Issue LOOP-AND-DISCREPANCY Writeup

Issue:		LOOP-AND-DISCREPANCY


References:	Loop Facility document X3J13/89-004


Related issues: 


Category:	CHANGE CLARIFICATION


Edit history:	Version 1, 15-Mar-88 by Steele


Problem description:


The treatment of the AND conjunction in FOR/AS and WITH clauses is not

consistent.  Examples of the use of WITH are also not consistent in this

respect.


Page 2-5 implies by example that when AND is used to join two

FOR/AS clauses, the word FOR or AS must occur after the word AND.


Page 2-31 has formal syntax specifying that when AND is used to join two

WITH clauses, the word WITH must *not* occur after the word AND.  Examples

on that page are consistent with this specification.


Page 2-41 has an example in which WITH is repeated after AND.



Proposal (LOOP-AND-DISCREPANCY:NO-REITERATION):


Let stand the formal syntax for WITH.


Change the description of FOR/AS clauses to specify that when

two or more such clauses are joined with AND, clauses after the

first do not have FOR or AS before them.


The complete formal syntax for FOR/AS may be described as follows:


for-as ::= {FOR | AS} for-as-subclause {AND for-as-subclause}*


for-as-subclause ::= for-as-arithmetic | for-as-in-list

		   | for-as-on-list | for-as-equals-then

		   | for-as-across | for-as-hash | for-as-package


for-as-arithmetic ::= var [type-spec] ...


and so on.


Examples:


> (loop for x from 1 to 10		;Corrected from X3J13/89-004, page 2-5

        and y = nil then x

        collect (list x y))

((1 NIL) (2 1) (3 2) (4 3) (5 4) (6 5) (7 6) (8 7) (9 8) (10 9))


> (loop with (a b) float = '(1.0 2.0)	;Corrected from X3J13/89-004, page 2-41

        and (c d) integer = '(3 4)

        and (e f)

        return (list a b c d e f))

(1.0 2.0 3 4 nil nil)



Rationale:


The treatment of AND should be internally consistent.  There is no reason

to repeat the FOR/AS keyword.  Not repeating the keyword emphasizes that

the subclauses are functionally linked under the heading of WITH or FOR.

(Compare to the third use of AND in LOOP, to link clauses controlled

by WHEN/IF/UNLESS.  One does not repeat the WHEN; rather, the clauses

grouped by AND are controlled by a single WHEN.)



Current practice:


Symbolics LOOP allows FOR to be included or omitted after AND,

with identical meanings.  WITH may not be repeated after AND.



Cost to Implementors: Small?


Cost to Users: Possible incompatibility with existing implementors' extensions.


Cost of non-adoption:  Utter confusion.


Performance impact:  None.


Benefits:  Consistent treatment of AND within LOOP.


Esthetics:


Absolutely none.  We're talking about LOOP here.


Discussion:


Steele supports this proposal.  It is a reversal of his previous

suggestion on the topic, thanks to feedback from Moon.