🚀 go-pugleaf

Title: Extrinsic Blocks

Advocate: Hokey <hokey@plus5.com>

History: 20 Mar 88 - Submitted

Abstract:
	MUMPS presently lacks common control-flow mechanisms found in many
	other languages.  Additionally, the "block structure" mechanism yields
	its benefits only to code invoked as a subroutine; neither the
	mechanism nor the benefits are available to code invoked as an
	extrinsic function.

	The following proposal, in conjunction with my "Implicit DO extension"
	proposal, solves both of these shortcomings.

Examples:
	;WHILE clause
	F  Q:while  D $$
	.	;code

	;UNTIL clause
	F  D $$ Q:until
	.	;code

	;repeat until subroutine says stop
	F  Q:$$
	() .	;code
	   .	QUIT $S(more:0,1:1)

	;WHILE with BREAK and CONTINUE
	S I=0 F  Q:(while!(I=-1))  S I=$$ I I=0 ;more code...
	() .	;code
	   .	QUIT $S(normal:0,break:-1,continue:1)

Proposal: (Page and section numbers are from X11/SC1/88-14)

	Add to the definition of line in 3.4:

		| exblkline |

	Add the following definition to section 3.4:

		exblkline ::= formallist ls li ... linebody

	Renumber the sections:

		3.2.10 Extrinsic Special Variable
		3.2.11 Extrinsic Function

	as:

		3.2.11 Extrinsic Special Variable
		3.2.12 Extrinsic Function

	Add a new section:

		3.2.10 Extrinsic Block

		exblk ::= $$

		Extrinsic blocks invoke an inner block of lines to return a
		value.  When an extrinsic block is executed, the current
		value of $T, the current execution level, the current
		execution level are saved in an exvar frame on the
		process-stack, and the execution level is increased by one.

		Execution continues at the first command of the next line
		in the routine, which must be an exblkline having the
		current execution level.

		Upon return from the subroutine the value of $T and the
		execution level are restored, and the value of the argument
		of the QUIT command that terminated the subroutine is returned
		as the value of the exblk.

	Add:
		exblk

	to the implicit/explicit QUIT list in the second-to-last sentence of
	the first paragraph of 3.6.3 and to the similar list in the second
	sentence of subparagraph d in that same section.
-- 
Hokey