<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> <HTML> <HEAD> <META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.6"> <TITLE>The Hugs-GHC Extension Libraries: Stable: Stable Names</TITLE> <LINK HREF="libs-24.html" REL=next> <LINK HREF="libs-22.html" REL=previous> <LINK HREF="libs.html#toc14" REL=contents> </HEAD> <BODY> <A HREF="libs-24.html">Next</A> <A HREF="libs-22.html">Previous</A> <A HREF="libs.html#toc14">Contents</A> <HR> <H2><A NAME="sec:stable-pointers"></A> 14.2 Stable Names</H2> <P> <P>A haskell object can be given a <EM>stable name</EM> by calling <CODE>makeStableName</CODE> on it. Stable names solve the following problem: suppose you want to build a hash table with Haskell objects as keys, but you want to use pointer equality for comparison; maybe because the keys are large and hashing would be slow, or perhaps because the keys are infinite in size. We can't build a hash table using the address of the object as the key, because objects get moved around by the garbage collector, meaning a re-hash would be necessary after every garbage collection. <P>Enter stable names. A stable name is an abstract entity that supports equality and hashing, with the following interface: <P> <BLOCKQUOTE><CODE> <PRE> data StableName a -- abstract, instance Eq. makeStableName :: a -> IO (StableName a) hashStableName :: StableName a -> Int </PRE> </CODE></BLOCKQUOTE> <P>All these operations run in constant time. <P>Stable names have the following properties: <P> <OL> <LI> If <CODE>sn1 :: StablePtr</CODE> and <CODE>sn2 :: StablePtr</CODE> and <CODE>sn1 == sn2</CODE> then <CODE>sn1</CODE> and <CODE>sn2</CODE> are either the same stable name, or they were created by calls to <CODE>makeStableName</CODE> on the same object.</LI> <LI> The reverse is not necessarily true: if two stable names are not equal, it doesn't mean that they don't refer to the same Haskell object (although they probably don't).</LI> <LI> There is no <CODE>freeStableName</CODE> operation. Stable names are reclaimed by the runtime system when they are no longer needed.</LI> <LI> There is no <CODE>deRefStableName</CODE> operation. You can't get back from a stable name to the original Haskell object. The reason for this is that the existence of a stable name for an object doesn't guarantee the existence of the object itself; it can still be garbage collected. </LI> <LI> There is a <CODE>hashStableName</CODE> operation, which converts a stable name to an <CODE>Int</CODE>. The <CODE>Int</CODE> returned is not necessarily unique (that is, it doesn't satisfy property (1) above), but it can be used for building hash tables of stable names.</LI> </OL> <P>Properties (1) and (2) are similar to stable pointers, but the key differences are that you can't get back to the original object from a stable name, and you can convert one to an <CODE>Int</CODE> for hashing. <P> <HR> <A HREF="libs-24.html">Next</A> <A HREF="libs-22.html">Previous</A> <A HREF="libs.html#toc14">Contents</A> </BODY> </HTML>
