Re: Type Specifier: (OR (FUNCTION ...) ...)

[Guy Steele <gls@THINK-AQUINAS.ARPA>]

    Date: 19 Apr 1986 17:08-EST
    From: NGALL@G.BBN.COM
	
	Date: Sat, 19 Apr 1986  16:24 EST
	From: "Scott E. Fahlman" <Fahlman@C.CS.CMU.EDU>
    
	Regarding your proposal that we allow forms like the following
    
	    (or (function (list) list)
		(function (vector) vector))
    
	as a way of specifying that the output type of a function matches its
	input type...

	Your proposal seems to take one small step in the direction
	of such a declarative language, and then it stops.
	...
    ...

I don't think it is a proposal.  I can find nothing in CLtL that forbids
using such a form right now.  I find no justification in CLtL for a
compiler's insisting that a type specifier given to FTYPE be of the form
(FUNCTION ...).  Unless a compiler can prove that a declaration is
actually inconsistent, it should not signal an error.  (Warnings are
another matter and, as always, subject to judements of taste.)  A compiler
is not justified in signalling an error just because it doesn't know
how to make use of a perfectly legitimate declaration.

					... If someone were to say
    
	(or (function (list sequence) foo)
	    (function (sequence list) bar))
    
	then what do we do?  

    According to the definition of the OR type-spec on pg. 45, "it always
    tests each of the component types in order from left to right and
    stops procesing as soon as one component of the union has been found
    to which the object belongs."

But hold on here, now.  That specification applies specifically to the case
of TYPEP processing the type specifier, not to the case of using the type
specifier in a declaration.  Also, do not make the mistake of thinking that
that one can decide which of several function type specifiers applies to
a function call by examining the argument types only.  If function BAZ has
the type shown above, then we are entitled only to conclude that the
call (BAZ '(a) '(b)) will return an object of type (OR FOO BAR).

But here is a more subtle point.  The above type states that BAZ is either
a function of type (FUNCTION (LIST SEQUENCE) FOO) or a function of type
(FUNCTION (SEQUENCE LIST) BAR).  Assume FOO and BAR to be disjoint types.
Suppose BAZ is called twice in succession, and the result of the first call
is determined to be of type FOO:
	(WHEN (TYPEP (BAZ '(a) '(b)) 'FOO)
	  (BAZ '(c) '(d)))

We have no cause to believe that BAZ will change between the two calls,
and so we are entitled to deduce that the second call to BAZ, if
executed, will necessarily return a FOO and not a BAR, because the first
call produced a FOO and thus BAZ is of type (FUNCTION (LIST SEQUENCE) FOO).

So maybe Nick isn't actually expressing quite what he wanted to in the
first place.
--Guy