Issue DECLARE-FUNCTION-AMBIGUITY Writeup

Forum:        Cleanup

Issue:        DECLARE-FUNCTION-AMBIGUITY


References:   CLtL pp 43 (Table 4-1), 158-159

		Issue FUNCTION-TYPE (passed X3J13/June 1988)


Category:     CHANGE


Edit history: #1, 21 Sept 1988, Walter van Roggen

              #2, 29 Sept 1988, Walter van Roggen (renamed; lessened ambiguity)

              #3, 30-Sep-88, Masinter

              #4,  5-Dec-88, Masinter (append Oct x3j13 comments)


Problem description:


CLtL permits confusing and ambiguous FUNCTION declarations.  One can say

  (DECLARE (FUNCTION F (VECTOR INTEGER) T))

to indicate that the function binding for F is of a certain type.  Yet

one can also say

  (DECLARE (FUNCTION X Y Z))

to indicate that the variables X, Y, and Z have values which are functions.


The former is an abbreviation for

  (DECLARE (FTYPE (FUNCTION (VECTOR INTEGER) T) F))

The latter is an abbreviation for

  (DECLARE (TYPE FUNCTION X Y Z))


The ambiguity arises in a case like

  (DECLARE (FUNCTION F NIL STRING))

However, it would be an error to declare the type of the constant NIL to be a

FUNCTION, so technically there is no ambiguity--there is just a peculiar

special case.


Using the same declaration for two completely different purposes can lead

 to confusion among people writing or reading such code.


It is now more useful to declare that variables have a value which

is of type FUNCTION, since the type FUNCTION has a more well-specified

meaning.


Proposal (DECLARE-FUNCTION-AMBIGUITY:DELETE-FTYPE-ABBREVIATION)


The declaration (FUNCTION name argtypes valtypes) is no longer permitted

to be an abbreviation for (FTYPE (FUNCTION argtypes valtypes) name).


The declaration (FUNCTION var1 var2) would just be an abbreviation for

(TYPE FUNCTION var1 var2).


Rationale:


Continuing to allow all the predefined atomic type specifiers as declaration

abbreviations for (TYPE type var1 var2 ...) is simpler for users to understand.

In other words, all the normal type declarations describe variable bindings;

only the FTYPE declaration describes function bindings.  This is a more

uniform solution than making an exception to table 4-1.


Since the old use of the FUNCTION declaration for function bindings was just

an abbreviation for the FTYPE declaration, no expressivity is lost.

Furthermore one is able to say that a variable's value is of type FUNCTION,

something that hadn't been clearly possible without using the TYPE declaration.


Current Practice:


Many current implementations treat FUNCTION declarations 

only as abbreviations for FTYPE declarations, primarily because

the proposal FUNCTION-TYPE has not been widely implemented.


Cost to Implementors:


Likely to be small to those implementations that heed these kinds of

declarations; none for those that don't.


Cost to Users:


Existing uses of the FUNCTION declaration for function bindings will need

to be changed to FTYPE declarations.


Cost of Non-Adoption:


People will continue to be confused by function declarations.


Benefits:


A simpler language.


Esthetics:



Discussion:


Making it clear that only FTYPE declarations describe function bindings will

make it easier to add new kinds of declarations that support incremental or

additional descriptions, as is needed for describing methods.


Since all cases can be disambiguated after all, compatibility considerations

might deem this proposal to be unnecessary.  However, making declarations

simpler and less confusing is possibly more important than compatibility.


There is no consensus on the cleanup committee.


Comments from October 1988 X3J13 meeting:


...    opposed but not vehemently--the incompatible change is gratuitous

...    prefer to document the

	 issue rather than change it."

...    a number of implementations accidentally implement

   	 this incorrectly. They first check the type table and then handle

	 the elaborate function declaration style. But, as it happens,

	 they never reach the code for the second case because function is

	 in the type table!

...    Having both styles is worse than having either one or the other.