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<H2>Issue DEFSTRUCT-DEFAULT-VALUE-EVALUATION Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss114.htm">DEFSTRUCT-DEFAULT-VALUE-EVALUATION</A><P>
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<B>References:</B> CLtL p.308-10 & 86-003 p.4<P>
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<B>Category:</B> CLARIFICATION<P>
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<B>Edit history:</B> Revision 1 by Skona Brittain 05/13/88<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 some confusion over whether default initialization<P>
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forms for <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> slots get evaluated, when they are not needed<P>
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because a keyword argument was supplied to the constructor function.<P>
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As a consequence of this confusion, there is confusion over whether<P>
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there can be a type-mismatch error between the specified type of the slot<P>
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and the type of the default value.<P>
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<P>
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On page 308, it says "The default-init is a form that is evaluated <P>
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each time a <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> is to be constructed; the value is used as the <P>
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initial value of the slot. If no default-init is specified, then the<P>
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initial contents of the slot are undefined and implementation-dependent."<P>
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<P>
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On the next page, however, it says that the default-init is evaluated if<P>
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the keyword argument is not supplied and the converse, although not stated,<P>
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is intended and informally implied.<P>
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<P>
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<P>
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<B>Proposal (DEFSTRUCT-DEFAULT-VALUE-EVALUATION:IFF-NEEDED):<P>
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</B><P>
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Clarify that the converse is true. i.e that the default-init is not evaluated <P>
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if the keyword argument is supplied.<P>
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<P>
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In the quote from page 308, delete the second sentence and replace <P>
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"a <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> is to be constructed; the value is" by "its value is to be".<P>
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<P>
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To section 19.3, add a clarification,<P>
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such as the following from Guy's issues file:<P>
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"The default value in a <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> slot is not evaluated <P>
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unless it is needed in the creation of a particular <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A><P>
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instance. If it is never needed, there can be no type-mismatch<P>
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error, even if the type of the slot is specified, and no warning<P>
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should be issued."<P>
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<P>
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<P>
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<B>Test Case:<P>
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</B><P>
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In the following sequence, only the last call is an error.<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> person (name 007 :type <A REL=DEFINITION HREF="../Body/a_string.htm#string"><B>string</B></A>)) <P>
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(make-person :name "James")<P>
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(make-person)<P>
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<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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It is inefficient, and inconsistent with the rest of the language, for the <P>
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default initialization form to be evaluated when it is not needed.<P>
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Consequently, when it's not needed, such type-mismatch errors should not be <P>
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detectable in general.<P>
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<P>
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Any existing confusion should be clarified by this proposal.<P>
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<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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KCL does not evaluate the default initialization form unless it is needed;<P>
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even when it is needed, the type checking is not done at all.<P>
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<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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If there are any implementations that currently behave differently from<P>
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the proposed way, then they need some slight modification. <P>
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<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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<P>
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<B>Benefits:<P>
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</B><P>
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Clarity and portability. In particular, clarifying that the unaesthetic <P>
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situation mentioned in the next section is allowed should be reassuring.<P>
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<P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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It appears slightly unaesthetic to have a default value that violates a <P>
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type specification. <P>
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<P>
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<P>
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<B>Discussion: <P>
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</B><P>
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Although this issue was mentioned in Guy's original issues file, it has<P>
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not been officially discussed since.<P>
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<P>
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!<P>
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Additional notes:<P>
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<P>
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Several members of the cleanup committee endorsed this proposal.<P>
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<P>
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JonL added:<P>
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<P>
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You can add to the "Current Practice:" section:<P>
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<P>
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LUCID does not evaluate the default initialization form unless it is <P>
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needed; even when it is needed, the type checking is not done at all.<P>
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However, at <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> definition time, if an initial value form is<P>
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constanp and doesn't satisfy the :type specification, a warning message<P>
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is printed.<P>
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<P>
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Oddly enough, Lucid's interpreter ensures that <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A>'s of slots obey the<P>
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:type specifier, even though init-forms aren't checked. Furthermore, in <P>
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safety levels 2 or higher, the compiled code will do minimal "memory-<P>
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integrity" type checking for <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A>'s (which is what I suspect the various <P>
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special-purpose microcoded machines do); however except for low-level numeric<P>
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types, this is rarely equivalent to what a full type check would do.<P>
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<P>
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I have long suggested that there should be at least one mode of operation <P>
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such that all :type information is checked when setting values into <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A><P>
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slots (<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> as well as initialization). Some have suggested that this mode <P>
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could be "when running interpretively, or when when compiled with the highest<P>
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degree of <A REL=DEFINITION HREF="../Body/d_optimi.htm#safety"><B>SAFETY</B></A> and lower degrees of <A REL=DEFINITION HREF="../Body/d_optimi.htm#speed"><B>SPEED</B></A>." However, since the wording of <P>
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CLtL p310 suggests that the :type slot options is merely a <A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>DECLARE</B></A>, and since<P>
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some vendors effectively ignore any and all declarations [except for SPECIAL],<P>
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then this suggestion hasn't reached proposal stage yet.<P>
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<P>
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</PRE>
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