The Grinnell Scheme Web: Quote-expressions

I still don't understand exactly what the single quotation mark in a pair literal is for. Is there another way to explain it?

Yes. The single quotation mark is actually an abbreviation for another of Scheme's keywords, quote. Pair literals are a kind of quote-expression.

A quote-expression consists of a left parenthesis, the keyword quote, a datum, and a right parenthesis. The datum can be a numeral, a Boolean literal, or a pair (that is, two data separated by a dot and enclosed in parentheses), or indeed any kind of direct literal representation of a Scheme value.

The result of evaluating any quote-expression is the datum it contains. No attempt is made to evaluate that datum itself; quote is not a procedure being applied to an operand, but just a way of marking the fact that what it encloses is to be taken literally as a value in its own right:

> (quote 5)
5
> (quote #f)
#f
> (quote (1 . 2))
(1 . 2)

Since it's not going to be evaluated, does the datum even have to be a legal expression of Scheme?

No, it does not. The syntax rules for data allow lots of things to qualify as data that are not legal expressions in Scheme:

> (quote if)
if
> (quote (let (lambda . let)))
(let (lambda . let))
> (quote ())
()

And the single quotation mark is just a shorthand way of writing quote-expressions?

That's right. The single quotation mark replaces the outermost parentheses of a quote-expression and the keyword quote. They are completely interchangeable. Your Scheme interpreter may even substitute one for the other when echoing data:

> (quote (quote (1 . 2)))
'(1 . 2)

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created August 6, 1995
last revised December 30, 1995