Tuesday, May 10, 2011

Seven Languages: Io

I took the train from home in Upstate New York to New York City today to
go to the Google I/O streaming sessions tomorrow, and it's a beautiful
day along the Hudson. I took the opportunity to work through the Io
chapter of Seven Languages.

Io is a prototype language, meaning that to create new objects, one just
clones an existing object, called a prototype, rather than instantiating
a class. I found this to be a simple model that makes reflection very
simple and natural. There is no reason to have, for example, Python
meta-classes, when you don't even have classes! To "inherit"
functionality, you clone your base object, add some functionality
dynamically, and then clone from your new prototype.

Another feature of Io I found distinguishing was message passing as a
means to call methods on objects. I had used messages before, but only
in Objective-C. I don't know if I was just doing it wrong, but message
passing in Objective-C basically amounted to normal functions calls with
a weird syntax, less compile-time checks, and named parameters.

Message passing in Io was much more obviously potent. Message passing
in Io is clearly sending a chunk of data to an object which may (or may
not) result in the execution of a method at some point. For example, to
pass parameters in function calls in most languages, the callee
evaluates expressions in the parameters, pushes the arguments and return
address onto the stack, and jumps into the callee.

This means that if you tried to write a function in C like

return_type if(cond, then_code, else_code)

That evaluated condition then returned one of the 2 branches, you would
have big problems with side-effects because both sides of the branch
would be executed before entering the if function.

In Io, the code in both of those clauses is simply passed in as part of
the message arguments. The callee can check the condition then execute
the side that they wish to. This could be done in C by wrapping that
code up in some kinda of data like a function-pointer + context, but in
Io it's automatic. Similarly, message passing can trivially handle
things like co-routines and asynchronous, deferred, or delayed
invocation, because they very nicely bundle up that act of calling a

One annoyance is that there isn't much syntactical support for the
collections classes. They still offer all the expected features as well
as nice transformation operations like map and select, but it still
requires a little more imperative "add this to a list, then this, then
this" rather than really nice declarative collections processing.

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