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>Control.Monad.Reader.Class</TITLE
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>mtl-1.1.0.2: Monad transformer library</TD
><TD CLASS="topbut"
><A HREF="index.html"
>Contents</A
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>Index</A
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><FONT SIZE="6"
>Control.Monad.Reader.Class</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 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
>Reader [(String,Value)] a</TT
>
</DD
></DL
><P
>The Reader 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 Reader 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"
><SPAN CLASS="keyword"
>class</SPAN
> <A HREF="../base/Control-Monad.html#t%3AMonad"
>Monad</A
> m =&gt; <A HREF="#t%3AMonadReader"
>MonadReader</A
> r m | m -&gt; r <SPAN CLASS="keyword"
>where</SPAN
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="#v%3Aask"
>ask</A
> :: m r</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Alocal"
>local</A
> ::  (r -&gt; r) -&gt; m a -&gt; m a</TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Aasks"
>asks</A
> :: <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r m =&gt; (r -&gt; a) -&gt; m 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"
><SPAN CLASS="keyword"
>class</SPAN
> <A HREF="../base/Control-Monad.html#t%3AMonad"
>Monad</A
> m =&gt; <A NAME="t:MonadReader"
><A NAME="t%3AMonadReader"
></A
></A
><B
>MonadReader</B
> r m | m -&gt; r <SPAN CLASS="keyword"
>where</SPAN
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
>See examples in <A HREF="Control-Monad-Reader.html"
>Control.Monad.Reader</A
>.
Note, the partially applied function type <TT
>(-&gt;) r</TT
> is a simple reader monad.
See the <TT
>instance</TT
> declaration below.
</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="section4"
>Methods</TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A NAME="v:ask"
><A NAME="v%3Aask"
></A
></A
><B
>ask</B
> :: m r</TD
></TR
><TR
><TD CLASS="doc"
>Retrieves the monad environment.
</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:local"
><A NAME="v%3Alocal"
></A
></A
><B
>local</B
> ::  (r -&gt; r) -&gt; m a -&gt; m a</TD
></TR
><TR
><TD CLASS="doc"
><P
>Executes a computation in a modified environment. Parameters:
</P
><UL
><LI
> The function to modify the environment.
</LI
><LI
> <TT
>Reader</TT
> to run.
</LI
><LI
> The resulting <TT
>Reader</TT
>.
</LI
></UL
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:MonadReader')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:MonadReader" 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
> s m =&gt; <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> s (<A HREF="Control-Monad-List.html#t%3AListT"
>ListT</A
> m)</TD
></TR
><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="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r ((-&gt;) r)</TD
></TR
><TR
><TD CLASS="decl"
>(<A HREF="Control-Monad-Error-Class.html#t%3AError"
>Error</A
> e, <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r m) =&gt; <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r (<A HREF="Control-Monad-Error.html#t%3AErrorT"
>ErrorT</A
> e 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-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r m =&gt; <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r (<A HREF="Control-Monad-State-Lazy.html#t%3AStateT"
>StateT</A
> s m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r m =&gt; <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r (<A HREF="Control-Monad-State-Strict.html#t%3AStateT"
>StateT</A
> s m)</TD
></TR
><TR
><TD CLASS="decl"
>(<A HREF="../base/Data-Monoid.html#t%3AMonoid"
>Monoid</A
> w, <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r m) =&gt; <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r (<A HREF="Control-Monad-Writer-Lazy.html#t%3AWriterT"
>WriterT</A
> w m)</TD
></TR
><TR
><TD CLASS="decl"
>(<A HREF="../base/Data-Monoid.html#t%3AMonoid"
>Monoid</A
> w, <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r m) =&gt; <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r (<A HREF="Control-Monad-Writer-Strict.html#t%3AWriterT"
>WriterT</A
> w m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r' m =&gt; <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r' (<A HREF="Control-Monad-Cont.html#t%3AContT"
>ContT</A
> r m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="../base/Data-Monoid.html#t%3AMonoid"
>Monoid</A
> w =&gt; <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r (<A HREF="Control-Monad-RWS-Lazy.html#t%3ARWS"
>RWS</A
> r w s)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="../base/Data-Monoid.html#t%3AMonoid"
>Monoid</A
> w =&gt; <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r (<A HREF="Control-Monad-RWS-Strict.html#t%3ARWS"
>RWS</A
> r w s)</TD
></TR
><TR
><TD CLASS="decl"
>(<A HREF="../base/Data-Monoid.html#t%3AMonoid"
>Monoid</A
> w, <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-RWS-Lazy.html#t%3ARWST"
>RWST</A
> r w s m)</TD
></TR
><TR
><TD CLASS="decl"
>(<A HREF="../base/Data-Monoid.html#t%3AMonoid"
>Monoid</A
> w, <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-RWS-Strict.html#t%3ARWST"
>RWST</A
> r w s m)</TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:asks"
><A NAME="v%3Aasks"
></A
></A
><B
>asks</B
> :: <A HREF="Control-Monad-Reader-Class.html#t%3AMonadReader"
>MonadReader</A
> r m =&gt; (r -&gt; a) -&gt; m a</TD
></TR
><TR
><TD CLASS="doc"
><P
>Retrieves a function of the current environment. Parameters:
</P
><UL
><LI
> The selector function to apply to the environment.
</LI
></UL
><P
>See an example in <A HREF="Control-Monad-Reader.html"
>Control.Monad.Reader</A
>.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
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>Produced by <A HREF="http://www.haskell.org/haddock/"
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