Status: passed, Jun 89 X3J13
References: TYPE-OF (CLtL)
88-005, p 43f, Condition system types
Related issues: SUBTYPEP-TOO-VAGUE
Edit history: Version 1, 1-Dec-88, Masinter
Version 2, 9-Dec-88, Masinter
Version 3, 12-Dec-88, Masinter (fix "egregious bug")
Version 4, 3-14-89 Steele
Version 5, 16-Mar-89, Masinter (add other amendments)
Version 6, 22-Jun-89, Masinter (fix as per Moon)
The specification of TYPE-OF in CLtL is so weak as to leave it
nearly useless, if any implementor actually took the specification
seriously.In particular, there are almost no constraints on the
value that TYPE-OF might return for any of the built in
types. The only constraint placed is that for objects created by the
constructor function of DEFSTRUCT, the result of TYPE-OF is the name of the
This means that implementations could return T for all other objects.
Specify that the result of TYPE-OF satisfies the following:
(a) For any object for which (typep object built-in-type) is true when
built-in-type is one of
INTEGER, RATIO, FLOAT, COMPLEX, NUMBER, NULL, SYMBOL,
STRING, BIT-VECTOR, VECTOR, ARRAY, RANDOM-STATE, SEQUENCE,CONS,
STREAM, PACKAGE, CHARACTER, FUNCTION, READTABLE, HASH-TABLE, PATHNAME,
RATIONAL, SHORT-FLOAT, LONG-FLOAT, SINGLE-FLOAT, DOUBLE-FLOAT
the result of TYPE-OF will be a type specifier for which
(subtypep (type-of object) built-in-type) returns T T, i.e., either the
build-in-type or a subtype of it (a subtype that the
implementation's SUBTYPEP can recognize.)
(b) For all objects, (TYPEP object (TYPE-OF object)) will be true.
(this implies that it will not be NIL, since no
object is of type NIL.)
(c) will not be T, or use SATISFIES, AND, OR, NOT or VALUES.
(d) The type returned by TYPE-OF is always a subtype of the class
returned by CLASS-OF, and is a subtype that the implementation's
SUBTYPEP can recognize.
(e) For objects of metaclass STRUCTURE-CLASS or STANDARD-CLASS,
TYPE-OF returns the proper name of the class returned by CLASS-OF
if it has a proper name, and otherwise returns the class itself.
In particular, for objects created by the "constructor" function
of a structure defined with DEFSTRUCT without a :TYPE option,
TYPE-OF will return the structure name.
This proposal is intended to be consistent with 88-002R,
and not to conflict with any of the definitions in that document.
It is legal for (TYPE-OF "abc") to return (SIMPLE-STRING 3), or just
STRING. It is legal for (TYPE-OF 112312) to return SI:MEDIUM-SIZE-FIXNUM,
as long as (SUBTYPEP 'SI:MEDIUM-SIZE-FIXNUM 'INTEGER).
(defvar *foo* (make-array 5 :element-type t))
(class-name (class-of *foo*)) => SIMPLE-VECTOR
It is legal for
(type-of *foo*) => (SIMPLE-VECTOR 5)
The original specification for "TYPE-OF" was written to allow
implementation freedom, but the wording is in fact more vague than was
Most Common Lisp implementations seem to implement the proposal, at least
for the types we've tried them on.
Cost to Implementors:
Even if there are implementations that do not currently implement the
proposal, the required changes for them are likely to be minimal.
Cost to Users:
Cost of non-adoption:
TYPE-OF would remain potentially inconsistent.
Bring the specification of TYPE-OF in like with its common
implementation, while requiring it to be generally useful.
Originally, TYPE-OF was concieved as being useful for information display.
CLOS specified CLASS-OF far more precisely, in that it must return exactly
the class of an object. Unless TYPE-OF is removed from the language, it
seems reasonable to require it to be at least as precise as CLASS-OF.
The accepted CLOS specification does not define CLASS-OF
in terms of TYPE-OF, but an earlier draft did.
This proposal presumes the (passed) proposals
DATA-TYPES-HIERARCHY-UNDERSPECIFIED:DISJOINT, FUNCTION-TYPE, and
accomodates the Condition System (version 18) and CLOS.
If ARRAY-TYPE-ELEMENT-TYPE-SEMANTICS does not pass,
and this proposal does pass, it may be that the only way an
implementation can satisfy (TYPEP array (TYPE-OF array))
would be to have (TYPE-OF array) return VECTOR or ARRAY
as appropriate, i.e., with no element type qualification.
It is possible to constrain TYPE-OF further, e.g., to eliminate
the possibility that it might return a non-portable
implementation-specific value. For example,
(TYPE-OF 112312) should not return SI:MEDIUM-SIZE-FIXNUM, but rather some
thing like (SIGNED-BYTE 17) [if that's what SI:MEDIUM-SIZE-FIXNUM means.]
We would like to make this as an implementation recommendation,
however, rather than as a constraint, at this point.
About part (c): the restriction on T is meaningless, since the CLASS-OF
restriction dominates it. The restriction on MEMBER is OK, just to keep
TYPE-OF from being the silly definition that (type-of x) = `(member ,x). I
think we're running out of good reasons to leave out SATISFIES, AND, OR,
NOT, and VALUES; they're probably no more useful, but we're probably
overspecifying for no good reason.
Some types are left out of the list in (a), including:
BIGNUM and FIXNUM were left out because their division was implementation
KEYWORD was left out because under odd circumstances it is possible to
dynamically change the 'type' (e.g., by UNINTERN).
STANDARD-CHAR and STRING-CHAR were left out for the same reasons they
aren't built-in classes.
(These reasonas are not 100% compelling.)