From: Ben Giddings <bg-rubytalk@...>
Date: 2003-11-22T04:28:34+09:00
Subject: Interface checking

I've been skimming over the thousands of emails about "stereotyping" for 
the last few days, and I thought I'd step in now and say something.

First of all, please Austin and Sean, keep the personal attacks off the 
list.  There's no need for that.  You both have strong views, and that's 
great, but there's no need to expose the rest of us to your personal 
attacks.  (And Sean, that does include complaining about Austin's off-list 
personal attacks as well).  Just keep it professional on the list, please.

Now for the actual issue.

If I try to compile the following C program, I'll get some compile-time errors:

float convert(float temperature) {
   return (temperature - 32) * 0.6;
}

int main(int argc, char **argv) {
   printf("Temp is %d\n", convert(32.0));
   printf("Temp is %d\n", convert(32));
   printf("Temp is %d\n", convert("hot"));
   return 0;
}

It will tell me that "hot" is an incompatible argument to "convert".  In 
this case, that's true.  The C compiler does a great job of catching this 
error.  It is helpful because it identifies the function  If "convert" were 
declared in an external library and the documentation of it were somehow 
inaccessible this is just the sort of information you'd want, before you 
actually tried to use the program.

If you try a brain-dead simple Ruby implementation of the same thing, you 
get an error, but it isn't nearly as informative:

def convert(temp)
   (temp - 32.0) / 0.6
end

puts "Temp is " + convert(32.0).to_s
puts "Temp is " + convert(32).to_s
puts "Temp is " + convert("hot").to_s

foo.rb:2:in `convert': undefined method `-' for "hot":String (NoMethodError)
         from foo.rb:7


If you use "duck typing", in the sense that you trust whatever is passed to 
the "convert" method can be converted to a float, you get no errors.  On 
the other hand, the result probably isn't what you want.

def convert(temp)
   (temp.to_f - 32.0) / 0.6
end
...

Temp is 0.0
Temp is 0.0
Temp is -53.3333333333333


Things are actually worse if you assume whatever is passed to the program 
can be converted to farenheit:


def convert(temp)
   (temp.to_farenheit - 32.0) / 0.6
end
...

foo.rb:2:in `convert': undefined method `to_farenheit' for 32.0:Float 
(NoMethodError)
         from foo.rb:5

The problem I see with all these errors is twofold:

1) There's no way to spot them until you actually call the "convert" method
2) The error message isn't very informative compared to the C error message 
which tells you the name of the function and that you're passing an 
inappropriate type.

Now, a lot of these problems can be overcome by making a less brain-dead 
"convert" method, but IMHO that isn't the best strategy.  That approach 
requires the person writing the library method to be able to anticipate all 
the ways in which someone might misuse the methods they provide, but at the 
same time, not fail when the method is used in an unexpected way that would 
actually have worked.  Most library writers are either not smart enough 
(me) or too lazy (me) to do that.  I also don't think that testing is the 
answer either, laziness and difficulty in writing complete tests being the 
reason there too.

Obviously, for Ruby the solution isn't to implement static type checking. 
The general solution isn't to check the class of the argument inside the 
method either.  In a dynamic language like Ruby that's either too 
restrictive or prone to error.  I don't think checking "respond_to?" is a 
solution either.  Take the case above where I ended up doing "hot".to_f. 
It responds to that method, but it is unlikely that the person passing 
"hot" in as an argument wanted it interpreted as 0 farenheit.

If we want to get from Ruby what we get from static type checking in C, we 
need a way to get the two things above: the name of the method that failed 
and the reason it failed.

Consider the error message you get in the first example:

foo.rb:2:in `convert': undefined method `-' for "cold":String (NoMethodError)

The problem I have with this error message is that it isn't clear that 
"cold" is the argument to the method, and it isn't clear why exactly it 
failed.  The error message says that '-' is an undefined message, but (at 
least to me) that doesn't translate to "you passed me a String but I was 
expecting something like a number".

Now maybe it is possible to modify Ruby so that when you define a method, 
it automatically wraps it in a begin/end block that catches NoMethodErrors 
and, if it determines that the object that caused the NoMethodError is an 
argument to the method it modifies the error to say something like:

foo.rb:2:in `convert': undefined method `-' for "cold":String; Possibly a 
an illegal argument to convert?  (NoMethodError)

What would make things even more interesting is if somehow it were possible 
to find out what methods an object is expected to implement in the course 
of a method, so given a method like:

def convert(temp)
   if temp.respond_to?(:to_farenheit)
     (temp.to_farenheit - 32.0) / 0.6
   else
     (temp.to_f - 32.0) / 0.6
   end
end

It might know that temp needs to implement "respond_to?" and one of 
"to_farenheit" or "to_f".  Then you could get an error message like:

foo.rb:2:in `convert': undefined method '-' for "cold":String; Possibly a 
an illegal argument to convert?  Required methods: [:respond_to?, 
:to_farenheit] or [:respond_to?, :to_f]  (NoMethodError)

I think that would be incredibly useful, but most likely very, very 
difficult to implement.

Anyhow, there's still the other problem: C finds argument-type errors at 
compile time, Ruby finds them at runtime.  Is there a way to have Ruby find 
them immediately rather than when the method is called?  Because of the 
dynamic nature of Ruby, the only possible answer to this is 'no'.  But, 
having said that, there are other possibilities.

When you run ruby with the '-d' flag set, it prints to stderr all 
exceptions, whether caught or not.  This means that it often prints things 
to stderr that don't need to be there.  Maybe it is possible to come up 
with a similar type checking mechanism.  At "compile time" (whatever that 
means in an interpreted language) it would try to look for type 
inconsistencies, meaning arguments to methods that don't implement the 
required methods.  It could print those out as warnings, then continue, 
hoping that by the time it actually calls those methods, things will have 
been resolved.  How this would be done?  I have *no* idea.  It sure sounds 
difficult, but it would sure be useful.

Now even of all the above could be done, there would still be errors.  Just 
because a method exists doesn't mean it does the right thing.  A method 
named 'read' might be a verb telling an IO-type object to read something, 
or it might represent a boolean flag indicating a message-type object has 
been read.

Maybe the easy solution for the short term is to do like Python and emacs-lisp:

def foo(bar)
"Foo takes a 'Bar' as a parameter.  It will try to do X, Y and Z with it"
   ...
end

irb> foo("hot")
in `convert': undefined method `-' for "hot":String (NoMethodError)
irb> foo.docstring
"Foo takes a 'Bar' as a parameter.  It will try to do X, Y and Z with it"
irb> foo(25.0)
5.0
irb>

This docstring type functionality wouldn't help much when running programs 
on the commandline, but it would be really useful when debugging or poking 
at things with irb.

Comments?

Ben