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GeneralizedEqualCowan

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== Generalized `equal?` predicate == `(generalized-equal? `''obj1 obj2''` . `''comparator-list''`)` Compares ''obj1'' and ''obj2'' for equality. A ''comparator'' is a procedure that is given two arguments to compare. It returns `#t` if its arguments are to be considered equal, `#f` if they are to be considered unequal, and the symbol `pass` if it cannot decide. The third argument passed to a comparator is ''comparator-list'', to be used in recursive calls to `generalized-equal?`. First, each element of ''comparator-list'' is invoked on ''obj1'' and ''obj2'', passing ''comparator-list'' as its third argument. If the comparator returns `#t` or `#f`, that is the result. If all comparators in the list have been invoked with a `pass` result, then `generalized-equal?` determines if both ''obj1'' and ''obj2'' are ordered containers of the same type. This determination is partly implementation-dependent, but pairs, strings, vectors, and bytevectors must be treated as ordered containers of distinct types. If they are not both ordered containers of the same type, then `generalized-equal?` returns what `eqv?` returns on ''obj1'' and ''obj2''. Otherwise, if the containers have different numbers of elements, the result is `#f`. Otherwise, `generalized-equal?` invokes itself recursively on each corresponding element of the containers, passing itself the same comparators. If any recursive call returns `#f`, that is the result; if all recursive calls return `#t`, that is the result. `(make-atomic-comparator `''type-predicate compare-predicate''`)` Returns a comparator that invokes ''type-predicate'' on its first and its second arguments. If they both return `#t`, then they are assumed to be of the same type, and ''compare-predicate'' is invoked on the first and second arguments together. If the result is `#t` or `#f`, then the comparator returns `#t` or `#f` respectively. If they are not of the same type, a third value is returned. The resulting comparator always ignores its third argument. == Standard comparators == `(numeric-comparator `''obj1 obj2 comparators-list''`)` A comparator that returns `#t` if ''obj1'' and ''obj2'' are numbers that are equal by `=`, `#f` if they are not equal by `=`, and a third value otherwise. The ''comparators-list'' argument is ignored. `(char-ci-comparator `''obj1'' ''obj2 comparators-list''`)` A comparator that returns `#t` if ''obj1'' and ''obj2'' are both characters that are equal by `char-ci=?`, `#f` if they are not equal by `char-ci=?`, and a third value otherwise. The ''comparators-list'' argument is ignored. `(string-ci-comparator `''obj1 obj2 comparators-list''`)` A comparator that returns `#t` if ''obj1'' and ''obj2'' are both strings that are equal by `string-ci=?`, `#f` if they are not equal by `string-ci=?`, and a third value otherwise. The ''comparators-list'' argument is ignored. == Issues == Should the third value be specified? As designed, if a badly written comparator returns nonsense, it's just ignored rather than giving the implementation of `generalized-equal?` a chance to report an error. The symbol `pass` has been suggested.

2013-05-22 19:53:55

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