Mirah Office Hours: Bam!

Last week was a busy week in Mirah land. We reopened github issues and consiliens awesomely went and moved over all the open issues from code.google.com. There were quite a lot of them and consiliens went through marking dups and closing issues he could not reproduce/were fixed. Plus, with the changes I checked in on Sunday during my office hours, the tests are passing again (on my machine anyway).

Office Hours.

I set high goals for myself and I met some of them. My main goal was to fix the issue where block arguments to a macro were scoped incorrectly and Bam! fix it I did.

I also wanted to start prepping for an 0.0.8 release, as well as fixing/verifying some of the issues that had been pulled over from google code.

On my second attempt to get blocks working with macros, I gave up on the deep dive approach and looked for the simplest thing that could possibly work. While I was diving in the code, I noticed that the block node was not a parameter. I’d figured out last week that all the parameters for the macro call were wrapped in a new ScopedBody, but I hadn’t realized that the block wasn’t and THAT was why look up was failing for it. Once I saw that, the answer was simple.

I fixed it by doing the same thing to the block body that was done to the rest of the parameters. I wrapped it in the correct scope before it got typed and that made the look ups work correctly. I’m not saying that my fix is the best way of ensuring that scoping works like you’d expect–but it is consistent with what the code was doing before, and isn’t a huge hack and it has tests–well one, anyway.

So, what does macro block scoping mean anyway?

Well, when you write a macro, you want to be able to reference variables defined where you called the macro from within the macro. Prior to the fix, this would only work with parameters. e.g. if you called my_macro some, expression, some and expression would be looked up in the outer scope. But this was not the case when you passed a block to the macro. In that case, the block would think it was in the macro’s scope. That meant that expressions in the block would look to the macro’s scope instead of where you’d expect–where the macro was called. That meant code like

a = 1
loop do
a += 1
break if a > 10

wouldn’t work because a += 1 would think it was in a different scope than a = 1, and wouldn’t know who or what a was supposed to be.

ARGV — Bout 1
I read this thread about command line args last week. It made me wonder how hard it would be to provide a way of accessing them in the body of a script. Right now mirah doesn’t do anything with additional commandline arguments. And, you can’t access them unless you are explicitly defining a main method on a class. I thought it would be nice to have an equivalent to Ruby’s ARGV in the implicit main method, for doing scripting. As things are right now, if you want to use commandline args, you need to define a main method explicitly:

class Main
def self.main args:string[] : void
puts args[0]

when what you’d like to do is more like

puts args[0]

which reads much better.

It turns out implementing that is a bit of a pain. The reason is because the implicit main method is not represented in the AST. Oh, the code that makes up the body of the implicit main method is in the AST, but it’s not there as a method body on a class. Generating the main method happens at code generation time, after the AST has been processed (more or less).

In order to access the argv in a script, the String[] args from Java’s public static void main(String[] args) need to be in the AST, or at least accessible to the typer when it is inferring the types of everything.

My first approach was to pull out all the AST nodes related to the implicit main(ie, everything not a class, interface, package definition, or import) and define a main method on the appropriate class with those nodes in the body. That actually worked ok, and allowed you to access argv because it was a parameter to the method whose body all those top level expressions ended up in.

I’m not sure this is the best way to do it, and it certainly isn’t the only one. But it was pretty straight forward. The problem I ran into was that in code like

puts args[0]

the class the main method hangs off of is never defined in the AST–which means you can’t define a new main method on it without creating it first. I didn’t get to that point. I don’t think it would be too hard though.

While, that is certainly a viable approach, you could also change the Script node so that argv returns something when you look it up. I’m not sure whether I like that better though, and I don’t quite know how I’d implement it that way, but it’d fun to try and see how it works out.

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