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3 points by raymyers 6149 days ago | link | parent

While we're on the subject, I know a number of people have been saying that Arc's unhygenic macros problems are a problem, because things like this break in a lisp-1:

    (mac break (a b) `(list ,a ,b))
    (let list nil (break 1 2))  =>  Error
However, you can simply unquote the function values in the macro.

    (mac dont-break (a b) `(,list ,a ,b))
    (let list nil (dont-break 1 2))  =>  (1 2)


3 points by cadaver 6149 days ago | link

To make this work for functions, you need to add ((procedure? s) s) to 'ac. Otherwise with arc2.tar:

  arc> (mac dont-break (a b) `(,list ,a ,b))
  #3(tagged mac #<procedure>)
  arc> (let list nil (dont-break 1 2))
  Error: "Bad object in expression #<procedure: list>"
Macros still remain a problem:

  (mac list-macro parms `(,list ,@parms))
  (mac break (a b) `(,list-macro ,a ,b))
  (break 1 2) => Error

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2 points by raymyers 6149 days ago | link

I stand corrected, it worked in Anarki :)

And yeah, it obviously wouldn't work for macros, but macros don't get shadowed by let blocks in the first place.

    (mac list-macro args `(list ,@args))
    (mac dont-break (a b) `(list-macro ,a ,b))
    (let list-macro nil (dont-break 1 2))  =>  (1 2)

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1 point by cadaver 6149 days ago | link

Then it is I who shall stand corrected :)

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2 points by bogomipz 6149 days ago | link

A problem with unquoting the function value is that if you redefine the function, any code that uses the macro will hold on to the original function as a literal. This means the solution doesn't lend itself well to exploratory programming.

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2 points by raymyers 6149 days ago | link

Yes, it is true that preventing redefinition will in fact prevent redefinition. Bear that in mind when preventing redefinition :)

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1 point by bogomipz 6149 days ago | link

Well, one could create a module system so that the macro would always see the function the way it was seen in the module where the macro was defined. Then, when the function name is rebound where the macro is applied, it doesn't affect the macro, but if the function is recompiled in its original module, it does affect the code produced by the macro. I'm not saying it's straight forward, but I do believe it's possible.

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