ISSUE: VARIABLE-LIST-ASYMMETRY

[Skona Brittain <skona%csilvax@hub.ucsb.edu>]

Issue:          VARIABLE-LIST-ASYMMETRY
References:     CLtL pgs. 110, 122, 131
Category:       ADDITION
Edit history:   Revision 1 by Skona Brittain 07/26/88
                Revision 2 by Skona Brittain 08/04/88 (change of category,
                                                       spelling & verbosity)

Problem Description:

The syntax of items in the variable-list for various control structues
(do, let, prog and their duals) varies.  This variation seems unnecessary.

The allowed variations are indicated in the following chart:

do & do*:             (var)    (var init)    (var init step)
prog & prog*:   var   (var)    (var init)       n.a.
let & let*:     var            (var val)        n.a.

Note that just plain `` var '' is prohibited in do forms
and that the case of ``(var)'' is prohibited in let forms.


Proposal (VARIABLE-LIST-ASYMMETRY:SYMMETRIZE):

Allow all the variations in all of the forms;
i.e. add the prohibited cases mentioned above.

I.e. change the variable-list in the syntax descriptions as follows:
do & do*:   ( { var | (var [init [step]] ) }* )
let & let*: ( { var | (var [value]       ) }* )


Test Cases:

(let (a (b) (c 3)) ... )  would be valid.

(do* (a (b) (c 3)) ... )  would be valid.


Rationale:

The asymmetry is unnecessary and impedes learning of CL.

Any other way to make these cases consistent, such as either
omitting just ``var'' from do & do* and prog & prog*, or
omitting ``(var)'' from let & let* and prog & prog*, 
would be an incompatible change to the language.  
This way just adds the flexibility found in some of the forms to all of them.


Current Practice:

KCL allows ``(var)'' in let & let* but not ``var'' in do & do*.

Symbolics Genera allows all three cases in all the forms; i.e. it conforms
to this proposal.


Cost to Implementors:

Extremely slight. (May involve subtracting code rather than adding it).


Cost to Users:

None.


Cost of Non-Adoption:

The variation in syntax makes them harder to learn.


Benefits:

Ease of learning.


Aesthetics:

Symmetry is more aesthetic than asymmetry, at least to some of us.


Discussion: 

Kent supports this proposal.

The issue about whether the atomic ``var'' should be allowed at all in the 
variable lists for let & let* is a separate issue.  (So is whether
it should mean that the var is initially bound to nil.)  Since it is allowed, 
this proposal merely says that the alternative syntax of an atom within a 
list with no initial value, ``(var)'', should also be allowed.