Re: What do we mean by "Identical" ?

newser.bbs@xxxxxxxxxxxxxxxxx wrote:
> Harald , I sincerely wish you have good time during your riding in the
> warm breeze.
> I think there is misunderstanding between us. I am not a pedagog. In
> fact , I have confessed that I am new to lisp.
>
> I believe that you are right that the arguments should be evaluated
> before the calling of the function , but this still makes me more and
> more confused . What do you think makes "eq" different from "eql"?
> Following those procedures you taught me , I can't tell "eq" apart from
> "eql".
>
> Again , in "Practical Common Lisp" , Peter tells us that "expression
> (eq 3 3) can legally evaluate to either true or false" . Following your
> procedures , I'm too foolish to see why it is possible for (eq 3 3 ) to
> evaluate to false.
>
> Many thanks if anyone can give me a hint !

Only because the spec says so: "numbers with the same value need not be
eq". If an implementation uses EQ to compare memory addresses, maybe
the two 3's are stored in different memory locations.
http://www.lisp.org/HyperSpec/Body/fun_eq.html

There are multiple equality operators in Common Lisp. After all, what
is the philosophical meaning of "equality"?

So:
(eq 3 3) is not always true, but might be.
http://www.lisp.org/HyperSpec/Body/fun_eq.html

(eql 3 3) is always true.
http://www.lisp.org/HyperSpec/Body/fun_eql.html#eql

(equal 3 3) is always true.
http://www.lisp.org/HyperSpec/Body/fun_equal.html#equal

(equalp 3 3) is always true.
http://www.lisp.org/HyperSpec/Body/fun_equalp.html#equalp

(= 3 3) is always true.
http://www.lisp.org/HyperSpec/Body/fun_eqcm_sleq__lteqcm_gteq.html#EQ


Tayssir

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