Arc Forumnew | comments | leaders | submitlogin
1 point by stefano 5930 days ago | link | parent

If the reader can be configured (e.g. by specifying wich read table to use) then two modules that uses different reading conventions can coexist by simply using their own configuration.


1 point by almkglor 5930 days ago | link

Now programmer C wants to use both programmer A's module and programmer B's module. Which readtable does he use so that he can freely intermix macros from A with macros from B, which have different expectations on the reader?

Reader hacking is nice, but I don't see it often in CL libraries (note: counterexamples are welcomed; it's not like I've made an exhaustive search for them). Any reader hack must make the cut of being a good, generic enough meaning that it will always be used by everyone; take for example the Arc-type [ ... _ ... ] syntax

-----

1 point by stefano 5930 days ago | link

A and B should provide macros to let C write:

  (with-A what%ever)
  ...
  (with-B what%ever)
CLSQL modifies the read table to let you write embedded SQL queries such as [select "A" [where [= ...]]] and similar (I've never studied the exact syntax, but this should give you the idea). The special reader in CLSQL can be activated/disactived through function calls that modifies the default reader.

-----

1 point by almkglor 5929 days ago | link

But B doesn't want to treat #\@ or whatever syntax specially. Why should B bend over backwards to support this?

For that matter: what if C wants to use a macro in A within the context of a macro in B or vice versa?

  (with-A
    (macro-A
       @foo
       (with-B
         (macro-B
           (prn "this is:" '@foo)))))
?

> The special reader in CLSQL can be activated/disactived through function calls that modifies the default reader.

Not a bad idea. Difficult in SNAP though - which default reader? I'd have to add yet another process-local variable.

-----

2 points by cchooper 5925 days ago | link

It looks like CLSQL needs reader macros to switch the syntax on and off locally. If Arc had reader macros, then you could do this:

  #.(with-A (mac macro-A ..blah..blah..in special A syntax))
Assuming 'with-A is a function that set the read table locally, and macro-A uses quasi-quote to generate its result, this will produce a macro that produces standard Arc syntax, even though it's written in A syntax.

With reader macros, 'w/html could be implemented even if de-sugaring were moved to the reader, although you'd have to call it with #. all the time.

It makes sense to me that macros should always expand to vanilla Arc syntax (or maybe even pure s-exps without any ssyntax) so that they are portable across environments.

-----