J3/06-380

To: J3
From: Malcolm Cohen
Subject: Macro Assignment SSE
Date: 2006/11/16

1. Introduction

This paper contains specifications, syntax and edits for macro assignment.

2. Specifications

(1) Assignment of a macro expression to a macro (integer) variable should
    be possible. The expression shall be an initialization expression of type integer.

Comment: This is sort-of possible now, by doing

  MACRO DO I=<expr>,<expr>

where you repeat the expression.  Within the DO block the macro variable I has
the value of the expression.  That is rather grotty.

(2) There should be macro variables that are capable of holding token sequences,
    not just integer values.  Expansion of such a macro variable produces its
    token sequence.  Such macro variables are initially empty.

Comment: This is the same as macro dummy arguments.

(3) That assignment to a macro "token sequence" variable should be possible.
    This assignment expands its right-hand-side but does not evaluate it, i.e.
    it is a token sequence not a macro expression.

Comment: This is similar to the way macro argument association works, where an actual
  argument is a token sequence, and the dummy argument gets associated with that
  sequence.  So this functionality is sort-of available now, but only by doing
  nested macro expansions.  That is rather arcane.

3. Syntax

Reminder: Macro local variables of type integer are declared by
   MACRO INTEGER [kind] :: <name-list>

(1) Macro local variables of "token sequence" type shall be declared by
       MACRO VARIABLE :: <name-list>

Comment: Since a token sequence doesn't have a type, just saying it is a variable
  seems reasonable.  Alternatively it could be "MACRO TOKEN SEQUENCE :: <name-list>"
  but that seems excessively verbose.

(2) Assignment to a macro integer variable shall be by
       MACRO <macro-integer-variable-name> = <macro-expr>
    Constraint: The macro-integer-variable-name shall be a macro local variable
                of type integer.

(3) Assignment to a macro token sequence variable shall be by
       MACRO <macro-token-variable-name> = [ <result-token> ]...
    Constraint: The <macro-token-variable-name> shall be a macro local variable
                that is not of type integer and is not a macro dummy argument.

Comment: The difference between these two forms depends only on the type of the
  macro variable being assigned to.  Note that assignment to an empty token
  sequence is allowed, and sets the variable to being an empty token sequence.

4. Edits

[33:15+] Add new production to R317 <macro-declaration-stmt>,
  "<<or>> <macro-variable-decl-stmt>"
{The new declaration.}

[33:18+] Add new production
  "R320a <macro-variable-decl-stmt> <<is>> MACRO VARIABLE :: <macro-local-variable-name-list>"
{The new declaration.}

[33:19] In C306, after "R318" insert "R320a".
{Same constraint as for MACRO INTEGER, viz that the macro-local-variable-name shall not
 be a macro dummy argument name.}

[33:26] Before "A" insert new sentence
  "A macro variable declaration statement declares untyped macro local variables;
   the value of an untyped macro local variable is a token sequence, and its
   initial value is an empty sequence (no tokens)."

[33:27] Append to sentence
  "or in a macro variable declaration statement".
{Allow untyped macro variables.}

[33:33+] Add new productions to R322 <macro-body-construct>,
  "<<or>> <macro-integer-assignment-stmt>
   <<or>> <macro-token-assignment-stmt>"

[34:24+] Add new productions
  "R332a <macro-integer-assignment-stmt> <<is>>
                 <macro-integer-variable-name> = <macro-expr>
   C312a (R332a) <macro-integer-variable-name> shall be the name of a macro local
         variable of type integer

   R332b <macro-token-assignment-stmt> <<is>>
                 <macro-token-variable-name> = <assignment-token-sequence>
   C312b (R332b) <macro-token-variable-name> shall be the name of an untyped macro
         local variable that is not a macro dummy argument.
   R332c <assignment-token-sequence> <<is>> [ <result-token> ]..."
{Syntax for macro assignment.}

[36:31] "or" -> ", assignment token sequence, or"
{Token replacement operates on the right-hand-side of macro assignment.}

[36:39] Append to paragraph
  "An untyped macro local variable expands to the sequence of tokens that was assigned
   to it, or to zero-length token sequence if it has never been assigned to."
{How untyped macro local variables expand.}

[38:6+] Insert new subclause
  "3.5.2.5a Macro assignment

   Processing a macro integer assignment statement sets the macro local variable value
   to that of the macro expression.

   Processing a macro token assignment statement sets the macro local variable value
   to be the sequence of tokens following the equals sign.  If no tokens appear after
   the equals sign, the macro local variable is set to the zero-length token sequence."
{Define effect of macro assignment.}

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