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<H2>Issue DEBUGGER-HOOK-VS-BREAK Writeup</H2>
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<PRE><B>Issue:</B> DEBUGGER-HOOK-BREAK<P>
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<B>Forum:</B> Editorial<P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/f_break.htm#break"><B>BREAK</B></A> (pp432-433), <A REL=DEFINITION HREF="../Body/v_debugg.htm#STdebugger-hookST"><B>*DEBUGGER-HOOK*</B></A> (Conditions, Rev18, p42)<P>
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<B>Category:</B> CHANGE<P>
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<B>Edit history:</B> 01-Mar-91, Version 1 by Pitman<P>
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<B>Status:</B> For X3J13 consideration<P>
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<P>
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<B>Problem Description:<P>
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</B><P>
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According to v18 of the Conditions Proposal (p42), when <A REL=DEFINITION HREF="../Body/f_invoke.htm#invoke-debugger"><B>INVOKE-DEBUGGER</B></A> <P>
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is executed, ``If the variable <A REL=DEFINITION HREF="../Body/v_debugg.htm#STdebugger-hookST"><B>*DEBUGGER-HOOK*</B></A> is not <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>, it will be <P>
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funcalled ...''<P>
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<P>
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According to p432-433 of CLtL, when <A REL=DEFINITION HREF="../Body/f_break.htm#break"><B>BREAK</B></A> is called, ``A particular <P>
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implementation may choose, according to its own style and needs, when<P>
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<A REL=DEFINITION HREF="../Body/f_break.htm#break"><B>BREAK</B></A> is called to go into a debugger different from the one used for <P>
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handling errors.''<P>
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<P>
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It isn't clear whether <A REL=DEFINITION HREF="../Body/f_break.htm#break"><B>BREAK</B></A> is affected by <A REL=DEFINITION HREF="../Body/v_debugg.htm#STdebugger-hookST"><B>*DEBUGGER-HOOK*</B></A>, or whether<P>
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it uses <A REL=DEFINITION HREF="../Body/v_debugg.htm#STdebugger-hookST"><B>*DEBUGGER-HOOK*</B></A> to implement any use of an alternate debugger.<P>
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<P>
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<B>Proposal (DEBUGGER-HOOK-BREAK:CLARIFY):<P>
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</B><P>
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1. Define the debugger implemented by <A REL=DEFINITION HREF="../Body/f_break.htm#break"><B>BREAK</B></A> is not affected by<P>
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the value of <A REL=DEFINITION HREF="../Body/v_debugg.htm#STdebugger-hookST"><B>*DEBUGGER-HOOK*</B></A> upon entry.<P>
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<P>
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2. Define that <A REL=DEFINITION HREF="../Body/f_break.htm#break"><B>BREAK</B></A> binds the value of <A REL=DEFINITION HREF="../Body/v_debugg.htm#STdebugger-hookST"><B>*DEBUGGER-HOOK*</B></A> to <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>.<P>
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<P>
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<B>Test Case:<P>
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</B><P>
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(<A REL=DEFINITION HREF="../Body/s_block.htm#block"><B>BLOCK</B></A> FOO<P>
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(<A REL=DEFINITION HREF="../Body/s_let_l.htm#let"><B>LET</B></A> ((<A REL=DEFINITION HREF="../Body/v_debugg.htm#STdebugger-hookST"><B>*DEBUGGER-HOOK*</B></A> #'(<A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>LAMBDA</B></A> (C D) (<A REL=DEFINITION HREF="../Body/s_ret_fr.htm#return-from"><B>RETURN-FROM</B></A> FOO <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>))))<P>
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(<A REL=DEFINITION HREF="../Body/f_break.htm#break"><B>BREAK</B></A>)))<P>
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<P>
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enters a breakpoint rather than just returning <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>.<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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1. <A REL=DEFINITION HREF="../Body/f_break.htm#break"><B>BREAK</B></A>'s primary role is to <A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> access to Lisp debugging.<P>
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Anyone who wants to enter the debugger through <A REL=DEFINITION HREF="../Body/v_debugg.htm#STdebugger-hookST"><B>*DEBUGGER-HOOK*</B></A><P>
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can use <A REL=DEFINITION HREF="../Body/f_invoke.htm#invoke-debugger"><B>INVOKE-DEBUGGER</B></A>.<P>
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<P>
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2. <A REL=DEFINITION HREF="../Body/v_debugg.htm#STdebugger-hookST"><B>*DEBUGGER-HOOK*</B></A> provides an application/end-user-oriented <P>
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debugging interface, while <A REL=DEFINITION HREF="../Body/f_break.htm#break"><B>BREAK</B></A> provides a Lisp debugging<P>
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interface. Since <A REL=DEFINITION HREF="../Body/f_break.htm#break"><B>BREAK</B></A> will generally lead to the typing <P>
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of Lisp expressions rather than application-specific commands,<P>
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it makes sense for the Lisp debugging environment to be visible<P>
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here rather than the application debugging environment.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Symbolics Genera returns <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> from the test case and would have to change.<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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Very small. They just have to make <A REL=DEFINITION HREF="../Body/f_break.htm#break"><B>BREAK</B></A> bind <A REL=DEFINITION HREF="../Body/v_debugg.htm#STdebugger-hookST"><B>*DEBUGGER-HOOK*</B></A> to <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A><P>
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before doing anything else.<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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Probably 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 know what to expect when moving from implementation to<P>
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implementation.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Less ambiguity in the spec.<P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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Negligible.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Pitman supports this proposal, but would consider any reasonable <P>
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alternative which made the expected behavior explicit.<P>
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</PRE>
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