What is the difference between quote and list?

Question

I know that you can use \' (aka quote) to create a list, and I use this all the time, like this:

> (car \'(1 2 3))
1
         


        

Answer 1

The behavior you are seeing is a consequence of Scheme not treating symbols as functions.

The expression '(+ - * /) produces a value which is a list of symbols. That's simply because (+ - * /) is a list of symbols, and we are just quoting it to suppress evaluation in order to get that object literally as a value.

The expression (list + - * /) produces a list of functions. This is because it is a function call. The symbolic expressions list, +, -, * and / are evaluated. They are all variables which denote functions, and so are reduced to those functions. The list function is then called, and returns a list of those remaining four functions.

In ANSI Common Lisp, calling symbols as functions works:

[1]> (mapcar (lambda (f) (funcall f 1)) '(+ - * /))
(1 -1 1 1)

When a symbol is used where a function is expected, the top-level function binding of the symbols is substituted, if it has one, and everything is cool.

If you want to use list to produce a list of symbols, just like '(+ - * /), you have to quote them individually to suppress their evaluation:

(list '+ '- '* '/)

you will see that if you map over this, it will fail in the same way.

The error message you are being shown is misleading:

expected a procedure that can be applied to arguments
given: '+

The applicator was not given (quote +) at all, but +. What's going on here is that the symbol + is being printed in "print as expression" mode, which is a feature of Racket, which is what I guess you're using.

In "print as expression" mode, objects are printed using a syntax which must be read and evaluated to produce a similar object. See the StackOverflow question "Why does the Racket interpreter write lists with an apostroph before?"