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      <title>Intermediate hash tables</title>
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      <h1>SRFI 125: Intermediate hash tables</h1>
      <p class="authors">by John Cowan and Will Clinger</p>
      <p class="based-on"></p>
      <p class="status">status: <em>final</em> (2016-05-28)</p>
      <p class="keywords">keywords: <a href="https://srfi.schemers.org/?keywords=data-structure">Data Structure</a>, <a href="https://srfi.schemers.org/?keywords=r7rs-large">R7RS Large</a>, <a href="https://srfi.schemers.org/?keywords=r7rs-large-red">R7RS Large: Red Edition</a></p><span class="see-also">See also <a href="/srfi-69/">SRFI 69: Basic hash tables</a> and <a href="/srfi-126/">SRFI 126: R6RS-based hashtables</a>.</span>
      <p class="library-name">library name: hashtables</p>
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      <h2>Abstract</h2>      <p>This SRFI defines an interface to hash tables,       which are widely recognized as a fundamental data structure for a wide       variety of applications. A hash table is a data structure that:       <ul>         <li>Is disjoint from all other types.         <li>           Provides a mapping from objects known as           <em>keys</em> to corresponding objects known as           <em>values</em>.           <ul>             <li>Keys may be any Scheme objects in some             kinds of hash tables, but are restricted in other kinds.             <li>Values may be any Scheme objects.           </ul>         <li>Has no intrinsic order for the key-value         <em>associations</em> it contains.         <li>Provides an <em>equality predicate</em>         which defines when a proposed key is the same as an existing key. No         table may contain more than one value for a given key.         <li>Provides a <em>hash function</em> which maps         a candidate key into a non-negative exact integer.         <li>Supports mutation as the primary means of         setting the contents of a table.         <li>Provides key lookup and destructive update         in (expected) amortized constant time, provided a satisfactory hash         function is available.         <li>Does not guarantee that whole-table         operations work in the presence of concurrent mutation of the whole         hash table (values may be safely mutated).       </ul></body></html>