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Issue SPECIAL-TYPE-SHADOWING Writeup

Issue:         SPECIAL-TYPE-SHADOWING

References: CLtL pages 156, 158

Related issues: DECLARE-TYPE-FREE

Category: CLARIFICATION

Edit history: Version 1, 04-Nov-88 by David Gray

Problem description:

A Common Lisp user raised the question of whether something like the

following is legal:

(PROCLAIM '(TYPE NUMBER *X*))

(DEFVAR *X*)

(DEFUN FOO ()

(LET ((*X* T))

(DECLARE (TYPE SYMBOL *X*))

(BAR)))

Page 156 of CLtL says that a proclamation is "always in force unless

locally shadowed" and page 158 says type declarations "only affect

variable bindings", which might be interpreted to mean that the DECLARE

locally shadows the PROCLAIM. However, that interpretation would make

the global type proclamation useless because it could not be relied on

when compiling a function such as BAR.

Proposal SPECIAL-TYPE-SHADOWING:CLARIFY

Clarify that if there is a local type declaration for a special

variable, and there is also a global type proclamation for that same

variable, then the value of the variable within the scope of the local

declaration must be a member of the intersection of the two declared

types.

Rationale:

Some restriction on local type declarations for special variables is

needed in order for type proclamations to be meaningful. The wording

used here was chosen for consistency with proposal DECLARE-TYPE-FREE.

Current practice:

The TI, Symbolics, and Lucid implementations do not report any error

on the example above, but it isn't clear that they really do anything

with type declarations for special variables anyway.

Cost to Implementors:

This is unlikely to require a change in any current implementation.

Cost to Users:

Anyone who has written code like the example above would have to

modify it if compilers started enforcing this restriction.

Cost of non-adoption:

A minor ambiguity in the language specification that could confuse

users.

Performance impact:

None.

Benefits:

A clearer definition of the meaning of type declarations for special

variables.

Discussion:

This is obviously very closely related to issue DECLARE-TYPE-FREE, but

this is an ambiguity in the existing language that should be resolved

even if the language extension of proposal DECLARE-TYPE-FREE is not

accepted. Note also that DECLARE-TYPE-FREE makes no mention of type

proclamations.

Other possible resolutions of the ambiguity would be to either rule

out use of local type declarations for special variables, or to say

that the local type must be a subtype of the global type.


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