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2 points by Pauan 3045 days ago | link | parent

You're right, if you allow for users to overload multiple macros onto the same name based upon type, then it gets very messy.

But I think if your language allows for that, it's inconsistent with the way that macros work.

If you want powerful Arc-style macros, then the macro must be run before any types are known.

Any system which allows for that kind of type-based dispatch is different from a Lisp-style macro system, and is probably less powerful.

For example, you won't be able to create macros which create new variable bindings (like "let", "with", etc.) because the type of the variable is not known until after the macro is run.

I'm not sure what a system like that would look like, or if I would even call it a macro system.

I think it makes more sense to have two separate systems: a macro system that deals with syntax only, and an overloading system that deals with types. That's what Nulan does.

But perhaps there is room for a third system, somewhere in between the two: that third system could do type-directed syntax manipulation.

2 points by Pauan 3042 days ago | link

I thought about this some more.

I have no experience with Idris or dependent types, so I may be completely wrong.

But from my understanding, a dependent type system allows for types to include values (which are evaluated at compile-time).

If so, then you might not even need macros at all to create the "," behavior.

Instead, you create a typeclass which will dispatch to the appropriate behavior:

  data Pair a b = pair a b

  interface Comma a b c where
    comma : a -> b -> c

  Comma Type Type Type where
    comma = Pair

  Comma a b (Pair a b) where
    comma = pair
Now whenever you use the "comma" function, it will dispatch depending on whether its arguments are Types or not:

  -- Type
  comma Integer Integer

  -- (Pair Integer Integer)
  comma 1 2
And since types may include arbitrary expressions, you can of course use the "comma" function inside of a type:

  foo : a -> b -> (comma a b)
Note: all of the above code is completely untested and probably wrong, but you get the idea.

Basically, your language needs first-class types, and the ability to run arbitrary expressions at compile-time (even within types), and then you can use an ordinary typeclass to get the desired type dispatch. No macros needed.


2 points by rocketnia 3040 days ago | link

"If so, then you might not even need macros at all to create the "," behavior."

I never said the overloading of "," was accomplished with macros.

The example I gave was of a macro whose name was overloaded in a type-directed way, similarly to the way "," is overloaded in Idris. (Maybe the macro itself is named ",".) My point is that if the macro system is designed to support that kind of overloading, then sometimes a macro call will depend on intermediate results obtained by the typechecker, so we can't run the macroexpander in a phase of its own.


For the sake of remembering Idris more accurately, I checked out the Idris docs to look for the particular "macro system" I was dealing with.

It turns out what I mean by the "macro system" in Idris is its system of elaborator extensions ( The elaborator's purpose is to fill in holes in the program, like the holes that remain when someone calls a function without supplying its implicit arguments.[1]

It's pretty natural to handle name overloading in the same phase as implicit arguments, because it's effectively a special case. Thanks to dependent types and implicit arguments, instead of having two differently typed functions named (,), you could have one function that implicitly takes a boolean:

  (,) : {isType : Bool} ->
    if isType then (... -> Type) else (... -> (a, b))
  (,) = ...
The elaborator phase is complex enough that I don't understand how to use it yet, but I think almost all that complexity belongs to implicit arguments. Name overloading is probably seamless relative to that.

Another "macro system" I encountered in the docs was Idris's system of syntax extensions ( As far as I can tell, these apply before the elaborator and typechecker, but they might not be able to run arbitrary code or break hygiene. Maybe someday they'll gain those abilities and become a Nulan-like macro system.

[1] Implicit arguments effectively generalize type classes. Idris still has something like type class declarations, which it calls "interfaces," but I bet this is primarily so a programmer can define the interface type and the method lookup functions together in one fell swoop.