReader</span><span class='hs-layout'>,</span> <span class='hs-varid'>mapReader</span><span class='hs-layout'>,</span> <span class='hs-varid'>withReader</span><span class='hs-layout'>,</span>
<a name="line-69"></a> <span class='hs-conid'>ReaderT</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span> <span class='hs-varid'>mapReaderT</span><span class='hs-layout'>,</span> <span class='hs-varid'>withReaderT</span><span class='hs-layout'>)</span>
<a name="line-70"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span>
<a name="line-71"></a>
<a name="line-72"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span>
<a name="line-73"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Fix</span>
<a name="line-74"></a>
<a name="line-75"></a><span class='hs-comment'>{- $simpleReaderExample
<a name="line-76"></a>
<a name="line-77"></a>In this example the @Reader@ monad provides access to variable bindings.
<a name="line-78"></a>Bindings are a @Map@ of integer variables.
<a name="line-79"></a>The variable @count@ contains number of variables in the bindings.
<a name="line-80"></a>You can see how to run a Reader monad and retrieve data from it
<a name="line-81"></a>with 'runReader', how to access the Reader data with 'ask' and 'asks'.
<a name="line-82"></a>
<a name="line-83"></a>> type Bindings = Map String Int;
<a name="line-84"></a>>
<a name="line-85"></a>>-- Returns True if the "count" variable contains correct bindings size.
<a name="line-86"></a>>isCountCorrect :: Bindings -> Bool
<a name="line-87"></a>>isCountCorrect bindings = runReader calc_isCountCorrect bindings
<a name="line-88"></a>>
<a name="line-89"></a>>-- The Reader monad, which implements this complicated check.
<a name="line-90"></a>>calc_isCountCorrect :: Reader Bindings Bool
<a name="line-91"></a>>calc_isCountCorrect = do
<a name="line-92"></a>> count <- asks (lookupVar "count")
<a name="line-93"></a>> bindings <- ask
<a name="line-94"></a>> return (count == (Map.size bindings))
<a name="line-95"></a>>
<a name="line-96"></a>>-- The selector function to use with 'asks'.
<a name="line-97"></a>>-- Returns value of the variable with specified name.
<a name="line-98"></a>>lookupVar :: String -> Bindings -> Int
<a name="line-99"></a>>lookupVar name bindings = fromJust (Map.lookup name bindings)
<a name="line-100"></a>>
<a name="line-101"></a>>sampleBindings = Map.fromList [("count",3), ("1",1), ("b",2)]
<a name="line-102"></a>>
<a name="line-103"></a>>main = do
<a name="line-104"></a>> putStr $ "Count is correct for bindings " ++ (show sampleBindings) ++ ": ";
<a name="line-105"></a>> putStrLn $ show (isCountCorrect sampleBindings);
<a name="line-106"></a>-}</span>
<a name="line-107"></a>
<a name="line-108"></a><span class='hs-comment'>{- $localExample
<a name="line-109"></a>
<a name="line-110"></a>Shows how to modify Reader content with 'local'.
<a name="line-111"></a>
<a name="line-112"></a>>calculateContentLen :: Reader String Int
<a name="line-113"></a>>calculateContentLen = do
<a name="line-114"></a>> content <- ask
<a name="line-115"></a>> return (length content);
<a name="line-116"></a>>
<a name="line-117"></a>>-- Calls calculateContentLen after adding a prefix to the Reader content.
<a name="line-118"></a>>calculateModifiedContentLen :: Reader String Int
<a name="line-119"></a>>calculateModifiedContentLen = local ("Prefix " ++) calculateContentLen
<a name="line-120"></a>>
<a name="line-121"></a>>main = do
<a name="line-122"></a>> let s = "12345";
<a name="line-123"></a>> let modifiedLen = runReader calculateModifiedContentLen s
<a name="line-124"></a>> let len = runReader calculateContentLen s
<a name="line-125"></a>> putStrLn $ "Modified 's' length: " ++ (show modifiedLen)
<a name="line-126"></a>> putStrLn $ "Original 's' length: " ++ (show len)
<a name="line-127"></a>-}</span>
<a name="line-128"></a>
<a name="line-129"></a><span class='hs-comment'>{- $ReaderTExample
<a name="line-130"></a>
<a name="line-131"></a>Now you are thinking: 'Wow, what a great monad! I wish I could use
<a name="line-132"></a>Reader functionality in MyFavoriteComplexMonad!'. Don't worry.
<a name="line-133"></a>This can be easy done with the 'ReaderT' monad transformer.
<a name="line-134"></a>This example shows how to combine @ReaderT@ with the IO monad.
<a name="line-135"></a>
<a name="line-136"></a>>-- The Reader/IO combined monad, where Reader stores a string.
<a name="line-137"></a>>printReaderContent :: ReaderT String IO ()
<a name="line-138"></a>>printReaderContent = do
<a name="line-139"></a>> content <- ask
<a name="line-140"></a>> liftIO $ putStrLn ("The Reader Content: " ++ content)
<a name="line-141"></a>>
<a name="line-142"></a>>main = do
<a name="line-143"></a>> runReaderT printReaderContent "Some Content"
<a name="line-144"></a>-}</span>
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