3 ms·

Can you describe more about how this works, specifically how it's an improvement over setitem and setattr? I'm interested in how various languages hand

by zwegner 12y ago

Can you describe more about how this works, specifically how it's an improvement over __setitem__ and __setattr__? I'm interested in how various languages handle this, mainly so I can steal ideas for my project Mutagen, a purely functional Pythonic language. I recently added modification operators that behave rather like a nestable setitem/setattr. Basically: list_of_objects = list_of_objects <- [index].attr = 'abc' I've been thinking about ways to make this extensible and flexible (e.g. returning a list of effects to apply to an object, or having some post-processing for any bookkeeping an object needs). The copatterns are pretty cool looking, but I'm not sure how I'd fit it into Mutagen's imperative-esque syntax.

sqrt17 12y ago
It's simply more general. And it's really easy to write macros that do interesting things. So, incf, which is CL's +=, could be defined as (defmacro (incf lexpr val) `(setf ,lexpr (+ ,lexpr val)) which would evaluate "lexpr" both as a place to put the new value in and as an expression to get the old value out. If you wanted to do that in terms of purely-functional monad application, it gets a bit hairier... Basically, you've got things like "+=" which are mutators (i.e. functions Int->Int). And you've got places, which either (i) take an X mutator and turn it into a mutator for Y, or (ii) take a Y and return an X (accessor). In your example, list_of_objects[index].attr = 'abc' would mean that you take a "constant value" mutator that takes some string and returns "abc" and turn it first into a mutator that changes the .attr value of something ... to something that changes the .attr value of index [index] of the list_of_objects member of the current data. Why would that kind of functional style be useful? Basically, because you could write something like a Python program that still allows for search/nondeterminism/backtracking etc. Some of the weirdness I see in Mutagen seems to come from a partial (or just differing) understanding of how Python and Haskell, respectively, do things. Shoot me a PM if you want to discuss more.
- zwegner 12y ago
  Haha, I was about to PM you and then realized that there aren't PMs on HN :) I always like to talk about this stuff though, if you want you can email me at zwegner@gmail.com. What you describe is somewhat similar to what I had in mind, but a more well-defined abstraction. I hadn't yet formulated it in terms of types, because for now it's just a hack in the bootstrap interpreter, and anyways a lot of Mutagen's type system is still in my head, waiting to be implemented once I have a better understanding of how/why other functional languages do this stuff.

Can you describe more about how this works, specifically how it's an improvement over __setitem__ and __setattr__? I'm interested in how various languages hand

by zwegner 12y ago

sqrt17 12y ago

It's simply more general. And it's really easy to write macros that do interesting things. So, incf, which is CL's +=, could be defined as (defmacro (incf lexpr val) `(setf ,lexpr (+ ,lexpr val)) which would evaluate "lexpr" both as a place to put the new value in and as an expression to get the old value out. If you wanted to do that in terms of purely-functional monad application, it gets a bit hairier... Basically, you've got things like "+=" which are mutators (i.e. functions Int->Int). And you've got places, which either (i) take an X mutator and turn it into a mutator for Y, or (ii) take a Y and return an X (accessor). In your example, list_of_objects[index].attr = 'abc' would mean that you take a "constant value" mutator that takes some string and returns "abc" and turn it first into a mutator that changes the .attr value of something ... to something that changes the .attr value of index [index] of the list_of_objects member of the current data. Why would that kind of functional style be useful? Basically, because you could write something like a Python program that still allows for search/nondeterminism/backtracking etc. Some of the weirdness I see in Mutagen seems to come from a partial (or just differing) understanding of how Python and Haskell, respectively, do things. Shoot me a PM if you want to discuss more.

zwegner 12y ago
Haha, I was about to PM you and then realized that there aren't PMs on HN :) I always like to talk about this stuff though, if you want you can email me at zwegner@gmail.com. What you describe is somewhat similar to what I had in mind, but a more well-defined abstraction. I hadn't yet formulated it in terms of types, because for now it's just a hack in the bootstrap interpreter, and anyways a lot of Mutagen's type system is still in my head, waiting to be implemented once I have a better understanding of how/why other functional languages do this stuff.