DECLARE-FUNCTION-AMBIGUITYFUNCTION 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.
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).
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.
FUNCTION declarations only as abbreviations for FTYPE declarations, primarily because the proposal FUNCTION-TYPE has not been widely implemented.FUNCTION declaration for function bindings will need to be changed to FTYPE declarations.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.