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On Tue, 23 Aug 2005, Marcin 'Qrczak' Kowalczyk wrote:
Andre van Tonder <andre@xxxxxxxxxxxxxxxxx> writes:Syntax as lists (or isomorphic to) represents the highest level of abstraction (on the input) that preserves the meaning of Scheme code.I would not call lists a higher level of abstraction than distinguished types for syntax.
What I meant was that you get a higher level of abstraction by ignoring details (location, comments, etc.) that do not affect the meaning. When you ignore precisely all these details, you get something that is isomorphic to a list, every time.
It's not orthogonal. If various instances of the syntax representing 5 are indistinguishable, it's impossible to attach different information to them. It would be a pity if the implementation could not report a compile time warning for (3 4) with source location only because it was passed through a macro.
As an example of what I mean, invoking syntax-error in the reference implementation will display the complete backtrace of all intermediate expansion steps back to the original offending source expression, for which a source location can be displayed with reader support. While I am not advocating any particular mechanism now, I personally find this more useful than just a source location, as some Schemes do, and it is orthogonal to the syntax-object representation, since nothing needs to be attached to the syntax data.