Issue FORMAT-COMMA-INTERVAL Writeup

Issue:        FORMAT-COMMA-INTERVAL

References:   FORMAT integer printing (p. 388-9)

Category:     ADDITION

Edit history: Version 1, Pavel, 10-Jun-87

              Version 2, Masinter, 15-Jun-87


Problem description:  


There are times when users would like to print out numbers with some punctuation

between groups of digits.  The "commachar" argument to the ~D, ~B, ~O, ~X, and

~R FORMAT directives was introduced to fill that need.  Unfortunately, the

interval at which the commachar should be printed is always every three digits.

This constraint is annoying when a different interval would be more appropriate.


Proposal (FORMAT-COMMA-INTERVAL:YES):


Add a fourth argument to the ~D, ~B, ~X, and ~O FORMAT directives, and a fifth

argument to the ~R directive, to be called the "comma-interval".  This value

must be an integer and defaults to 3.  When the : modifier is given to any of

these directives, the "commachar" is printed between groups of "comma-interval"

digits.


Test Cases:


Under the proposal, the following forms return the indicated values:

	(format nil "~,,' ,4:B" 13) => "1101"

	(format nil "~,,' ,4:B" 17) => "1 0001"

	(format nil "~19,0,' ,4:B" 3333) => "0000 1101 0000 0101"

	(format nil "~3,,,' ,2:R" 17) => "1 22"

	(format nil "~,,'|,2:D" #xFFFF) => "6|55|35"


Rationale: 


The current specification of FORMAT gives the user control over almost all of

the facets of printing integers.  Only the wired-in constant for the

comma-interval remains, even though there are uses for varying that number.  For

example, in many contexts, it would be convenient to be able to print out

numbers in binary with a space between each group of four bits.  FORMAT does not

currently allow specification of the commachar printing interval so users

needing this functionality must write it themselves, duplicating much of the

logic in every implementation's integer printing code.  Other uses, requiring

other intervals, can be imagined.  For example, using a "commachar" of #\Newline

and a "comma-interval" of, say, 72, very large bignums could be printed in such

a way as to ensure line-breaks at appropriate places.


Current practice:


No released implementations currently support this feature.


Cost to implementors: 


The change in the implementation of FORMAT is reasonably minor and, in most

cases, highly localized.  Usually, the change is as simple as taking an extra

parameter and using it instead of a wired-in value of 3.


Cost to users:


Since the proposal involves the addition of an argument to certain directives,

the change would be entirely upward-compatible. No user code would need to be

converted.


Cost of non-adoption: 


Users desiring this functionality will have to write it themselves, duplicating

much of the logic involved in printing integers at all.


Benefits: 


One of the few remaining inflexibilities in integer printing is eliminated with

a net increase in user-visible functionality.


Esthetics: 


By parameterizing one of the final pieces of wired-in behavior from integer

printing, this small part of the workings of FORMAT would be perceived as having

been cleaned up.


Discussion: 


Several members of the cleanup committee endorsed this proposal. No objections

were raised.