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Equality Predicates

Lisp has taught us that there are several kinds of equality of objects. In C++ the == operator can only express one kind of equality. Also without void* casting it can not be applied to any two objects dynamically.

The == operator in Lpp can be applied to any two Lpp objects dynamically and means the same as the eq function of Common Lisp. However an eq function is still supplied so that an eq function object can be passed to some functions that take an optional predicate function argument. Also note that eq as opposed to == is a true Lpp predicate in that it returns either t or nil.

eq x y Function

eq returns t if x is the same object as y and nil otherwise.

eql x y Function

eql is the same as eq except that if x and y are Characters or Numbers of the same type their values are compared.

equal x y Function

equal returns t if x and y are structurally similar (isomorphic) objects and nil otherwise. Roughly speaking, t means that x and y print the same way.

equalp x y Function

equalp returns t if x and y are are equal; if they are characters and are eql ignoring alphabetic case; if they are numbers and have the same numerical value, even if they are of different types; or if they have components that are all equalp.

Objects that have components are equalp if they are of the same type and corresponding components are equalp. This test is implemented in a recursive manner and may fail to terminate for circular structures. For conses, equalp is defined recursively as the two car's being equalp and the two cdr's being equalp.

Here are some example of all these equality predicates

     eq(L(23), L(23)) => nil
     eql(L(23), L(23)) => t
     equal(L(23), L(23)) => t
     eq(L('a'), L('a')) => t
     eql(L('a'), L('a')) => t
     equal(L('a'), L('A')) => nil
     equalp(L('a'), L('A')) => t
     eq(S(foo), S(foo)) => t
     eql(S(foo), S(foo)) => t
     equal(S(foo), S(foo)) => t
     equal(S(foo), S(Foo)) => nil
     equalp(S(foo), S(Foo)) => nil
     eql(L("foo"), L("foo")) => nil
     equal(L("foo"), L("foo")) => t
     equal(L("foo"), L("Foo")) => nil
     equalp(L("foo"), L("Foo")) => t
     equalp(S(foo), L("foo")) => nil