Issue RANGE-OF-START-AND-END-PARAMETERS Writeup

Issue:        RANGE-OF-START-AND-END-PARAMETERS

References:   COUNT (p257), COUNT-IF (p257), COUNT-IF-NOT (p257),

	      DELETE (p257), DELETE-DUPLICATES (p254), DELETE-IF (p254),

	      DELETE-IF-NOT (p254), FILL (p252), FIND (p257), FIND-IF (p257),

	      FIND-IF-NOT (p257), MAKE-STRING-INPUT-STREAM (p330),

	      MISMATCH (p257), NSTRING-CAPITALIZE (p304), 

	      NSTRING-DOWNCASE (p304), NSTRING-UPCASE (p304), SUBSTITUTE (p255),

	      NSUBSTITUTE-IF (p256), NSUBSTITUTE-IF-NOT (p256), 

	      PARSE-INTEGER (p381), PARSE-NAMESTRING (p414), POSITION (p257),

	      POSITION-IF (p257), POSITION-IF-NOT (p257), 

	      READ-FROM-STRING (p381), REDUCE (p251), REMOVE (p253), 

	      REMOVE-DUPLICATES (p254), REMOVE-IF (p253), REMOVE-IF-NOT (p253),

	      REPLACE (p252), SEARCH (p258), STRING-CAPITALIZE (p303),

	      STRING-EQUAL (p301), STRING-DOWNCASE (p303), STRING-GREATERP (p302),

	      STRING-LESSP (p302), STRING-NOT-EQUAL (p302),

	      STRING-NOT-GREATERP (p302), STRING-NOT-LESSP (p302),

	      STRING-UPCASE (p303), STRING/= (p301), STRING< (p301), 

	      STRING<= (p301), STRING= (p300), STRING> (p301), STRING>= (p301),

	      SUBSEQ (p248), SUBSTITUTE (p255), SUBSTITUTE-IF (p255), 

	      SUBSTITUTE-IF-NOT (p255), WRITE-LINE (p384), WRITE-STRING (p384)

Category:     CLARIFICATION

Edit history: 14-Sep-88, Version 1 by Pitman


Problem Description:


  CLtL is not always clear about the possible values which the START and END

  parameters of built-in Common Lisp can take.


Proposal (RANGE-OF-START-AND-END-PARAMETERS:INTEGER-AND-INTEGER-NIL):


  Clarify that for functions permitting a parameter named START, START1,

  or START2 which delimits the beginning point in a sequence to be

  considered for some operation, that paremeter must be a non-negative

  integer. If the argument is optional or key (as is the case for all

  functions referenced above except SUBSEQ), the value will default to

  0 if not supplied. It is not permissible to pass NIL as this argument.


  Clarify that for functions permitting a parameter named END, END1,

  or END2 which delimits the end point in a sequence to be considered

  for some operation, that paremeter must be a non-negative integer

  indicating a termination point or NIL indicating the last element

  in the sequence. If the argument is optional or key (as is the case

  for all functions referenced above), the value will default to NIL

  if not supplied. Supplying NIL is, therefore, equivalent to not

  supplying this argument.


Test Case:


  (SEARCH "F" "FOO" :START1 NIL) is an error.


  (SEARCH "F" "FOO" :START2 0) is permissible and is equivalent to

  (SEARCH "F" "FOO")


  (SEARCH "F" "FOO" :END2 NIL) is permissible and is equivalent to

  (SEARCH "F" "FOO")


Rationale:


  To keep data flow between programs from becoming excessively complicated,

  it's a good idea to specify what the default values are so that they can

  be passed explicitly rather than requiring an alternate calling sequence

  for all possible cases where the value might need to default.


Current Practice:


  Symbolics Genera implements the proposed behavior.


Cost to Implementors:


  Hopefully most implementations already do this. Those that do not will

  probably have to make quite a number of small changes to both the code

  for these functions and to any associated compiler optimizers.


Cost to Users:


  This change is upward compatible with existing user code. Any program

  which did not conform to this proposal was already not portable.


Cost of Non-Adoption:


  Subtle gratuitous differences in the handling of these arguments would

  continue to be a possibility and a barrier to portability.


Benefits:


  The language would be more regular and well-defined.


Aesthetics:


  If it makes things clearer, it's an improvement.


Discussion:


  Pitman supports RANGE-OF-START-AND-END-PARAMETERS:INTEGER-AND-INTEGER-NIL.