"Richard A. O'Keefe"
On 6/10/2015, at 10:16 pm, Kosyrev Serge <_deepfire@feelingofgreen.ru> wrote:
As an example of the opposite end, consider Common Lisp, which:
1. reifies READ as the third processing phase (macroexpansion[1] and actual compilation being the other two) 2. operates at expression granularity 3. makes the full power of the language available to support the decision process of what subexpressions are available in which contexts
Except that the syntax for conditional reading in Common Lisp, #+ <feature> <datum> #- <feature> <datum>
(a) is utterly different from conditional evaluation, like (if <expr> <expr> <expr>). (b) does NOT make the full power of the language available. The feature test is a Boolean combination, using and, or, not of atoms, where the value of an atom is true iff it is a member of *features* (the asterisks are part of the name).
There are several issues mixed up here: 1. Desirability of expression-level granularity of pre-processing. 2. The desired palette of options for this pre-processing (if desired), that varies in expressive power. 3. The difference between #+/#- and #. (aka *READ-EVAL*, http://clhs.lisp.se/Body/v_rd_eva.htm) in Common Lisp. My original message was about point #1, but point #2 deserves a similar kind of attention. The point #3 is purely didactic -- the READ-time processing is layered in Common Lisp into: 1. the basic layer, covering 95% of cases, which uses the following primitives, to decide if an expression suffix needs to be included: - *FEATURES*, a variable containing a list of features - ability to test for presence of a certain symbol within *FEATURES* - ability to combine atomic tests using AND, OR, NOT It is, indeed, what is covered in the example that I have provided. 2. the full language layer, available through #. -- whatever is returned by the expression following #. is inserted as program text. This layered approach has some merit: - It provides a kind of frugal brevity, that is sufficient in 95% of cases - Whenever the soft option fails, it unleashes the whole language to express basically anything during that pre-processing phase. Except, in the latter case it /probably/ doesn't count as pre-processing anymore, since the inserted text is no longer external to the pre-processor directives. -- с уважениeм / respectfully, Косырев Серёга -- “And those who were seen dancing were thought to be insane by those who could not hear the music.” – Friedrich Wilhelm Nietzsche