'(quote quote) in scheme

问题:

I'm trying to learn scheme by myself. Could anyone tell me why '(quote quote) will output 'quote, and '(quote 'quote) will output ''quote?

Thank you very much!

回答1:

This expression:

'(quote quote)

... after expanding '<something> to (quote <something>) is equivalent to (quote (quote quote)), notice that the symbol quote is being quoted two times, and this expression is evaluated and printed as ''quote.

On the other hand, this expression:

'(quote 'quote)

... is equivalent to (quote (quote (quote quote))), notice that the symbol quote is being quoted three times, and this expression is evaluated and printed as '''quote.

回答2:

Take a look at (free, online) How To Design Programs, intermezzo 2. It explains quote in terms of list and cons. If anything in that explanation doesn't make sense, just back up a bit in the textbook.