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<H2>Issue DISPLACED-ARRAY-PREDICATE Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss132.htm">DISPLACED-ARRAY-PREDICATE</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/f_mk_ar.htm#make-array"><B>MAKE-ARRAY</B></A><P>
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Related issues: <A HREF="iss003.htm">ADJUST-ARRAY-DISPLACEMENT</A><P>
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<B>Category:</B> ADDITION<P>
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<B>Edit history:</B> v1, 13 Feb 1991, Sandra Loosemore<P>
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v2, 11 Mar 1991, Sandra Loosemore<P>
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v3, 26 Mar 1991, Sandra Loosemore (amendment from meeting)<P>
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<B>Status:</B> v3 (proposal ADD) accepted by X3J13, Mar 91<P>
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<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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There is no way to determine whether an array is displaced to another<P>
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array. Having this information available would be helpful to some<P>
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applications that want to dump and restore array contents, either to <P>
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allow them to detect when sharing of displaced arrays is lost or to<P>
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reconstruct the arrays in such a way that sharing is preserved.<P>
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<P>
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<B>Proposal (DISPLACED-ARRAY-PREDICATE:ADD):<P>
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</B><P>
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Add a function:<P>
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<P>
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<A REL=DEFINITION HREF="../Body/f_ar_dis.htm#array-displacement"><B>ARRAY-DISPLACEMENT</B></A> <array> [Function]<P>
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<P>
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The <array> argument must be an array. If the array is a displaced<P>
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array, two values are returned: the array to which it is displaced<P>
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corresponding to the :DISPLACED-TO argument to <A REL=DEFINITION HREF="../Body/f_mk_ar.htm#make-array"><B>MAKE-ARRAY</B></A>, and an<P>
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integer corresponding to the :DISPLACED-INDEX-OFFSET argument to<P>
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<A REL=DEFINITION HREF="../Body/f_mk_ar.htm#make-array"><B>MAKE-ARRAY</B></A>. If the array is not a displaced array, this function<P>
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returns two values <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> and 0.<P>
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<P>
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If <A REL=DEFINITION HREF="../Body/f_ar_dis.htm#array-displacement"><B>ARRAY-DISPLACEMENT</B></A> is called on an array for which a non-NIL<P>
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object was provided as the :DISPLACED-TO argument to <A REL=DEFINITION HREF="../Body/f_mk_ar.htm#make-array"><B>MAKE-ARRAY</B></A> <P>
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or <A REL=DEFINITION HREF="../Body/f_adjust.htm#adjust-array"><B>ADJUST-ARRAY</B></A>, it must return that object as its first value. <P>
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It is implementation-dependent whether <A REL=DEFINITION HREF="../Body/f_ar_dis.htm#array-displacement"><B>ARRAY-DISPLACEMENT</B></A> returns<P>
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a non-NIL first value for any other array.<P>
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<P>
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<P>
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<B>Examples:<P>
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</B><P>
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(<A REL=DEFINITION HREF="../Body/f_ar_dis.htm#array-displacement"><B>array-displacement</B></A> '#(1 2 3))<P>
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=> the values are not specified, but are probably <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> and 0 in many<P>
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implementations.<P>
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(<A REL=DEFINITION HREF="../Body/f_ar_dis.htm#array-displacement"><B>array-displacement</B></A> (<A REL=DEFINITION HREF="../Body/f_mk_ar.htm#make-array"><B>make-array</B></A> 2 :displaced-to '#1=#(1 2 3)))<P>
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=> (values #1# 0)<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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Providing the function solves the problem.<P>
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<P>
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Some implementations implicitly displace some arrays. (For example,<P>
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adjustable arrays might be represented as displaced arrays.) Permitting<P>
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<A REL=DEFINITION HREF="../Body/f_ar_dis.htm#array-displacement"><B>ARRAY-DISPLACEMENT</B></A> to return non-NIL for those arrays allows those<P>
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implementations not to have to record some additional information <P>
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in the array about whether an explicit :DISPLACED-TO was provided.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Lucid version 4.0 and Apple's Macintosh Common Lisp implement such<P>
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a function, but call it DISPLACED-ARRAY-P.<P>
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<P>
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Symbolics has an ARRAY-DISPLACED-P predicate but it returns neither <P>
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the array nor an index, just a <A REL=DEFINITION HREF="../Body/t_ban.htm#boolean"><B>boolean</B></A>.<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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It's probably easy to add.<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, this is a new feature.<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 can't do some kind of manipulations on displaced arrays in a <P>
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portable way.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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Probably none.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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The cost of non-adoption is avoided.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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Looks OK to me.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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The addition of such a function was mentioned in the writeup for<P>
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proposal <A HREF="iss003.htm">ADJUST-ARRAY-DISPLACEMENT</A> as something that had been<P>
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considered at that time. I don't recall what the arguments were <P>
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against it.<P>
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<P>
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A more extreme proposal would also define <A REL=DEFINITION HREF="../Body/f_ar_dis.htm#array-displacement"><B>ARRAY-DISPLACEMENT</B></A> as<P>
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a <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> place, to allow these parameters of displaced arrays to <P>
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be changed.<P>
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<P>
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The original name suggested for this function was DISPLACED-ARRAY-P.<P>
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<P>
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JonL notes:<P>
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<P>
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The reason Lucid has DISPLACED-ARRAY-P is that is the name Guy Steele<P>
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gave it back it 6-Dec-85 -- the "Clarifications" and<P>
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"non-controversial ..." lists. Quite possibly, it had been suggested<P>
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already in the common-lisp@su-ai forum.<P>
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</PRE>
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