Crosstwine Labs

From: redacted

Hi,

I’m puzzled: I thought list and ' (the single quote operator) were equivalent, but they produce different results in the following examples:

1. a=1 b=2 c=list(a b)

   |- (1 2)

2. a=1 b=2 c='(a b)

   |- (a b)

Can you tell me the difference between these expressions?

From: Damien Diederen <dd@crosstwine.com>
Date: Wed, 23 Jan 2013 09:58:47 +0100

Hi redacted,

Yes. Example 1 is easy; focusing on the right side of the assignment,

list(a b)

is a normal function call. Each argument is to be evaluated, and the results—the contents of the a and b variables—applied to the list function, which builds a fresh list of its arguments. The result is thus:

(1 2)

Example 2 is a bit more complicated to understand. The first thing to remember is that:

'(a b)

is just a syntax shortcut for:

quote((a b))

This is very important, because quote is not a normal function. It is a special operator which admits a single argument, and returns it unevaluated. In this case, the single argument to quote is:

(a b)

which, by the rules of the SKILL/SKILL++ program text parser (the reader), is a list of two symbols, a and b. The result is thus simply:

(a b)

Note that that list is built once, by the reader, and never touched again. Every evaluation of quote will return that same list object. This is very different from e.g.:

list('a 'b)

which, while it produces a similar-looking result, allocates a fresh list on each call; that difference is crucial if the list if to be modified afterwards!

Hope this helps,
Damien Diederen