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>Control.Monad.State.Class</TITLE
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>mtl-1.1.0.2: Monad transformer library</TD
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>Control.Monad.State.Class</FONT
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><TD ALIGN="right"
><TABLE CLASS="narrow" CELLSPACING="0" CELLPADDING="0"
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><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
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><TD CLASS="section4"
><B
>Contents</B
></TD
></TR
><TR
><TD
><DL
><DT
><A HREF="#1"
>MonadState class
</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"
><P
>MonadState class.
</P
><P
>This module is 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
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><TR
><TD CLASS="s15"
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></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%3AMonadState"
>MonadState</A
> s m | m -&gt; s <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%3Aget"
>get</A
> :: m s</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Aput"
>put</A
> :: s -&gt; m <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Amodify"
>modify</A
> :: <A HREF="Control-Monad-State-Class.html#t%3AMonadState"
>MonadState</A
> s m =&gt; (s -&gt; s) -&gt; m <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3Agets"
>gets</A
> :: <A HREF="Control-Monad-State-Class.html#t%3AMonadState"
>MonadState</A
> s m =&gt; (s -&gt; a) -&gt; m a</TD
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></TD
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><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="1"
><A NAME="1"
>MonadState class
</A
></A
></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:MonadState"
><A NAME="t%3AMonadState"
></A
></A
><B
>MonadState</B
> s m | m -&gt; s <SPAN CLASS="keyword"
>where</SPAN
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
><P
><EM
>get</EM
> returns the state from the internals of the monad.
</P
><P
><EM
>put</EM
> replaces the state inside the monad.
</P
></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:get"
><A NAME="v%3Aget"
></A
></A
><B
>get</B
> :: m s</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:put"
><A NAME="v%3Aput"
></A
></A
><B
>put</B
> :: s -&gt; m <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></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:MonadState')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:MonadState" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><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-List.html#t%3AListT"
>ListT</A
> m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Monad-State-Class.html#t%3AMonadState"
>MonadState</A
> s (<A HREF="Control-Monad-State-Lazy.html#t%3AState"
>State</A
> s)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Monad-State-Class.html#t%3AMonadState"
>MonadState</A
> s (<A HREF="Control-Monad-State-Strict.html#t%3AState"
>State</A
> s)</TD
></TR
><TR
><TD CLASS="decl"
>(<A HREF="Control-Monad-Error-Class.html#t%3AError"
>Error</A
> e, <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-Error.html#t%3AErrorT"
>ErrorT</A
> e 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-State-Class.html#t%3AMonadState"
>MonadState</A
> s (<A HREF="Control-Monad-State-Lazy.html#t%3AStateT"
>StateT</A
> s m)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="../base/Control-Monad.html#t%3AMonad"
>Monad</A
> m =&gt; <A HREF="Control-Monad-State-Class.html#t%3AMonadState"
>MonadState</A
> s (<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-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-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-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-Writer-Strict.html#t%3AWriterT"
>WriterT</A
> w 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-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-State-Class.html#t%3AMonadState"
>MonadState</A
> s (<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-State-Class.html#t%3AMonadState"
>MonadState</A
> s (<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-State-Class.html#t%3AMonadState"
>MonadState</A
> s (<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-State-Class.html#t%3AMonadState"
>MonadState</A
> s (<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:modify"
><A NAME="v%3Amodify"
></A
></A
><B
>modify</B
> :: <A HREF="Control-Monad-State-Class.html#t%3AMonadState"
>MonadState</A
> s m =&gt; (s -&gt; s) -&gt; m <A HREF="../ghc-prim/GHC-Unit.html#t%3A%28%29"
>()</A
></TD
></TR
><TR
><TD CLASS="doc"
><P
>Monadic state transformer.
</P
><P
>Maps an old state to a new state inside a state monad.
      The old state is thrown away.
</P
><PRE
>      Main&gt; :t modify ((+1) :: Int -&gt; Int)
      modify (...) :: (MonadState Int a) =&gt; a ()
</PRE
><P
>This says that <TT
>modify (+1)</TT
> acts over any
    Monad that is a member of the <TT
>MonadState</TT
> class,
    with an <TT
>Int</TT
> state.
</P
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:gets"
><A NAME="v%3Agets"
></A
></A
><B
>gets</B
> :: <A HREF="Control-Monad-State-Class.html#t%3AMonadState"
>MonadState</A
> s m =&gt; (s -&gt; a) -&gt; m a</TD
></TR
><TR
><TD CLASS="doc"
>Gets specific component of the state, using a projection function
 supplied.
</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
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