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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="../Issues/iss186_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss187.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss188_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue HASH-TABLE-KEY-MODIFICATION Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss187.htm">HASH-TABLE-KEY-MODIFICATION</A><P>
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<B>References:</B> CLtL page 282, page 168 (last paragraph in 10.2)<P>
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X3J13 Proposal <A HREF="iss143.htm">EQUAL-STRUCTURE:MAYBE-STATUS-QUO</A> (passed 6/89)<P>
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<B>Category:</B> CLARIFICATION<P>
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<B>Edit history:</B> Version 1, 12-Sep-88, Moon, for discussion<P>
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Version 2, 05-Feb-90, by Kim A. Barrett,<P>
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rewrite with more explicit constraints<P>
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Version 3, 16-Feb-90, by Kim Barrett, KMP (respond to comments from Moon)<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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CLtL is silent about what happens if you modify a component of an object that<P>
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is used as a <A REL=DEFINITION HREF="../Body/t_hash_t.htm#hash-table"><B>hash-table</B></A> key.<P>
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<P>
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<B>Proposal (HASH-TABLE-KEY-MODIFICATION:SPECIFY):<P>
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</B><P>
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Define that the function supplied as the :TEST argument to <A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>MAKE-HASH-TABLE</B></A><P>
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specifies the `equivalence test' for the new hash table.<P>
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<P>
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Define that an object is `visibly modified' with regard to an equivalence test<P>
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if there exists some set of objects (or potential objects) which are equivalent<P>
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to the object before the modification but are no longer equivalent afterwards.<P>
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<P>
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If an object is used as a key in a hash table and is then visibly modified<P>
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with regard to the equivalence test of the hash table, then using the object<P>
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as a key in further operations on the hash table has unspecified consequences.<P>
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Moreover, this applies for other objects which either are or were equivalent<P>
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to the key object. Further, undoing the modification does not remove this<P>
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effect.<P>
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<P>
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Following are specifications of the modifications which are visible to the<P>
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equivalence tests which must be supported by hash tables. The modifications<P>
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are described in terms of modification of components, and are defined<P>
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recursively. Visible modifications of components of the object are visible<P>
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modifications of the object.<P>
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<P>
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1. <A REL=DEFINITION HREF="../Body/f_eq.htm#eq"><B>EQ</B></A> and <A REL=DEFINITION HREF="../Body/a_eql.htm#eql"><B>EQL</B></A><P>
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Common Lisp does not specify any <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> for visibly modifying any data type<P>
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with regard to either of these equivalence tests.<P>
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<P>
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2. <A REL=DEFINITION HREF="../Body/f_equal.htm#equal"><B>EQUAL</B></A><P>
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As a consequence of the behavior for <A REL=DEFINITION HREF="../Body/f_equal.htm#equal"><B>EQUAL</B></A> specified by Issue<P>
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<A HREF="iss143.htm">EQUAL-STRUCTURE</A>, the specification for this case reverts to the previous<P>
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description for <A REL=DEFINITION HREF="../Body/f_eq.htm#eq"><B>EQ</B></A> and <A REL=DEFINITION HREF="../Body/a_eql.htm#eql"><B>EQL</B></A> for many data types. The exceptions are<P>
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<P>
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a. <A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>CONS</B></A><P>
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The <A REL=DEFINITION HREF="../Body/f_car_c.htm#car"><B>car</B></A> and <A REL=DEFINITION HREF="../Body/f_car_c.htm#cdr"><B>cdr</B></A>.<P>
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<P>
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b. <A REL=DEFINITION HREF="../Body/t_bt_vec.htm#bit-vector"><B>BIT-VECTOR</B></A><P>
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Elements of the vector, limited by the <A REL=DEFINITION HREF="../Body/f_fill_p.htm#fill-pointer"><B>fill-pointer</B></A> if present.<P>
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The length of the vector, if adjustable or a <A REL=DEFINITION HREF="../Body/f_fill_p.htm#fill-pointer"><B>fill-pointer</B></A> is present.<P>
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<P>
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c. <A REL=DEFINITION HREF="../Body/a_string.htm#string"><B>STRING</B></A><P>
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Same as for <A REL=DEFINITION HREF="../Body/t_bt_vec.htm#bit-vector"><B>BIT-VECTOR</B></A>.<P>
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<P>
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d. <A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>PATHNAME</B></A><P>
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Common Lisp does not specify any <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> for visibly modifying a <A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>PATHNAME</B></A><P>
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with regard to <A REL=DEFINITION HREF="../Body/f_equal.htm#equal"><B>EQUAL</B></A>.<P>
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<P>
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3. <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A><P>
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As a consequence of the behavior for <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A> specified by Issue<P>
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<A HREF="iss143.htm">EQUAL-STRUCTURE</A>, the specification for this case reverts to the previous<P>
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description for <A REL=DEFINITION HREF="../Body/f_equal.htm#equal"><B>EQUAL</B></A> for many data types. The exceptions are<P>
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<P>
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a. <A REL=DEFINITION HREF="../Body/t_number.htm#number"><B>NUMBER</B></A><P>
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Common Lisp does not specify any <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> for visibly modifying a <A REL=DEFINITION HREF="../Body/t_number.htm#number"><B>NUMBER</B></A><P>
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with regard to <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A>.<P>
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<P>
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b. <A REL=DEFINITION HREF="../Body/a_ch.htm#character"><B>CHARACTER</B></A><P>
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Common Lisp does not specify any <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> for visibly modifying a <A REL=DEFINITION HREF="../Body/a_ch.htm#character"><B>CHARACTER</B></A><P>
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with regard to <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A>.<P>
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<P>
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c. <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>STRUCTURE</B></A><P>
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The values of the slots.<P>
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<P>
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d. <A REL=DEFINITION HREF="../Body/a_vector.htm#vector"><B>VECTOR</B></A><P>
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Elements of the vector, limited by the <A REL=DEFINITION HREF="../Body/f_fill_p.htm#fill-pointer"><B>fill-pointer</B></A> if present.<P>
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The length of the vector, if adjustable or a <A REL=DEFINITION HREF="../Body/f_fill_p.htm#fill-pointer"><B>fill-pointer</B></A> is present.<P>
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<P>
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e. <A REL=DEFINITION HREF="../Body/t_array.htm#array"><B>ARRAY</B></A><P>
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Elements of the array, limited by the <A REL=DEFINITION HREF="../Body/f_fill_p.htm#fill-pointer"><B>fill-pointer</B></A> if present.<P>
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The value of the <A REL=DEFINITION HREF="../Body/f_fill_p.htm#fill-pointer"><B>fill-pointer</B></A>, if present.<P>
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The dimensions, if adjustable.<P>
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<P>
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f. <A REL=DEFINITION HREF="../Body/t_hash_t.htm#hash-table"><B>HASH-TABLE</B></A><P>
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The count of entries in the table.<P>
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The keys. Note that the visibility of modifications to the keys depends<P>
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on the equivalence test of the hash table, not on the specification of<P>
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<A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A>. <P>
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The values associated with the keys.<P>
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<P>
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Implementations which extend the language by providing additional mutator<P>
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functions (or additional behavior for existing mutator functions) must<P>
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document how the use of these extensions interacts with equivalence tests and<P>
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hash table searches.<P>
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<P>
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Implementations which extend the language by defining additional acceptable<P>
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equivalence tests for hash tables (allowing additional values for the :TEST<P>
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argument to <A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>MAKE-HASH-TABLE</B></A>) must document the visible components of these<P>
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tests.<P>
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<P>
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<B>Test Cases/Examples:<P>
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</B><P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> ht (<A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>make-hash-table</B></A> :test #'<A REL=DEFINITION HREF="../Body/f_eq.htm#eq"><B>eq</B></A>))<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> a (<A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>cons</B></A> 1 2))<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> a ht) 'win)<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_car_c.htm#cdr"><B>cdr</B></A> a) 3)<P>
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(<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> a ht 'lose) => win t<P>
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<P>
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The same example with :test #'equal in the first line would be an error.<P>
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<P>
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The following example is not an error, because <A REL=DEFINITION HREF="../Body/f_equal.htm#equal"><B>EQUAL</B></A> does not examine the<P>
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components of general vectors:<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> ht (<A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>make-hash-table</B></A> :test #'<A REL=DEFINITION HREF="../Body/f_equal.htm#equal"><B>equal</B></A>))<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> a (<A REL=DEFINITION HREF="../Body/a_vector.htm#vector"><B>vector</B></A> 1 2))<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> a ht) 'win)<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_aref.htm#aref"><B>aref</B></A> a 1) 3)<P>
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(<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> a ht 'lose) => win t<P>
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<P>
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The following example is not an error, because <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A> is limited by the<P>
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<A REL=DEFINITION HREF="../Body/f_fill_p.htm#fill-pointer"><B>fill-pointer</B></A> when examining the elements of a vector:<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> ht (<A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>make-hash-table</B></A> :test #'<A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>equalp</B></A>))<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> a (<A REL=DEFINITION HREF="../Body/f_mk_ar.htm#make-array"><B>make-array</B></A> 3 :fill-pointer 2 :initial-contents '(1 2 3)))<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> a ht) 'win)<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_aref.htm#aref"><B>aref</B></A> a 2) 4)<P>
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(<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> a ht 'lose) => win t<P>
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<P>
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The following example is an error, because <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A> may examine the dimensions<P>
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of arrays:<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> ht (<A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>make-hash-table</B></A> :test #'<A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>equalp</B></A>))<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> a (<A REL=DEFINITION HREF="../Body/f_mk_ar.htm#make-array"><B>make-array</B></A> '(2 3) :adjustable t))<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> a ht) 'win)<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> a (<A REL=DEFINITION HREF="../Body/f_adjust.htm#adjust-array"><B>adjust-array</B></A> a '(3 2)))<P>
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(<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> a ht 'lose) => `unspecified'<P>
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<P>
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The following example is not an error, because <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A> is case insensitive:<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> ht (<A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>make-hash-table</B></A> :test #'<A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>equalp</B></A>))<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> a "ABC")<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> a ht) 'win)<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_char_.htm#char"><B>char</B></A> a 0) #\a)<P>
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(<A REL=DEFINITION HREF="../Body/f_gethas.htm#gethash"><B>gethash</B></A> a ht 'lose) => win t<P>
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<P>
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The same example with :test #'equal in the first line would be an error.<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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<A REL=DEFINITION HREF="../Body/f_eq.htm#eq"><B>EQ</B></A> and <A REL=DEFINITION HREF="../Body/a_eql.htm#eql"><B>EQL</B></A> hash tables use the identity of the object as the key, while <A REL=DEFINITION HREF="../Body/f_equal.htm#equal"><B>EQUAL</B></A><P>
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and <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A> hash tables use the <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> of the object as the key. Component<P>
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modification changes the <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> of an object, while the identity of an<P>
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object cannot be changed in Common Lisp.<P>
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<P>
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Specifying `unspecified consequences' provides implementation freedom, while<P>
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still providing guidance to users.<P>
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<P>
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This is a generalization of the warning on p.168 of CLtL.<P>
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<P>
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Note that this proposal implies that it is invalid to use an overly general<P>
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hash function, such as <A REL=DEFINITION HREF="../Body/f_sxhash.htm#sxhash"><B>SXHASH</B></A>, as the hash function for <A REL=DEFINITION HREF="../Body/f_eq.htm#eq"><B>EQ</B></A> or <A REL=DEFINITION HREF="../Body/a_eql.htm#eql"><B>EQL</B></A> hash tables.<P>
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The value of <A REL=DEFINITION HREF="../Body/f_sxhash.htm#sxhash"><B>SXHASH</B></A> can be affected by component modifications, and this is<P>
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likely to cause hash table entries to become inaccessible. The general rule<P>
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for this is that a hash function must not depend on the value of some<P>
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property of an object if modification of that property is not visible to the<P>
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associated equivalence function.<P>
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<P>
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The documentation requirements for extensions seems like the minimal necessary<P>
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thing, assuming we want to talk about extensions at all.<P>
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<P>
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<B>Current practice:<P>
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</B><P>
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I am not aware of any implementations that do not conform to the proposal.<P>
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<P>
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Some implementations which <A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> any of the described extensions may not<P>
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conform to the documentation requirement.<P>
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<P>
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<B>Cost to Implementors:<P>
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</B><P>
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Most implementations probably already conform. It is possible that some<P>
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implementations might have to use a different hash function in their<P>
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implementation of some hash tables in order to conform.<P>
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<P>
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<B>Cost to Users:<P>
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</B><P>
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None.<P>
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<P>
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<B>Cost of non-adoption:<P>
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</B><P>
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Users would not be sure whether they were allowed to perform side effects on<P>
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objects that might have been used as keys of hash tables.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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More specific language semantics.<P>
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<P>
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<B>Esthetics:<P>
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</B> <P>
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More specific language semantics.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Version 1<P>
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Discussion on the Common-Lisp mailing list was in favor of this.<P>
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<P>
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Version 2<P>
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Barrett:<P>
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The two paragraphs about language extensions could be stricken without really<P>
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affecting the proposal.<P>
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<P>
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This proposal does not deal with any of the performance issues addressed by<P>
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Issue HASH-TABLE-STABILITY. Rather, it provides guidance to users and<P>
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implementors as to the requirements for conforming programs.<P>
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<P>
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`Unspecified consequences' could be changed to 'undefined consequences',<P>
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though it seems unlikely that this is actually a 'crash and burn' situation.<P>
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(The program which was depending on <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>the</B></A> result of <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>the</B></A> operation may exhibit<P>
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undefined consequences as a result of <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>the</B></A> <A REL=DEFINITION HREF="../Body/t_hash_t.htm#hash-table"><B>hash-table</B></A> operation's having<P>
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unspecified consequences, but that's <A REL=DEFINITION HREF="../Body/a_not.htm#not"><B>not</B></A> <A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>the</B></A> same thing.)<P>
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<P>
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Moon:<P>
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Re: Define that an object is 'visibly modified' with regard to an<P>
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equivalence test if there exists some <A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A> of objects which were <P>
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equivalent to the object before the modification but are no longer <P>
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equivalent afterwards.<P>
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Except for ... the use of "class" to mean "set" rather than the<P>
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usual CLOS meaning, it looks okay. Actually where it says "some<P>
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<A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A> of objects", a single object would be sufficient. X is<P>
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visibly modified if there exists any Y such that (<A REL=DEFINITION HREF="../Body/f_funcal.htm#funcall"><B>funcall</B></A> test X Y)<P>
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produces a different result, provided (this is missing from Kim's<P>
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writeup) Y has not been modified [whatever that means exactly].<P>
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<P>
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Pitman and Barrett support this proposal (v3).<P>
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</PRE>
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