Tuesday, February 17, 2004
literal expressions and external representation
part 4.1.2 of the spec discusses the quote operator. It says,"
(quote <datum>) evalutates to <datum>. <Datum> may be any external
representation of a Scheme object." Hrm, so what's an external representation?
It's a "sequence of characters", like "28" or "(8 13)". These sequences of
characters "may be written in a program to obtain the corresponding object",
as in the use of quote above.
so the question from before was something like, if you evaluate a quoted expression do you get back a something other than the expression itself -- a 'literal expression' type thing? according to the above, the answer is /yes/, what you get back is the external representation of the thing.
oddness. so, when does an external representation become an internal one? interpreter I - IV converted external representations to internal ones at the time of parsing.. (that's just what parsing was for). Evaluating a quoted expression, in those interpreters, has the effect of returning the expression itself, not the sequence of characters that make up the expression.
Ah, so maybe the spec is being murky again (that is, maybe my mind is being murky again as i read the spec) because look here, in the bit about external representations it says this, "Note that the sequence of characters "(+ 2 6)" is /not/ an external representation of the integer 8, even though it /is/ an expression evaluating to the integer 9 ..." Ah ha, so they /are/ calling the sequence of characters an expression, which means that if they say a quoted expression returns an external representation then they mean that it's exactly the same as returning the expression itself?
But wait, the above quotation from the spec continues, "... rather, it is an external representation of a three-element list, ..." Blurp.
so, my question remains,
macros
Reading Petrofsky's article on syntax-rules and much more from the spec, it's making some sense. So, here are the two key bits from the spec on "hygienic" and "referentially transparent":- If a macro transformer inserts a binding for an identifier (variable or keyword), the identifier will in effect be renamed throughout the scope to avoid conflicts with other identifiers...
- If a macro transformer inserts a free reference to an identifier, the reference refers to the binding that was visible where the transformer was specified, regardless of any local bindings that may surround the use of the macro
He paused for a moment's reflection."Tricky," he said finally.
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