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>Control.Arrow.ArrowState</TITLE
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>hxt-7.1: </TD
><TD CLASS="topbut"
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>Contents</A
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>Index</A
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><FONT SIZE="6"
>Control.Arrow.ArrowState</FONT
></TD
><TD ALIGN="right"
><TABLE CLASS="narrow" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="infohead"
>Portability</TD
><TD CLASS="infoval"
>portable</TD
></TR
><TR
><TD CLASS="infohead"
>Stability</TD
><TD CLASS="infoval"
>experimental</TD
></TR
><TR
><TD CLASS="infohead"
>Maintainer</TD
><TD CLASS="infoval"
>Uwe Schmidt (uwe\@fh-wedel.de)</TD
></TR
></TABLE
></TD
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></TD
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><TD CLASS="s15"
></TD
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><TR
><TD CLASS="section1"
>Description</TD
></TR
><TR
><TD CLASS="doc"
><P
>Version : $Id: ArrowState.hs,v 1.5 2005<EM
>09</EM
>02 17:09:39 hxml Exp $
</P
><P
>Arrows for managing an explicit state
</P
><P
>State arrows work similar to state monads.
A state value is threaded through the application of arrows.
</P
></TD
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><TD CLASS="s15"
></TD
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><TR
><TD CLASS="section1"
>Synopsis</TD
></TR
><TR
><TD CLASS="s15"
></TD
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><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
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><TD CLASS="decl"
><SPAN CLASS="keyword"
>class</SPAN
> Arrow a => <A HREF="#t%3AArrowState"
>ArrowState</A
> s a | a -> s <SPAN CLASS="keyword"
>where</SPAN
></TD
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><TD CLASS="body"
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><TD CLASS="decl"
><A HREF="#v%3AchangeState"
>changeState</A
> :: (s -> b -> s) -> a b b</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AaccessState"
>accessState</A
> :: (s -> b -> c) -> a b c</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AgetState"
>getState</A
> :: a b s</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AsetState"
>setState</A
> :: a s s</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AnextState"
>nextState</A
> :: (s -> s) -> a b s</TD
></TR
></TABLE
></TD
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><TD CLASS="s15"
></TD
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><TR
><TD CLASS="section1"
>Documentation</TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>class</SPAN
> Arrow a => <A NAME="t%3AArrowState"
></A
><B
>ArrowState</B
> s a | a -> s <SPAN CLASS="keyword"
>where</SPAN
></TD
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><TD CLASS="ndoc"
><P
>The interface for accessing and changing the state component.
</P
><P
>Multi parameter classes and functional dependencies are required.
</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%3AchangeState"
></A
><B
>changeState</B
> :: (s -> b -> s) -> a b b</TD
></TR
><TR
><TD CLASS="doc"
>change the state of a state arrow by applying a function
for computing a new state from the old and the arrow input.
Result is the arrow input
</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v%3AaccessState"
></A
><B
>accessState</B
> :: (s -> b -> c) -> a b c</TD
></TR
><TR
><TD CLASS="doc"
>access the state with a function using the arrow input
as data for selecting state components.
</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v%3AgetState"
></A
><B
>getState</B
> :: a b s</TD
></TR
><TR
><TD CLASS="doc"
><P
>read the complete state, ignore arrow input
</P
><P
>definition: <TT
> getState = accessState (\ s x -> s) </TT
>
</P
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v%3AsetState"
></A
><B
>setState</B
> :: a s s</TD
></TR
><TR
><TD CLASS="doc"
><P
>overwrite the old state
</P
><P
>definition: <TT
> setState = changeState (\ s x -> x) </TT
>
</P
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v%3AnextState"
></A
><B
>nextState</B
> :: (s -> s) -> a b s</TD
></TR
><TR
><TD CLASS="doc"
><P
>change state (and ignore input) and return new state
</P
><P
>convenience function,
usefull for generating e.g. unique identifiers:
</P
><P
>example with SLA state list arrows
</P
><PRE
> newId :: SLA Int b String
newId = nextState (+1)
>>>
arr (('#':) . show)
runSLA 0 (newId <+> newId <+> newId) undefined
= ["#1", "#2", "#3"]
</PRE
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> Instances</TD
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><TR
><TD CLASS="body"
><DIV ID="i:ArrowState" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
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><TD CLASS="decl"
><A HREF="Control-Arrow-ArrowState.html#t%3AArrowState"
>ArrowState</A
> s (<A HREF="Control-Arrow-IOStateListArrow.html#t%3AIOSLA"
>IOSLA</A
> s)</TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Arrow-ArrowState.html#t%3AArrowState"
>ArrowState</A
> s (<A HREF="Control-Arrow-StateListArrow.html#t%3ASLA"
>SLA</A
> s)</TD
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