Issue SEQUENCE-TYPE-LENGTH Writeup

Issue:         SEQUENCE-TYPE-LENGTH


References:    CLtL p.51, p.249, p.260, p.252, p.354

               CONCATENATE, COERCE, MAKE-SEQUENCE, MAP, MERGE


Category:      CLARIFICATION


Edit history:  16-Jun-89, version 1, by Moon


Problem description:


  In several functions that take a type specifier as an argument and create

  a sequence of the specified type, it isn't clear what happens if the type

  specifier has an explicit length that doesn't match the length implied by

  the other arguments.


Proposal (SEQUENCE-TYPE-LENGTH:MUST-MATCH):


  COERCE should signal an error if the new sequence type specifies the

  number of elements and the old sequence has a different length.


  MAKE-SEQUENCE should signal an error if the sequence type specifies the

  number of elements and the size argument is different.


  CONCATENATE should signal an error if the sequence type specifies the

  number of elements and the sum of the argument lengths is different.


  MAP should signal an error if the sequence type specifies the number of

  elements and the minimum of the argument lengths is different.


  MERGE should signal an error if the sequence type specifies the number of

  elements and the sum of the lengths of the two sequence arguments is

  different.


Examples:


  ;; All of the following forms should signal an error

  (coerce '(a b c) '(vector * 4))

  (coerce #(a b c) '(vector * 4))

  (coerce '(a b c) '(vector * 2))

  (coerce #(a b c) '(vector * 2))

  (coerce "foo" '(string 2))

  (coerce #(#\a #\b #\c) '(string 2))

  (coerce '(0 1) '(simple-bit-vector 3))

  (make-sequence '(vector * 2) 3)

  (make-sequence '(vector * 4) 3)

  (concatenate '(vector * 2) "a" "bc")

  (map '(vector * 4) #'cons "abc" "de")

  (merge '(vector * 4) '(1 5) '(2 4 6) #'<)


Rationale:


  If CLtL hadn't overlooked this situation, it's likely that it would have

  said it "is an error".  The best translation of that to ANSI CL error

  terminology seemed to be "should signal".  There doesn't seem to be any

  reason to require signalling this error even in unsafe code.  There

  doesn't seem to be any reason to define this situation to do something

  other than signalling an error, such as ignoring the length in the

  type specifier or forcing the sequence to have the correct length by

  truncating or extending it with elements of implementation-dependent

  value.


Current practice:


  Symbolics Genera 7.2 and 7.4 usually ignore the length in the type

  specifier in the above situations, but sometimes signal an error.

  The type of error signalled is sometimes somewhat random.

  Other implementations were not surveyed.


Cost to Implementors:


  This does not seem like difficult checking to add.  I have not examined

  the code in any implementation to try to evaluate what it would cost.


Cost to Users:


  None.


Cost of non-adoption:


  Aesthetic.


Performance impact:


  Probably small, just have to keep track of the length when dealing

  with sequence type specifiers in safe code.  I have not attempted to

  evaluate the exact impact.


Benefits:


  Less ambiguity in the language specification.  Less deviation among

  implementations, hence fewer porting problems.


Esthetics:


  Since the length field is present in sequence type specifiers, it

  seems unesthetic to ignore it, and even more unesthetic not to say

  what is done with it.


Discussion:


  Moon doesn't know what error condition is appropriate.  TYPE-ERROR

  doesn't seem quite appropriate here.  One idea is not to say, just let it

  be any subtype of ERROR.  Another idea is to produce the result object

  and then signal a TYPE-ERROR that this object doesn't match the

  type-specifier for the result type.


  Cassels points out that two similar operations are defined in CLtL to be

  inconsistent with each other:


  (replace (make-array 4) #(1 2 3)) just picks the shortest length, and

     "the extra elements near the end of the longer subsequence are not

     involved in the operation" so the result is #(1 2 3 NIL)


  #4(1 2 3) duplicates the last element, so it's like #(1 2 3 3)

  #2(1 2 3) "is an error".