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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/iss228_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss229.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss230_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue MACRO-ENVIRONMENT-EXTENT Writeup</H2>
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<PRE><B>Forum:</B> Compiler<P>
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<B>Issue:</B> <A HREF="iss229.htm">MACRO-ENVIRONMENT-EXTENT</A><P>
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<B>References:</B> CLtL p. 145-146<P>
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Issue <A HREF="iss066.htm">COMPILER-LET-CONFUSION</A><P>
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Issue MACRO-CACHING<P>
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Issue <A HREF="iss147.htm">EVAL-WHEN-NON-TOP-LEVEL</A><P>
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Issue <A HREF="iss342.htm">SYNTACTIC-ENVIRONMENT-ACCESS</A><P>
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CLOS Chapter 3 (89-003)<P>
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<B>Category:</B> CLARIFICATION,CHANGE<P>
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<B>Edit History:</B> V1, 10 Jan 1989, Sandra Loosemore<P>
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V2, 09 Mar 1989, Sandra Loosemore<P>
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V3, 13 Mar 1989, Sandra Loosemore (last-minute discussion)<P>
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<B>Status:</B> **DRAFT**<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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What is the extent of environment objects received as the <A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&ENVIRONMENT</B></A><P>
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argument of a macro function?<P>
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<P>
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CLtL says that <A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&ENVIRONMENT</B></A> is "useful primarily in the rare cases<P>
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where a macro definition must explicitly expand any macros in a<P>
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subform of the macro call before computing its own expansion". While<P>
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this suggests that macro environment objects are typically used within<P>
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the dynamic scope of the macro function, the use of the word<P>
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"primarily" (rather than "only" or "exclusively" or some similarly<P>
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strong language) suggests that there may be other legitimate uses for<P>
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environment objects. But, because CLtL is silent about what those<P>
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uses might be, many users and implementors are under the impression<P>
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that environment objects have only dynamic extent.<P>
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<P>
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There are some situations where using environment objects as if they<P>
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had indefinite extent provides a very useful viewpoint from which to<P>
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solve a problem. Consider the following example:<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/m_defmac.htm#defmacro"><B>defmacro</B></A> typed-var (var) var)<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/m_defmac.htm#defmacro"><B>defmacro</B></A> local-type-declare (declarations <A REL=DEFINITION HREF="../Body/03_dd.htm#AMbody"><B>&body</B></A> forms <A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&environment</B></A> env)<P>
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`(<A REL=DEFINITION HREF="../Body/s_flet_.htm#macrolet"><B>macrolet</B></A> ((typed-var (<A REL=DEFINITION HREF="../Body/03_dd.htm#AMwhole"><B>&whole</B></A> w var)<P>
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(<A REL=DEFINITION HREF="../Body/s_let_l.htm#let"><B>let</B></A> ((<A REL=DEFINITION HREF="../Body/a_type.htm#type"><B>type</B></A> (<A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc"><B>assoc</B></A> var ',declarations)))<P>
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(<A REL=DEFINITION HREF="../Body/s_if.htm#if"><B>if</B></A> <A REL=DEFINITION HREF="../Body/a_type.htm#type"><B>type</B></A> <P>
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`(<A REL=DEFINITION HREF="../Body/s_the.htm#the"><B>the</B></A> ,(<A REL=DEFINITION HREF="../Body/f_car_c.htm#cadr"><B>cadr</B></A> <A REL=DEFINITION HREF="../Body/a_type.htm#type"><B>type</B></A>) ,var)<P>
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(<A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>macroexpand</B></A> w ',env)))))<P>
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,@forms))<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/m_defun.htm#defun"><B>defun</B></A> f (x y)<P>
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(local-type-declare ((x <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>fixnum</B></A>) (y <A REL=DEFINITION HREF="../Body/a_float.htm#float"><B>float</B></A>))<P>
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(+ (typed-var x) (typed-var y))))<P>
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<P>
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Here, local macro TYPED-VAR is defined to look first in the innermost<P>
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lexical environment for information about the variable, and if there<P>
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isn't any then it recurses on the next outermost lexical environment.<P>
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The global definition of TYPED-VAR provides a terminal case to stop<P>
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the recursion.<P>
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<P>
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There are other situations where the extent of macro environment<P>
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objects comes into play. For example, if we allow caching of macro<P>
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expansions (issue MACRO-CACHING), environments must have indefinite<P>
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extent. It is unclear whether CLOS would be affected by allowing<P>
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macro environments to have only dynamic extent. (The descriptions of<P>
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the CLOS defining macros in document 89-003 seem to imply that the<P>
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value of the <A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&ENVIRONMENT</B></A> argument appears in the expansion of the<P>
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macro, but there now seems to be sentiment that the model of how the<P>
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defining macros work that is presented there is broken.)<P>
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<P>
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<P>
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<B>Proposal MACRO-ENVIRONMENT-EXTENT:INDEFINITE:<P>
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</B><P>
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State that macro environment objects received with the <A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&ENVIRONMENT</B></A><P>
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argument of a macro function or as the argument to a <A REL=DEFINITION HREF="../Body/v_mexp_h.htm#STmacroexpand-hookST"><B>*MACROEXPAND-HOOK*</B></A><P>
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function have indefinite extent.<P>
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<P>
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Note that implementations are not permitted to destructively modify<P>
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lexical information in environment objects once they have been passed<P>
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to a macro function. It is, however, permissible to add or remove<P>
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global definitions that are accessible through the environment.<P>
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<P>
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Rationale:<P>
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<P>
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This legitimizes the use of idioms which depend on macro environments<P>
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having indefinite extent.<P>
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<P>
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Since data objects in Lisp otherwise have indefinite extent, it is<P>
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more consistent to give environment objects indefinite extent as<P>
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well.<P>
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<P>
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<P>
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<B>Proposal MACRO-ENVIRONMENT-EXTENT:DYNAMIC:<P>
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</B><P>
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State that macro environment objects received with the <A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&ENVIRONMENT</B></A><P>
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argument of a macro function or with a <A REL=DEFINITION HREF="../Body/v_mexp_h.htm#STmacroexpand-hookST"><B>*MACROEXPAND-HOOK*</B></A> function<P>
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have only dynamic extent; the consequences are undefined if they are<P>
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referred to outside the dynamic extent of that macro function or hook<P>
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function.<P>
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<P>
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Rationale:<P>
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<P>
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This allows implementations to use somewhat more efficient techniques<P>
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for representing environment objects. For example, the storage could<P>
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be stack-allocated, or environments could be bashed destructively<P>
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instead of always being freshly heap-allocated.<P>
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<P>
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<P>
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<B>Proposal MACRO-ENVIRONMENT-EXTENT:DYNAMIC-WITH-COPIER:<P>
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</B><P>
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State that macro environment objects received with the <A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&ENVIRONMENT</B></A><P>
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argument of a macro function or with a <A REL=DEFINITION HREF="../Body/v_mexp_h.htm#STmacroexpand-hookST"><B>*MACROEXPAND-HOOK*</B></A> function<P>
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have only dynamic extent; the consequences are undefined if they are<P>
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referred to outside the dynamic extent of that macro function or hook<P>
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function.<P>
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<P>
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Add a new function:<P>
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<P>
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COPY-ENVIRONMENT environment [function]<P>
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<P>
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This function returns an environment object that is semantically<P>
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equivalent to "environment" (which must be an object of the type<P>
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received with an <A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&ENVIRONMENT</B></A> argument to a macro or as an argument to<P>
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a <A REL=DEFINITION HREF="../Body/v_mexp_h.htm#STmacroexpand-hookST"><B>*MACROEXPAND-HOOK*</B></A> function), but which may safely be referred to<P>
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outside the dynamic extent of the macro function. This function is<P>
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permitted to return an object that is <A REL=DEFINITION HREF="../Body/f_eq.htm#eq"><B>EQ</B></A> to its argument if that <P>
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object may be safely used.<P>
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<P>
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Rationale:<P>
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<P>
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This allows implementations to use somewhat more efficient techniques<P>
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for representing environment objects. For example, the storage could<P>
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be stack-allocated, or environments could be bashed destructively<P>
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instead of always being freshly heap-allocated.<P>
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<P>
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It also allows programmers to use idioms that rely on environment<P>
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objects having indefinite extent.<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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Macro environments appear to have indefinite extent in Lucid Common<P>
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Lisp, Kyoto Common Lisp, CMU Common Lisp (at least in the<P>
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interpreter), Utah Common Lisp, and A-Lisp. A-Lisp internally uses<P>
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the kind of idiom shown in the example above to implement <A REL=DEFINITION HREF="../Body/s_flet_.htm#flet"><B>FLET</B></A>,<P>
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<A REL=DEFINITION HREF="../Body/s_flet_.htm#labels"><B>LABELS</B></A>, and <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>FUNCTION</B></A> as macros.<P>
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<P>
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Macro environments are stack-allocated in Symbolics Genera, and hence<P>
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have dynamic extent.<P>
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<P>
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Macro environments also have dynamic extent on the TI Explorer,<P>
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because the compiler uses special variables are used to keep track of<P>
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lexical definitions.<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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For proposal INDEFINITE, some implementations would need to change. A<P>
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simple implementation of macro environments that would fit the<P>
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requirements of this proposal is to represent them as lists, pushing<P>
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information for inner contours onto the front of the list as the<P>
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contour is entered and popping the list when the contour is exited.<P>
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<P>
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For proposal DYNAMIC, there is no associated implementation cost.<P>
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<P>
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For proposal DYNAMIC-WITH-COPIER, the implementation cost is unknown<P>
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but probably fairly small in most implementations.<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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For proposal INDEFINITE, there is no associated cost to users.<P>
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<P>
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For proposal DYNAMIC, users would not be able to portably use a<P>
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simple and elegant approach to solving certain kinds of problems.<P>
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<P>
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For proposal DYNAMIC-WITH-COPIER, users would have to remember to make<P>
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a copy of an environment object in some situations.<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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It is made clear whether treating environment objects as if they had<P>
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indefinite extent is portable usage.<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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Proposal SYNTACTIC-ENVIRONMENT-ACCESS:ADD-FUNCTIONAL-INTERFACE<P>
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includes adding a function called AUGMENT-ENVIRONMENT which could also<P>
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be used to create a copy of an environment. However, it returns an<P>
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object with the same extent as its argument, and therefore can not<P>
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replace the function COPY-ENVIRONMENT under proposal<P>
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DYNAMIC-WITH-COPIER.<P>
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<P>
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We have also considered a couple of other alternatives on this <P>
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issue.<P>
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<P>
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One alternative would be to give "remote" environments created by<P>
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<A REL=DEFINITION HREF="../Body/f_cmp_fi.htm#compile-file"><B>COMPILE-FILE</B></A> the extent of that call to <A REL=DEFINITION HREF="../Body/f_cmp_fi.htm#compile-file"><B>COMPILE-FILE</B></A>, while "local"<P>
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environments (the null lexical environment and environments created by<P>
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<A REL=DEFINITION HREF="../Body/f_cmp.htm#compile"><B>COMPILE</B></A> and <A REL=DEFINITION HREF="../Body/f_eval.htm#eval"><B>EVAL</B></A>) would have indefinite extent.<P>
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<P>
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Another alternative would be to say that environments created by<P>
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<A REL=DEFINITION HREF="../Body/f_cmp_fi.htm#compile-file"><B>COMPILE-FILE</B></A>, <A REL=DEFINITION HREF="../Body/f_cmp.htm#compile"><B>COMPILE</B></A>, or <A REL=DEFINITION HREF="../Body/f_eval.htm#eval"><B>EVAL</B></A> have a dynamic extent that includes the<P>
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time when any other macro calls appearing lexically within the<P>
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expansion returned by the macro function are expanded. This is<P>
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similar to the extent of the special bindings made by COMPILER-LET.<P>
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<P>
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Both of these proposals could be combined with adding a copier<P>
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function to deal with those implementations where environments are<P>
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stack-allocated. They would both solve the extent problem for the<P>
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example given in the problem description section, but not the general<P>
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problem of macro caching. In conjunction with the proposals for issue<P>
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<A HREF="iss342.htm">SYNTACTIC-ENVIRONMENT-ACCESS</A>, they would both <A REL=DEFINITION HREF="../Body/f_provid.htm#require"><B>require</B></A> some<P>
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modifications to implementations that currently give macro<P>
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environments dynamic extent.<P>
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<P>
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Loosemore supports proposal MACRO-ENVIRONMENT-EXTENT:INDEFINITE.<P>
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<P>
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Moon says:<P>
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My opinion is that anything in CLOS that seems to depend on indefinite<P>
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extent for macro environments is broken and needs to be fixed. It's not<P>
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broken because of the environment extent, but for other reasons.<P>
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Thus I believe in dynamic extent for environments.<P>
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<P>
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Neil Goldman says:<P>
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In my code walker I have a pretty ugly way of dealing with <A REL=DEFINITION HREF="../Body/s_flet_.htm#macrolet"><B>MACROLET</B></A><P>
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that would have been trivial if I could have counted on the<P>
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ENVIRONMENT having indefinite extent. There may be some cleaner way<P>
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than what I did, but I just looked at PCL's code walker, and it also<P>
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is much more complex than would be necessary if environments had<P>
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indefinite extent.<P>
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<P>
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</PRE>
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