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## Issue CONCATENATE-SEQUENCE Writeup

    Issue:        CONCATENATE-SEQUENCE
    Forum:	      Editorial
    References:   CONCATENATE (p249), COERCE (p51),
    	      MAKE-SEQUENCE (p249), MAP (p249), MERGE (p260),
    	      Issue SUBTYPEP-TOO-VAGUE
    Category:     CLARIFICATION/CHANGE
    Edit history: 01-Mar-91, Version 1 by Pitman
    	        (only about (CONCATENATE 'SEQUENCE ...))
    	      15-Mar-91, Version 2 by Pitman
    	        (generalize per JonL, Moon, Barmar, MacLachlan)
    	      17-Mar-91, Version 3 by Pitman
    	        (clarify for MacLachlan; also, include MAKE-SEQUENCE)
    	      31-May-91, Version 4 by Pitman (amendments per X3J13)
    Status:	      X3J13 passed v3 with amendments on 10-1 vote, Mar 91.
    	      v4 reflects those amendments.

    Problem Description:

      CLtL (p249) says that the RESULT-TYPE argument to CONCATENATE
       ``must be a subtype of SEQUENCE, as for the function COERCE''

      CLtL (p51) says:
       ``Any sequence type may be converted to any other sequence
         type, provided the new sequence can contain all actual
         elements of the old sequence ... If the sequence is already
         the specified type, it may be returned without copying it;
         in this, (COERCE sequence type) differs from
         (CONCATENATE type sequence), for the latter is required to
         copy the argument seqeunce.  In particular, if one specifies
         SEQUENCE, then the argument may simply be returned if it
         already is a sequence.

      The passage about CONCATENATE suggests that sequence subtypes
      valid to COERCE are valid to CONCATENATE.  But the type SEQUENCE
      is a subtype of SEQUENCE that is clearly permissible to COERCE.

      COERCE, MAKE-SEQUENCE, MAP, and MERGE are also impacted similarly.

    Terminology:

      Draft 8.81 of the specification defines the following term:

        recognizable subtype n. (of a <type>)
         a <subtype> of the <type> which can be reliably detected
         to be such by the <implementation>.

      The intent is that SUBTYPEP will be described as a function which
      returns "T, T" when the first type argument is a "recognizable subtype"
      of the second.

    Proposal (CONCATENATE-SEQUENCE:X3J13-MAR-91):

      1. Specify that CONCATENATE, MAKE-SEQUENCE, MAP, and MERGE are
         defined only when the target type argument is either a
         recognizable subtype of LIST or a recognizable subtype of
         VECTOR.  [MAP is also permitted to special case NIL as a
         target type in a manner consisting with existing practice.]
         Otherwise, an error is signaled.

      2. Specify that if COERCE receives a target type argument which is
         a recognizable subtype of SEQUENCE, but which is neither a
         recognizable subtype of LIST nor a recognizable subtype of
         VECTOR, then the argument to be coerced must already be
         of that type (in which case, COERCE behaves like IDENTITY for that
         argument).  Otherwise, an error is signaled.

      3. a. Specify that if COERCE, CONCATENATE, MAKE-SEQUENCE, MAP,
    	or MERGE receives a target type argument which is any subtype
    	of LIST, then the result is of type LIST.

         b. Specify that if COERCE receives a target type argument that
            is a subtype of array, and the object is already of that type,
    	COERCE returns that object as its result.

         c. For COERCE in situations not covered by 3b,
    	CONCATENATE, MAKE-SEQUENCE, MAP and MERGE
    	when the target type argument is a subtype of ARRAY:

    	 * If the implementation can determine the element type
     	   specified for the target type, the element type of the
    	   resulting array is the upgraded array element type of
    	   the specified element type.

    	 * Else if the implementation can determine that the element
    	   type is unspecified, (or ``*'') the element type of the
    	   resulting array defaults as for MAKE-ARRAY (to T).

    	 * Else an error is signaled.

    Rationale:

      This gives users a sense of what they can expect at minimum,
      and also gives implementors a sense for how they can go about
      making extensions without violating user expectations.

      Note that although that specific terminology is not used, the
      specific constraints on recognizable subtypes are effectively
      spelled out in issue SUBTYPEP-TOO-VAGUE.

    Test Case:

     #1: (CONCATENATE 'SEQUENCE '(A B C) "DEF")

     #2: (COERCE '#1=#(1 0 1) '(OR NULL (EQL (1 0 1))))

     #3: (FLET ((SAME-TYPE-P (X Y)
    	      (VALUES (AND (SUBTYPEP X Y) (SUBTYPEP Y X)))))
           (SAME-TYPE-P
    	 (ARRAY-ELEMENT-TYPE (MAKE-SEQUENCE '(ARRAY (UNSIGNED-BYTE 5)) 3))
    	 (UPGRADED-ARRAY-ELEMENT-TYPE '(UNSIGNED-BYTE 5))))
         => T  ;or signals an error, if (UNSIGNED-BYTE 5) is not upgradable.

    Current Practice:

     Symbolics Genera seems to approximately implement this proposal,
     although it isn't quite right on the LIST side (e.g., it fails on test
     case #2).

    Cost to Implementors:

      Small.

    Cost to Users:

      None.  This was already not portable.

    Cost of Non-Adoption:

      The language is left ambiguous.

    Benefits:

      A tighter specification--better portability.

    Aesthetics:

      Negligible.

    Discussion:

     Pitman supports this.

     An earlier version of this proposal worried specifically about the case
     of function CONCATENATE and specifically about the argument named SEQUENCE
     but it was shown that the problem was bigger along both axes, so v2 starts
     over trying to do things a different way.

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