Sophie: ghc-doc-0:6.10.4-1mdv2010.0 i586

ghc-doc-6.10.4-1mdv2010.0.i586.rpm

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<!--Rendered using the Haskell Html Library v0.2-->
<HTML
><HEAD
><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=UTF-8"
><TITLE
>Control.Monad.Reader</TITLE
><LINK HREF="haddock.css" REL="stylesheet" TYPE="text/css"
><SCRIPT SRC="haddock-util.js" TYPE="text/javascript"
></SCRIPT
><SCRIPT TYPE="text/javascript"
>window.onload = function () {setSynopsis("mini_Control-Monad-Reader.html")};</SCRIPT
></HEAD
><BODY
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="topbar"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD
><IMG SRC="haskell_icon.gif" WIDTH="16" HEIGHT="16" ALT=" "
></TD
><TD CLASS="title"
>mtl-1.1.0.2: Monad transformer library</TD
><TD CLASS="topbut"
><A HREF="index.html"
>Contents</A
></TD
><TD CLASS="topbut"
><A HREF="doc-index.html"
>Index</A
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="modulebar"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD
><FONT SIZE="6"
>Control.Monad.Reader</FONT
></TD
><TD ALIGN="right"
><TABLE CLASS="narrow" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="infohead"
>Portability</TD
><TD CLASS="infoval"
>non-portable (multi-param classes, functional dependencies)</TD
></TR
><TR
><TD CLASS="infohead"
>Stability</TD
><TD CLASS="infoval"
>experimental</TD
></TR
><TR
><TD CLASS="infohead"
>Maintainer</TD
><TD CLASS="infoval"
>libraries@haskell.org</TD
></TR
></TABLE
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="section4"
><B
>Contents</B
></TD
></TR
><TR
><TD
><DL
><DT
><A HREF="#1"
>Example 1: Simple Reader Usage
</A
></DT
><DT
><A HREF="#2"
>Example 2: Modifying Reader Content With <TT
>local</TT
>
</A
></DT
><DT
><A HREF="#3"
>Example 3: <TT
>ReaderT</TT
> Monad Transformer
</A
></DT
></DL
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
>Description</TD
></TR
><TR
><TD CLASS="doc"
><DL
><DT
>Computation type:</DT
><DD
> Computations which read values from a shared environment.
</DD
><DT
>Binding strategy:</DT
><DD
> Monad values are functions from the environment to a value.
The bound function is applied to the bound value, and both have access
to the shared environment.
</DD
><DT
>Useful for:</DT
><DD
> Maintaining variable bindings, or other shared environment.
</DD
><DT
>Zero and plus:</DT
><DD
> None.
</DD
><DT
>Example type:</DT
><DD
> <TT
><TT
><A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
></TT
> [(String,Value)] a</TT
>
</DD
></DL
><P
>The <TT
><A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
></TT
> monad (also called the Environment monad).
Represents a computation, which can read values from
a shared environment, pass values from function to function,
and execute sub-computations in a modified environment.
Using <TT
><A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
></TT
> monad for such computations is often clearer and easier
than using the Control.Monad.State.State monad.
</P
><P
>Inspired by the paper
  /Functional Programming with Overloading and
      Higher-Order Polymorphism/, 
    Mark P Jones (<A HREF="http://web.cecs.pdx.edu/~mpj/"
>http://web.cecs.pdx.edu/~mpj/</A
>)
    Advanced School of Functional Programming, 1995.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
>Synopsis</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
>module <A HREF="Control-Monad-Reader-Class.html"
>Control.Monad.Reader.Class</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>newtype</SPAN
>  <A HREF="#t%3AReader"
>Reader</A
> r a = <A HREF="#v%3AReader"
>Reader</A
> {<TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="recfield"
><A HREF="#v%3ArunReader"
>runReader</A
> :: r -&gt; a</TD
></TR
></TABLE
>}</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AmapReader"
>mapReader</A
> ::  (a -&gt; b) -&gt; <A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
> r a -&gt; <A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
> r b</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AwithReader"
>withReader</A
> ::  (r' -&gt; r) -&gt; <A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
> r a -&gt; <A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
> r' a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>newtype</SPAN
>  <A HREF="#t%3AReaderT"
>ReaderT</A
> r m a = <A HREF="#v%3AReaderT"
>ReaderT</A
> {<TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="recfield"
><A HREF="#v%3ArunReaderT"
>runReaderT</A
> :: r -&gt; m a</TD
></TR
></TABLE
>}</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AmapReaderT"
>mapReaderT</A
> ::  (m a -&gt; n b) -&gt; <A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> w m a -&gt; <A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> w n b</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AwithReaderT"
>withReaderT</A
> ::  (r' -&gt; r) -&gt; <A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r m a -&gt; <A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r' m a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="../base/Control-Monad.html"
>Control.Monad</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="../base/Control-Monad-Fix.html"
>Control.Monad.Fix</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Control-Monad-Trans.html"
>Control.Monad.Trans</A
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
>Documentation</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Control-Monad-Reader-Class.html"
>Control.Monad.Reader.Class</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>newtype</SPAN
>  <A NAME="t:Reader"
><A NAME="t%3AReader"
></A
></A
><B
>Reader</B
> r a </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
><P
>The parameterizable reader monad.
</P
><P
>The <TT
>return</TT
> function creates a <TT
>Reader</TT
> that ignores the environment,
and produces the given value.
</P
><P
>The binding operator <TT
>&gt;&gt;=</TT
> produces a <TT
>Reader</TT
> that uses the environment
to extract the value its left-hand side,
and then applies the bound function to that value in the same environment.
</P
></TD
></TR
><TR
><TD CLASS="section4"
>Constructors</TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="5" CELLPADDING="0"
><TR
><TD CLASS="arg"
><A NAME="v:Reader"
><A NAME="v%3AReader"
></A
></A
><B
>Reader</B
></TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="body" COLSPAN="2"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="arg"
><A NAME="v:runReader"
><A NAME="v%3ArunReader"
></A
></A
><B
>runReader</B
> :: r -&gt; a</TD
><TD CLASS="rdoc"
><P
>Runs <TT
>Reader</TT
> and extracts the final value from it.
    To extract the value apply <TT
>(runReader reader)</TT
> to an environment value.  
    Parameters:
</P
><UL
><LI
> A <TT
>Reader</TT
> to run.
</LI
><LI
> An initial environment.
</LI
></UL
></TD
></TR
></TABLE
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:Reader')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:Reader" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r (<A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
> r)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="../base/Control-Monad.html#t%3AMonad"
>Monad</A
> (<A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
> r)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="../base/Control-Monad.html#t%3AFunctor"
>Functor</A
> (<A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
> r)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="../base/Control-Monad-Fix.html#t%3AMonadFix"
>MonadFix</A
> (<A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
> r)</TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:mapReader"
><A NAME="v%3AmapReader"
></A
></A
><B
>mapReader</B
> ::  (a -&gt; b) -&gt; <A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
> r a -&gt; <A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
> r b</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:withReader"
><A NAME="v%3AwithReader"
></A
></A
><B
>withReader</B
> ::  (r' -&gt; r) -&gt; <A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
> r a -&gt; <A HREF="Control-Monad-Reader.html#t%3AReader"
>Reader</A
> r' a</TD
></TR
><TR
><TD CLASS="doc"
>A more general version of <TT
><A HREF="Control-Monad-Reader-Class.html#v%3Alocal"
>local</A
></TT
>.
</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>newtype</SPAN
>  <A NAME="t:ReaderT"
><A NAME="t%3AReaderT"
></A
></A
><B
>ReaderT</B
> r m a </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
>The reader monad transformer.
Can be used to add environment reading functionality to other monads.
</TD
></TR
><TR
><TD CLASS="section4"
>Constructors</TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="5" CELLPADDING="0"
><TR
><TD CLASS="arg"
><A NAME="v:ReaderT"
><A NAME="v%3AReaderT"
></A
></A
><B
>ReaderT</B
></TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="body" COLSPAN="2"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="arg"
><A NAME="v:runReaderT"
><A NAME="v%3ArunReaderT"
></A
></A
><B
>runReaderT</B
> :: r -&gt; m a</TD
><TD CLASS="rdoc"
></TD
></TR
></TABLE
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:ReaderT')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:ReaderT" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="Control-Monad-Writer-Class.html#t%3AMonadWriter"
>MonadWriter</A
> w m =&gt; <A HREF="Control-Monad-Writer-Class.html#t%3AMonadWriter"
>MonadWriter</A
> w (<A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Monad-State-Class.html#t%3AMonadState"
>MonadState</A
> s m =&gt; <A HREF="Control-Monad-State-Class.html#t%3AMonadState"
>MonadState</A
> s (<A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="../base/Control-Monad.html#t%3AMonad"
>Monad</A
> m =&gt; <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r (<A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Monad-Error-Class.html#t%3AMonadError"
>MonadError</A
> e m =&gt; <A HREF="Control-Monad-Error-Class.html#t%3AMonadError"
>MonadError</A
> e (<A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Monad-Trans.html#t%3AMonadTrans"
>MonadTrans</A
> (<A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="../base/Control-Monad.html#t%3AMonad"
>Monad</A
> m =&gt; <A HREF="../base/Control-Monad.html#t%3AMonad"
>Monad</A
> (<A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="../base/Control-Monad.html#t%3AMonad"
>Monad</A
> m =&gt; <A HREF="../base/Control-Monad.html#t%3AFunctor"
>Functor</A
> (<A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="../base/Control-Monad-Fix.html#t%3AMonadFix"
>MonadFix</A
> m =&gt; <A HREF="../base/Control-Monad-Fix.html#t%3AMonadFix"
>MonadFix</A
> (<A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="../base/Control-Monad.html#t%3AMonadPlus"
>MonadPlus</A
> m =&gt; <A HREF="../base/Control-Monad.html#t%3AMonadPlus"
>MonadPlus</A
> (<A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Monad-Trans.html#t%3AMonadIO"
>MonadIO</A
> m =&gt; <A HREF="Control-Monad-Trans.html#t%3AMonadIO"
>MonadIO</A
> (<A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Monad-Cont-Class.html#t%3AMonadCont"
>MonadCont</A
> m =&gt; <A HREF="Control-Monad-Cont-Class.html#t%3AMonadCont"
>MonadCont</A
> (<A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r m)</TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:mapReaderT"
><A NAME="v%3AmapReaderT"
></A
></A
><B
>mapReaderT</B
> ::  (m a -&gt; n b) -&gt; <A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> w m a -&gt; <A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> w n b</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:withReaderT"
><A NAME="v%3AwithReaderT"
></A
></A
><B
>withReaderT</B
> ::  (r' -&gt; r) -&gt; <A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r m a -&gt; <A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
> r' m a</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="../base/Control-Monad.html"
>Control.Monad</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="../base/Control-Monad-Fix.html"
>Control.Monad.Fix</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
>module <A HREF="Control-Monad-Trans.html"
>Control.Monad.Trans</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="1"
><A NAME="1"
>Example 1: Simple Reader Usage
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="doc"
><P
>In this example the <TT
>Reader</TT
> monad provides access to variable bindings.
Bindings are a Map of integer variables.
The variable <TT
>count</TT
> contains number of variables in the bindings.
You can see how to run a Reader monad and retrieve data from it
with <TT
><A HREF="Control-Monad-Reader.html#v%3ArunReader"
>runReader</A
></TT
>, how to access the Reader data with <TT
><A HREF="Control-Monad-Reader-Class.html#v%3Aask"
>ask</A
></TT
> and <TT
><A HREF="Control-Monad-Reader-Class.html#v%3Aasks"
>asks</A
></TT
>.
</P
><PRE
> type Bindings = Map String Int;

-- Returns True if the &quot;count&quot; variable contains correct bindings size.
isCountCorrect :: Bindings -&gt; Bool
isCountCorrect bindings = runReader calc_isCountCorrect bindings

-- The Reader monad, which implements this complicated check.
calc_isCountCorrect :: Reader Bindings Bool
calc_isCountCorrect = do
    count &lt;- asks (lookupVar &quot;count&quot;)
    bindings &lt;- ask
    return (count == (Map.size bindings))

-- The selector function to  use with 'asks'.
-- Returns value of the variable with specified name.
lookupVar :: String -&gt; Bindings -&gt; Int
lookupVar name bindings = fromJust (Map.lookup name bindings)

sampleBindings = Map.fromList [(&quot;count&quot;,3), (&quot;1&quot;,1), (&quot;b&quot;,2)]

main = do
    putStr $ &quot;Count is correct for bindings &quot; ++ (show sampleBindings) ++ &quot;: &quot;;
    putStrLn $ show (isCountCorrect sampleBindings);
</PRE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="2"
><A NAME="2"
>Example 2: Modifying Reader Content With <TT
>local</TT
>
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="doc"
><P
>Shows how to modify Reader content with <TT
><A HREF="Control-Monad-Reader-Class.html#v%3Alocal"
>local</A
></TT
>.
</P
><PRE
>calculateContentLen :: Reader String Int
calculateContentLen = do
    content &lt;- ask
    return (length content);

-- Calls calculateContentLen after adding a prefix to the Reader content.
calculateModifiedContentLen :: Reader String Int
calculateModifiedContentLen = local (&quot;Prefix &quot; ++) calculateContentLen

main = do
    let s = &quot;12345&quot;;
    let modifiedLen = runReader calculateModifiedContentLen s
    let len = runReader calculateContentLen s
    putStrLn $ &quot;Modified 's' length: &quot; ++ (show modifiedLen)
    putStrLn $ &quot;Original 's' length: &quot; ++ (show len)
</PRE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="3"
><A NAME="3"
>Example 3: <TT
>ReaderT</TT
> Monad Transformer
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="doc"
><P
>Now you are thinking: 'Wow, what a great monad! I wish I could use
Reader functionality in MyFavoriteComplexMonad!'. Don't worry.
This can be easy done with the <TT
><A HREF="Control-Monad-Reader.html#t%3AReaderT"
>ReaderT</A
></TT
> monad transformer.
This example shows how to combine <TT
>ReaderT</TT
> with the IO monad.
</P
><PRE
>-- The Reader/IO combined monad, where Reader stores a string.
printReaderContent :: ReaderT String IO ()
printReaderContent = do
    content &lt;- ask
    liftIO $ putStrLn (&quot;The Reader Content: &quot; ++ content)

main = do
    runReaderT printReaderContent &quot;Some Content&quot;
</PRE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="botbar"
>Produced by <A HREF="http://www.haskell.org/haddock/"
>Haddock</A
> version 2.4.2</TD
></TR
></TABLE
></BODY
></HTML
>