hxt-8.2.0: doc/hdoc/src/Data-Atom.html
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<pre><a name="line-1"></a><span class='comment'>-- ------------------------------------------------------------</span>
<a name="line-2"></a>
<a name="line-3"></a><span class='comment'>{- |
<a name="line-4"></a> Module : Data.Atom
<a name="line-5"></a> Copyright : Copyright (C) 2008 Uwe Schmidt
<a name="line-6"></a> License : MIT
<a name="line-7"></a>
<a name="line-8"></a> Maintainer : Uwe Schmidt (uwe\@fh-wedel.de)
<a name="line-9"></a> Stability : experimental
<a name="line-10"></a> Portability: non-portable
<a name="line-11"></a>
<a name="line-12"></a> Unique Atoms generated from Strings and
<a name="line-13"></a> managed as flyweights
<a name="line-14"></a>
<a name="line-15"></a> Data.Atom can be used for caching and storage optimisation
<a name="line-16"></a> of frequently used strings. An @Atom@ is constructed from a @String@.
<a name="line-17"></a> For two equal strings the identical atom is returned.
<a name="line-18"></a>
<a name="line-19"></a> This module can be used for optimizing memory usage when working with
<a name="line-20"></a> strings or names. Many applications use data types like
<a name="line-21"></a> @Map String SomeAttribute@ where a rather fixed set of keys is used.
<a name="line-22"></a> Especially XML applications often work with a limited set of element and attribute names.
<a name="line-23"></a> For these applications it becomes more memory efficient when working with types like
<a name="line-24"></a> @Map Atom SomeAttribute@ and convert the keys into atoms before operating
<a name="line-25"></a> on such a map.
<a name="line-26"></a>
<a name="line-27"></a> Internally this module manages a map of atoms. The atoms are internally represented
<a name="line-28"></a> by @ByteString@s. When creating a new atom from a string, the string is first converted
<a name="line-29"></a> into an UTF8 @Word8@ sequence, which is packed into a @ByteString@. This @ByteString@ is looked
<a name="line-30"></a> up in the table of atoms. If it is already there, the value in the map is used as atom, else
<a name="line-31"></a> the new @ByteString@ is inserted into the map.
<a name="line-32"></a>
<a name="line-33"></a> Of course the implementation of this name cache uses @unsavePerformIO@ and @MVar@s
<a name="line-34"></a> for managing this kind of global state.
<a name="line-35"></a>
<a name="line-36"></a> The following laws hold for atoms
<a name="line-37"></a>
<a name="line-38"></a> >
<a name="line-39"></a> > s == t => newAtom s == newAtom t
<a name="line-40"></a> > s `compare` t => newAtom s `compare` newAtom t
<a name="line-41"></a> > show . newAtom == id
<a name="line-42"></a>
<a name="line-43"></a> Equality test for @Atom@s runs in /O(1)/, it is just a pointer comarison.
<a name="line-44"></a> The @Ord@ comparisons have the same runtime like the @ByteString@ comparisons.
<a name="line-45"></a> Internally there is an UTF8 comparison, but UTF8 encoding preserves the total order.
<a name="line-46"></a>
<a name="line-47"></a> Warning: The internal cache never shrinks during execution. So using it in a
<a name="line-48"></a> undisciplined way can lead to memory leaks.
<a name="line-49"></a>-}</span>
<a name="line-50"></a>
<a name="line-51"></a><span class='comment'>-----------------------------------------------------------------------------</span>
<a name="line-52"></a>
<a name="line-53"></a><span class='keyword'>module</span> <span class='conid'>Data</span><span class='varop'>.</span><span class='conid'>Atom</span> <span class='layout'>(</span>
<a name="line-54"></a> <span class='comment'>-- * Atom objects</span>
<a name="line-55"></a> <span class='conid'>Atom</span><span class='layout'>,</span> <span class='comment'>-- instance (Eq, Ord, Read, Show)</span>
<a name="line-56"></a> <span class='varid'>newAtom</span><span class='layout'>,</span> <span class='comment'>-- :: String -> Atom</span>
<a name="line-57"></a> <span class='varid'>share</span> <span class='comment'>-- :: String -> String</span>
<a name="line-58"></a> <span class='layout'>)</span> <span class='keyword'>where</span>
<a name="line-59"></a>
<a name="line-60"></a><span class='keyword'>import</span> <span class='conid'>Control</span><span class='varop'>.</span><span class='conid'>Concurrent</span><span class='varop'>.</span><span class='conid'>MVar</span>
<a name="line-61"></a><span class='keyword'>import</span> <span class='conid'>Control</span><span class='varop'>.</span><span class='conid'>Parallel</span><span class='varop'>.</span><span class='conid'>Strategies</span>
<a name="line-62"></a>
<a name="line-63"></a><span class='keyword'>import</span> <span class='conid'>Data</span><span class='varop'>.</span><span class='conid'>ByteString</span><span class='varop'>.</span><span class='conid'>Internal</span> <span class='layout'>(</span> <span class='varid'>toForeignPtr</span><span class='layout'>,</span> <span class='varid'>c2w</span><span class='layout'>,</span> <span class='varid'>w2c</span> <span class='layout'>)</span>
<a name="line-64"></a><span class='keyword'>import</span> <span class='conid'>Data</span><span class='varop'>.</span><span class='conid'>ByteString</span> <span class='layout'>(</span> <span class='conid'>ByteString</span><span class='layout'>,</span> <span class='varid'>pack</span><span class='layout'>,</span> <span class='varid'>unpack</span> <span class='layout'>)</span>
<a name="line-65"></a><span class='keyword'>import</span> <span class='keyword'>qualified</span> <span class='conid'>Data</span><span class='varop'>.</span><span class='conid'>Map</span> <span class='keyword'>as</span> <span class='conid'>M</span>
<a name="line-66"></a><span class='keyword'>import</span> <span class='conid'>Data</span><span class='varop'>.</span><span class='conid'>Typeable</span>
<a name="line-67"></a>
<a name="line-68"></a><span class='keyword'>import</span> <span class='conid'>System</span><span class='varop'>.</span><span class='conid'>IO</span><span class='varop'>.</span><span class='conid'>Unsafe</span> <span class='layout'>(</span> <span class='varid'>unsafePerformIO</span> <span class='layout'>)</span>
<a name="line-69"></a>
<a name="line-70"></a><span class='keyword'>import</span> <span class='conid'>Text</span><span class='varop'>.</span><span class='conid'>XML</span><span class='varop'>.</span><span class='conid'>HXT</span><span class='varop'>.</span><span class='conid'>DOM</span><span class='varop'>.</span><span class='conid'>Unicode</span> <span class='layout'>(</span> <span class='varid'>unicodeToUtf8</span> <span class='layout'>)</span>
<a name="line-71"></a><span class='keyword'>import</span> <span class='conid'>Text</span><span class='varop'>.</span><span class='conid'>XML</span><span class='varop'>.</span><span class='conid'>HXT</span><span class='varop'>.</span><span class='conid'>DOM</span><span class='varop'>.</span><span class='conid'>UTF8Decoding</span> <span class='layout'>(</span> <span class='varid'>decodeUtf8</span> <span class='layout'>)</span>
<a name="line-72"></a>
<a name="line-73"></a><span class='comment'>-- ------------------------------------------------------------</span>
<a name="line-74"></a>
<a name="line-75"></a><a name="Atoms"></a><span class='keyword'>type</span> <span class='conid'>Atoms</span> <span class='keyglyph'>=</span> <span class='conid'>M</span><span class='varop'>.</span><span class='conid'>Map</span> <span class='conid'>ByteString</span> <span class='conid'>ByteString</span>
<a name="line-76"></a>
<a name="line-77"></a><a name="Atom"></a><span class='keyword'>newtype</span> <span class='conid'>Atom</span> <span class='keyglyph'>=</span> <span class='conid'>A</span> <span class='layout'>{</span> <span class='varid'>bs</span> <span class='keyglyph'>::</span> <span class='conid'>ByteString</span> <span class='layout'>}</span>
<a name="line-78"></a> <span class='keyword'>deriving</span> <span class='layout'>(</span><span class='conid'>Typeable</span><span class='layout'>)</span>
<a name="line-79"></a>
<a name="line-80"></a><span class='comment'>-- ------------------------------------------------------------</span>
<a name="line-81"></a>
<a name="line-82"></a><span class='comment'>-- | the internal cache for the strings</span>
<a name="line-83"></a>
<a name="line-84"></a><a name="theAtoms"></a><span class='definition'>theAtoms</span> <span class='keyglyph'>::</span> <span class='conid'>MVar</span> <span class='conid'>Atoms</span>
<a name="line-85"></a><span class='definition'>theAtoms</span> <span class='keyglyph'>=</span> <span class='varid'>unsafePerformIO</span> <span class='layout'>(</span><span class='varid'>newMVar</span> <span class='conid'>M</span><span class='varop'>.</span><span class='varid'>empty</span><span class='layout'>)</span>
<a name="line-86"></a><span class='comment'>{-# NOINLINE theAtoms #-}</span>
<a name="line-87"></a>
<a name="line-88"></a><span class='comment'>-- | insert a bytestring into the atom cache</span>
<a name="line-89"></a>
<a name="line-90"></a><a name="insertAtom"></a><span class='definition'>insertAtom</span> <span class='keyglyph'>::</span> <span class='conid'>ByteString</span> <span class='keyglyph'>-></span> <span class='conid'>Atoms</span> <span class='keyglyph'>-></span> <span class='layout'>(</span><span class='conid'>Atoms</span><span class='layout'>,</span> <span class='conid'>Atom</span><span class='layout'>)</span>
<a name="line-91"></a><span class='definition'>insertAtom</span> <span class='varid'>s</span> <span class='varid'>m</span> <span class='keyglyph'>=</span> <span class='varid'>maybe</span> <span class='layout'>(</span><span class='conid'>M</span><span class='varop'>.</span><span class='varid'>insert</span> <span class='varid'>s</span> <span class='varid'>s</span> <span class='varid'>m</span><span class='layout'>,</span> <span class='conid'>A</span> <span class='varid'>s</span><span class='layout'>)</span>
<a name="line-92"></a> <span class='layout'>(</span><span class='keyglyph'>\</span> <span class='varid'>s'</span> <span class='keyglyph'>-></span> <span class='layout'>(</span><span class='varid'>m</span><span class='layout'>,</span> <span class='conid'>A</span> <span class='varid'>s'</span><span class='layout'>)</span><span class='layout'>)</span>
<a name="line-93"></a> <span class='varop'>.</span>
<a name="line-94"></a> <span class='conid'>M</span><span class='varop'>.</span><span class='varid'>lookup</span> <span class='varid'>s</span> <span class='varop'>$</span> <span class='varid'>m</span>
<a name="line-95"></a>
<a name="line-96"></a><span class='comment'>-- | creation of an @Atom@ from a @String@</span>
<a name="line-97"></a>
<a name="line-98"></a><a name="newAtom"></a><span class='definition'>newAtom</span> <span class='keyglyph'>::</span> <span class='conid'>String</span> <span class='keyglyph'>-></span> <span class='conid'>Atom</span>
<a name="line-99"></a><span class='definition'>newAtom</span> <span class='keyglyph'>=</span> <span class='varid'>unsafePerformIO</span> <span class='varop'>.</span> <span class='varid'>newAtom'</span>
<a name="line-100"></a><span class='comment'>{-# NOINLINE newAtom #-}</span>
<a name="line-101"></a>
<a name="line-102"></a><a name="newAtom'"></a><span class='comment'>-- | The internal operation running in the IO monad</span>
<a name="line-103"></a><span class='definition'>newAtom'</span> <span class='keyglyph'>::</span> <span class='conid'>String</span> <span class='keyglyph'>-></span> <span class='conid'>IO</span> <span class='conid'>Atom</span>
<a name="line-104"></a><span class='definition'>newAtom'</span> <span class='varid'>s</span> <span class='keyglyph'>=</span> <span class='keyword'>do</span>
<a name="line-105"></a> <span class='varid'>m</span> <span class='keyglyph'><-</span> <span class='varid'>takeMVar</span> <span class='varid'>theAtoms</span>
<a name="line-106"></a> <span class='keyword'>let</span> <span class='layout'>(</span><span class='varid'>m'</span><span class='layout'>,</span> <span class='varid'>a</span><span class='layout'>)</span> <span class='keyglyph'>=</span> <span class='varid'>insertAtom</span> <span class='layout'>(</span><span class='varid'>pack</span><span class='varop'>.</span> <span class='varid'>map</span> <span class='varid'>c2w</span> <span class='varop'>.</span> <span class='varid'>unicodeToUtf8</span> <span class='varop'>$</span> <span class='varid'>s</span><span class='layout'>)</span> <span class='varid'>m</span>
<a name="line-107"></a> <span class='varid'>putMVar</span> <span class='varid'>theAtoms</span> <span class='varid'>m'</span>
<a name="line-108"></a> <span class='varid'>return</span> <span class='varid'>a</span>
<a name="line-109"></a>
<a name="line-110"></a><span class='comment'>-- | Insert a @String@ into the atom cache and convert the atom back into a @String@.</span>
<a name="line-111"></a><span class='comment'>--</span>
<a name="line-112"></a><span class='comment'>-- locically @share == id@ holds, but internally equal strings share the same memory.</span>
<a name="line-113"></a>
<a name="line-114"></a><a name="share"></a><span class='definition'>share</span> <span class='keyglyph'>::</span> <span class='conid'>String</span> <span class='keyglyph'>-></span> <span class='conid'>String</span>
<a name="line-115"></a><span class='definition'>share</span> <span class='keyglyph'>=</span> <span class='varid'>show</span> <span class='varop'>.</span> <span class='varid'>newAtom</span>
<a name="line-116"></a>
<a name="line-117"></a><span class='keyword'>instance</span> <span class='conid'>Eq</span> <span class='conid'>Atom</span> <span class='keyword'>where</span>
<a name="line-118"></a> <span class='varid'>a1</span> <span class='varop'>==</span> <span class='varid'>a2</span> <span class='keyglyph'>=</span> <span class='varid'>fp1</span> <span class='varop'>==</span> <span class='varid'>fp2</span>
<a name="line-119"></a> <span class='keyword'>where</span>
<a name="line-120"></a> <span class='layout'>(</span><span class='varid'>fp1</span><span class='layout'>,</span> <span class='keyword'>_</span><span class='layout'>,</span> <span class='keyword'>_</span><span class='layout'>)</span> <span class='keyglyph'>=</span> <span class='varid'>toForeignPtr</span> <span class='varop'>.</span> <span class='varid'>bs</span> <span class='varop'>$</span> <span class='varid'>a1</span>
<a name="line-121"></a> <span class='layout'>(</span><span class='varid'>fp2</span><span class='layout'>,</span> <span class='keyword'>_</span><span class='layout'>,</span> <span class='keyword'>_</span><span class='layout'>)</span> <span class='keyglyph'>=</span> <span class='varid'>toForeignPtr</span> <span class='varop'>.</span> <span class='varid'>bs</span> <span class='varop'>$</span> <span class='varid'>a2</span>
<a name="line-122"></a>
<a name="line-123"></a><span class='keyword'>instance</span> <span class='conid'>Ord</span> <span class='conid'>Atom</span> <span class='keyword'>where</span>
<a name="line-124"></a> <span class='varid'>compare</span> <span class='varid'>a1</span> <span class='varid'>a2</span>
<a name="line-125"></a> <span class='keyglyph'>|</span> <span class='varid'>a1</span> <span class='varop'>==</span> <span class='varid'>a2</span> <span class='keyglyph'>=</span> <span class='conid'>EQ</span>
<a name="line-126"></a> <span class='keyglyph'>|</span> <span class='varid'>otherwise</span> <span class='keyglyph'>=</span> <span class='varid'>compare</span> <span class='layout'>(</span><span class='varid'>bs</span> <span class='varid'>a1</span><span class='layout'>)</span> <span class='layout'>(</span><span class='varid'>bs</span> <span class='varid'>a2</span><span class='layout'>)</span>
<a name="line-127"></a>
<a name="line-128"></a><span class='keyword'>instance</span> <span class='conid'>Read</span> <span class='conid'>Atom</span> <span class='keyword'>where</span>
<a name="line-129"></a> <span class='varid'>readsPrec</span> <span class='varid'>p</span> <span class='varid'>str</span> <span class='keyglyph'>=</span> <span class='keyglyph'>[</span> <span class='layout'>(</span><span class='varid'>newAtom</span> <span class='varid'>x</span><span class='layout'>,</span> <span class='varid'>y</span><span class='layout'>)</span> <span class='keyglyph'>|</span> <span class='layout'>(</span><span class='varid'>x</span><span class='layout'>,</span> <span class='varid'>y</span><span class='layout'>)</span> <span class='keyglyph'><-</span> <span class='varid'>readsPrec</span> <span class='varid'>p</span> <span class='varid'>str</span> <span class='keyglyph'>]</span>
<a name="line-130"></a>
<a name="line-131"></a><span class='keyword'>instance</span> <span class='conid'>Show</span> <span class='conid'>Atom</span> <span class='keyword'>where</span>
<a name="line-132"></a> <span class='varid'>show</span> <span class='keyglyph'>=</span> <span class='varid'>fst</span> <span class='varop'>.</span> <span class='varid'>decodeUtf8</span> <span class='varop'>.</span> <span class='varid'>map</span> <span class='varid'>w2c</span> <span class='varop'>.</span> <span class='varid'>unpack</span> <span class='varop'>.</span> <span class='varid'>bs</span>
<a name="line-133"></a> <span class='comment'>-- show = show . toForeignPtr . bs -- for debug only</span>
<a name="line-134"></a>
<a name="line-135"></a><span class='keyword'>instance</span> <span class='conid'>NFData</span> <span class='conid'>Atom</span> <span class='keyword'>where</span>
<a name="line-136"></a>
<a name="line-137"></a><span class='comment'>-----------------------------------------------------------------------------</span>
</pre></body>
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