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schemequote

What happened in scheme when defining a symbol to be something else?

I'm a beginner of the Scheme language.

Recently I found that the data type symbol can be displayed using quote, like this:

> 'E
E
> (quote E)
E

However, if the code below executed, every kind of quote may fail:

> (define 'E 123)
> 'E
E: undefined;
 cannot reference an identifier before its definition
> 'abc
abc: undefined;
 cannot reference an identifier before its definition

So what happend when code (define 'E 123) being executed?

Solution

  • First you asked Scheme to evaluate (define 'E 123). Let's put a quote in front of that, to see what it looks like without the ' shorthand. You can always do this: quote any expression to ask Scheme, "What do you think this value is?"

     > '(define 'E 123)
=> (define (quote E) 123)

    Well, in Scheme, (define (x ...) ...) is a shorthand for (define x (lambda (...) ...)): it's just a convenient shorthand for defining a function. So in this case (define (quote E) 123) is the same as (define quote (lambda (E) 123)). Thus, the symbol you are redefining is quote, and you define it to be a function of one parameter which always returns 123.

    Next you asked to evaluate 'E. Again let's expand that to look through the shorthand:

     > ''E
=> (quote E)

    You now call the quote function you defined, and pass it the variable E as an argument. But E has not previously been defined, so this fails. If you wanted to, you could first define E to have any value, and then perhaps 'E would return 123. It rather depends on what Scheme evaluator you are using: the one I found does not much appreciate it when you try to redefine quote, but apparently yours does not mind, so I suspect you would get 123, and that you would get the same result if you defined abc and then evaluated 'abc.