CLHS: Issue COMMON-TYPE Writeup

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## Issue COMMON-TYPE Writeup

    Issue:         COMMON-TYPE

    References:    CLtL p.35, p.76

    Category:      CHANGE

    Edit history:  Version 1, 20-Mar-89, by Moon

    Problem description:

      The type COMMON is defined in a very peculiar way and does not seem to
      be useful for anything.  It can be extended by users using DEFSTRUCT,
      but not DEFTYPE nor DEFCLASS, but it cannot be extended by implementations.
      Whether certain types such as NUMBER and ARRAY are subtypes of COMMON
      is implementation-dependent.  The goal of having the COMMON type was
      probably to improve portability, but it is unclear how it could actually
      be used in that way.

    Proposal (COMMON-TYPE:REMOVE):

      Remove COMMON and COMMONP from the language.

    Rationale:

      Keeping the definition of COMMON accurate in the new specification, in
      the face of changes elsewhere in the language such as the introduction
      of CLOS and the possible introduction of character registries, is
      difficult when no one is sure what COMMON is for.  If no one uses COMMON,
      it would be less work to just get rid of it.

    Current practice:

      Every implementation probably implements COMMON.  Moon has never seen it
      used except in a program to test whether its implementation matched CLtL.

    Cost to Implementors:

      None.

    Cost to Users:

      None unless they are actually using COMMON.

    Cost of non-adoption:

      Implementors would have to maintain COMMON.  Users would have to try to
      understand it, or figure out that they didn't care about it.

    Performance impact:

      None.

    Benefits:

      Simpler language.

    Esthetics:

      Simpler language.

    Discussion:

      None.

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