wiki:GeneralizedEqualCowan

Version 7 (modified by cowan, 5 years ago) (diff)

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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 any other value if it cannot decide. The third argument of a comparators consists of a list of comparators 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 without a #t or #f 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 a recursive call returns #f, that is the result; if all recursive calls return #t, that is the result.

(predicates->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 comparator always ignores its third argument.

Sample comparators

These comparators may or may not be part of this package.

(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.

(hash-table-comparator obj1 obj2 comparators-list)

A comparator that returns #t if obj1 and obj2 are both hash tables using the same comparison key, contain the same keys (in the sense of that comparison key), and map each key to the same value (in the sense of generalized-equal?, using the comparators in comparators-list); returns #f if obj1 and obj2 are both hash tables but are distinct in the above sense; and returns a third value in all other cases.

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. My implementation uses 0.