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>Data.Dynamic</TITLE
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>base-4.1.0.0: Basic libraries</TD
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>Contents</A
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>Data.Dynamic</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"
>libraries@haskell.org</TD
></TR
></TABLE
></TD
></TR
></TABLE
></TD
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><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
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><TD CLASS="section4"
><B
>Contents</B
></TD
></TR
><TR
><TD
><DL
><DT
><A HREF="#1"
>The <TT
>Dynamic</TT
> type
</A
></DT
><DT
><A HREF="#2"
>Converting to and from <TT
>Dynamic</TT
>
</A
></DT
><DT
><A HREF="#3"
>Applying functions of dynamic type
</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
>The Dynamic interface provides basic support for dynamic types.
</P
><P
>Operations for injecting values of arbitrary type into
 a dynamically typed value, Dynamic, are provided, together
 with operations for converting dynamic values into a concrete
 (monomorphic) type.
</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="Data-Typeable.html"
>Data.Typeable</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
>  <A HREF="#t%3ADynamic"
>Dynamic</A
> </TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AtoDyn"
>toDyn</A
> :: <A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> a =&gt; a -&gt; <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AfromDyn"
>fromDyn</A
> :: <A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> a =&gt; <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
> -&gt; a -&gt; a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AfromDynamic"
>fromDynamic</A
> :: <A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> a =&gt; <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
> -&gt; <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AdynApply"
>dynApply</A
> :: <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
> -&gt; <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
> -&gt; <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AdynApp"
>dynApp</A
> :: <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
> -&gt; <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
> -&gt; <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AdynTypeRep"
>dynTypeRep</A
> :: <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
> -&gt; <A HREF="Data-Typeable.html#t%3ATypeRep"
>TypeRep</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="Data-Typeable.html"
>Data.Typeable</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="1"
><A NAME="1"
>The <TT
>Dynamic</TT
> type
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><SPAN CLASS="keyword"
>data</SPAN
>  <A NAME="t:Dynamic"
><A NAME="t%3ADynamic"
></A
></A
><B
>Dynamic</B
>  </TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="ndoc"
><P
>A value of type <TT
><A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TT
> is an object encapsulated together with its type.
</P
><P
>A <TT
><A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TT
> may only represent a monomorphic value; an attempt to
  create a value of type <TT
><A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TT
> from a polymorphically-typed
  expression will result in an ambiguity error (see <TT
><A HREF="Data-Dynamic.html#v%3AtoDyn"
>toDyn</A
></TT
>).
</P
><P
><TT
><A HREF="Text-Show.html#t%3AShow"
>Show</A
></TT
>ing a value of type <TT
><A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TT
> returns a pretty-printed representation
  of the object's type; useful for debugging.
</P
></TD
></TR
><TR
><TD CLASS="section4"
><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:Dynamic')" ALT="show/hide"
> Instances</TD
></TR
><TR
><TD CLASS="body"
><DIV ID="i:Dynamic" STYLE="display:block;"
><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
><TR
><TD CLASS="decl"
><A HREF="Text-Show.html#t%3AShow"
>Show</A
> <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="Control-Exception-Base.html#t%3AException"
>Exception</A
> <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="2"
><A NAME="2"
>Converting to and from <TT
>Dynamic</TT
>
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:toDyn"
><A NAME="v%3AtoDyn"
></A
></A
><B
>toDyn</B
> :: <A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> a =&gt; a -&gt; <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
></TR
><TR
><TD CLASS="doc"
><P
>Converts an arbitrary value into an object of type <TT
><A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TT
>.  
</P
><P
>The type of the object must be an instance of <TT
><A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
></TT
>, which
 ensures that only monomorphically-typed objects may be converted to
 <TT
><A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TT
>.  To convert a polymorphic object into <TT
><A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TT
>, give it
 a monomorphic type signature.  For example:
</P
><PRE
>    toDyn (id :: Int -&gt; Int)
</PRE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:fromDyn"
><A NAME="v%3AfromDyn"
></A
></A
><B
>fromDyn</B
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="arg"
>:: <A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> a</TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="arg"
>=&gt; <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
><TD CLASS="rdoc"
>the dynamically-typed object
</TD
></TR
><TR
><TD CLASS="arg"
>-&gt; a</TD
><TD CLASS="rdoc"
>a default value 
</TD
></TR
><TR
><TD CLASS="arg"
>-&gt; a</TD
><TD CLASS="rdoc"
>returns: the value of the first argument, if
 it has the correct type, otherwise the value of
 the second argument.
</TD
></TR
><TR
><TD CLASS="ndoc" COLSPAN="2"
>Converts a <TT
><A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TT
> object back into an ordinary Haskell value of
 the correct type.  See also <TT
><A HREF="Data-Dynamic.html#v%3AfromDynamic"
>fromDynamic</A
></TT
>.
</TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:fromDynamic"
><A NAME="v%3AfromDynamic"
></A
></A
><B
>fromDynamic</B
></TD
></TR
><TR
><TD CLASS="body"
><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
><TR
><TD CLASS="arg"
>:: <A HREF="Data-Typeable.html#t%3ATypeable"
>Typeable</A
> a</TD
><TD CLASS="rdoc"
></TD
></TR
><TR
><TD CLASS="arg"
>=&gt; <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
><TD CLASS="rdoc"
>the dynamically-typed object
</TD
></TR
><TR
><TD CLASS="arg"
>-&gt; <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> a</TD
><TD CLASS="rdoc"
>returns: <TT
><TT
><A HREF="Data-Maybe.html#v%3AJust"
>Just</A
></TT
> a</TT
>, if the dynamically-typed
 object has the correct type (and <TT
>a</TT
> is its value), 
 or <TT
><A HREF="Data-Maybe.html#v%3ANothing"
>Nothing</A
></TT
> otherwise.
</TD
></TR
><TR
><TD CLASS="ndoc" COLSPAN="2"
>Converts a <TT
><A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TT
> object back into an ordinary Haskell value of
 the correct type.  See also <TT
><A HREF="Data-Dynamic.html#v%3AfromDyn"
>fromDyn</A
></TT
>.
</TD
></TR
></TABLE
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="section1"
><A NAME="3"
><A NAME="3"
>Applying functions of dynamic type
</A
></A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:dynApply"
><A NAME="v%3AdynApply"
></A
></A
><B
>dynApply</B
> :: <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
> -&gt; <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
> -&gt; <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:dynApp"
><A NAME="v%3AdynApp"
></A
></A
><B
>dynApp</B
> :: <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
> -&gt; <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
> -&gt; <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="decl"
><A NAME="v:dynTypeRep"
><A NAME="v%3AdynTypeRep"
></A
></A
><B
>dynTypeRep</B
> :: <A HREF="Data-Dynamic.html#t%3ADynamic"
>Dynamic</A
> -&gt; <A HREF="Data-Typeable.html#t%3ATypeRep"
>TypeRep</A
></TD
></TR
><TR
><TD CLASS="s15"
></TD
></TR
><TR
><TD CLASS="botbar"
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