CODEWITHSUNDEEP
bindinglispcommon-lisp
Andy
Andy

Reputation: 10830

What function in Lisp allows you to compare variable names and not what it holds?

Note: First time using lisp*

I have a homework, and we are suppose to essentially bind expressions with either a 0 or 1. Example:

(defun orexp (a b) (list 'or a b))

(setq p3 (orexp 1 'a)) ;which equals (or 1 a)

Say I wanted to create a function to evaluate them by binding them and simplify them, but ignoring the simplification, how can I bind them doing something like this:

(evalexp p3 '((a 0)))

and end up with:

(or 1 0)

I tried searching it, but I can't find anything. Please let me know if it needs a better explanation, but I am leaving information out because its a homework and I do not want answers, hints on how to go about my problem. Thanks.

Upvotes: 1

Views: 236

Answers (3)

jcc333
jcc333

Reputation: 769

Actually, eq is implementation-dependent on its specificity isn't it?:

The effect is that Common Lisp makes no guarantee that eq is true even when both its arguments are ``the same thing'' if that thing is a character or number. (from the hyperspec)

So eq should work if you quote the arguments, involved, but I think you could write a small little macro to do what you want without hand-quoting if you wanted:

(defmacro es (x y)
          `(eq (quote ,x) (quote ,y)))

works I think.

Upvotes: 0

Svante
Svante

Reputation: 51501

You can compare symbols with eq.

Your function is something like (defun evalexp (expression bindings) #| ... |#), where the commented part (#| ... |#) would contain your code. It would have to walk the expression tree, and for each symbol it finds, check whether a binding of that symbol exists in bindings, replacing the symbol with the value when that is the case.

Upvotes: 1

John Pick
John Pick

Reputation: 5650

In the title, you say you want to compare symbols (what you call variable names).

> (eq 'a 'a)
true
> (eq 'a 'b)
false

Is that what you mean?

Upvotes: 2

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