SUBTYPEP-TOO-VAGUE
The description of SUBTYPEP allows it to return a second value of NIL when the relationship between the two types cannot be determined. In some cases this is a reasonable thing to do because it is impossible to tell (if the SATISFIES type specifier is involved), and in other cases the relationships between types are not well-defined (for example, the VALUES type specifier or the list form of the FUNCTION type specifier).
Some implementations, however, have apparently interpreted this to mean that it is permissible for SUBTYPEP to "give up" and return a second value of NIL in some cases where it actually would be possible to determine the relationship. This makes it difficult to depend on subtype relationships in portable code.
[Addition for version 5]
There are two problems with version 4. First is that of the first three bulleted points in the version 4 proposal:
* Clarify that SUBTYPEP will return a second value of NIL only when either of the type specifiers involves the SATISFIES, NOT, AND, OR, MEMBER. SUBTYPEP will not return a second value of NIL when both arguments involve only the words in Table 4-1, or names of DEFSTRUCT- or DEFCLASS-defined types, or user-defined deftypes that expand into only those words and/or names.
* SUBTYPEP should signal an error when handed (for either argument) a type specifier that involves VALUES or the list form of the FUNCTION type.
* SUBTYPEP must always return values T T in the case where the two type specifiers (or their expansions) are EQUAL.
any two have significant overlap, and indeed all three can overlap; version 4 contained no indication of how this conflict should be resolved.
Second is that version 4 calls for SUBTYPEP to signal an error (at least at high safety)even when the arguments are valid type specifiers, but this can make it harder to use SUBTYPEP. These are cases that returning NIL NIL was supposed to cover.
This version replaces the three bulleted points above with a single point and some observations about its consequences. This version eliminates the requirement to signal an error.
SATISFIES, MEMBER, NOT, etc. if it either contains it directly as a type specifier, or as the result of expansion of a DEFTYPE defined type specifier. (While the type specifiers listed in CLtL Table 4-1 and names of DEFSTRUCT or DEFCLASS-defined types may, in some cases, be implemented in terms of DEFTYPE, they are to be regarded for this purpose as not being "user-defined". Therefore, SUBTYPEP must regard elements as primitive with respect to the question of returning NIL NIL.)
* Clarify that SUBTYPEP is permitted to return NIL NIL only when at least one argument involves SATISFIES, AND, OR, NOT, MEMBER, VALUES, or the list form of FUNCTION.
Note that one consequence of this is that if neither argument involves any of these type specifiers, then SUBTYPEP is obliged to determine the relationship accurately. In particular, SUBTYPEP must return T T if the arguments are EQUAL and do not involve any of the above-stated type specifiers.
* Clarify that the relationships between types reflected by SUBTYPEP are those specific to the particular implementation. For example, if an implementation supports only a single type of floating-point numbers, in that implementation (SUBTYPEP 'FLOAT 'LONG-FLOAT) would return T T (since the two types would be identical).
SUBTYPEP makes it more useful. Otherwise, programs cannot rely on any more than NIL NIL as return values.
It is generally conceded that it is impossible to determine the relationships between types defined with the SATISFIES specifier. MEMBER, AND, OR, and NOT are messy to deal with.
SUBTYPEP in (the original) HPCL does not try to expand type specifiers defined with DEFTYPE and does not recognize EQUAL type specifiers as being equivalent. Most other implementations appear to be substantially in conformance with the proposal.SUBTYPEP.SUBTYPEP. This proposal does not require any implementation to "handle" fewer cases of SUBTYPEP.SUBTYPEP will be more portable.FLOAT and SINGLE-FLOAT appeared to be the consensus from a discussion on the common-lisp mailing list some time ago.
It would not be too onerous to require implementations to handle the cases where one but not the other type specifier involves OR, AND, NOT or MEMBER, but the specification becomes cumbersome.
A related issue is clarifying what kinds of type specifiers must be recognized by functions such as MAKE-SEQUENCE and COERCE. For example, HPCL complains that (SIMPLE-ARRAY (UNSIGNED-BYTE *) (*)) is not a valid sequence type when passed to MAKE-SEQUENCE, although SUBTYPEP does recognize it to be a subtype of SEQUENCE. Should this proposal be extended to deal with these issues, or is another proposal in order?
The rules for comparing the various type specifiers (such as ARRAY) need to be spelled out in detail.