So I was listening to the discussion about opApply in the "lazy" thread.  Somebody mentioned that opApply would be better if we could eliminate the int return code.

So I thought, "well, you could make the unusual situations (return, break, etc.) exceptions, but that would be too slow..."

Then I thought, "wouldn't it be cool if the compiler could recognize that we had a void return value, and somehow convert the exception into a simple return code..."

Then I thought, "what if we could delcare a function's return type to be 'exception'?"

The idea here is that if you had return type 'exception', then exceptions coming out of that function would be returned as return values rather than thrown with the ordinary mechanism.  The calling function would be responsible for checking that return code and handling it properly if an exception were returned.

So, inside this type of function, "throw" statements would be converted (by the compiler) into "return"s.  And when you called a function of this type, it would check the return code and throw the proper exception if one was returned.  In something like opApply, where you have this type of function calling this type of delegate, the compiler-generated code would look like opApply does now.

PROPOSED SYNTAX
	exception opApply(exception delegate(ref MyType) dg)
	{
		<do some looping stuff>
			dg(<thing>)
			// compiler checks the return code for us!
	}
END SYNTAX

Thoughts?

Hello Russell,

> The idea here is that if you had return type 'exception', then
> exceptions coming out of that function would be returned as return
> values rather than thrown with the ordinary mechanism.
[...]
>Thoughts?
> 

An interesting idea. Might be of particular use if you have a function (or a stack of them) that throws a lot it could result in a major speed boost.

== Quote from Russell Lewis (webmaster@villagersonline.com)'s article
> So I was listening to the discussion about opApply in the "lazy" thread.
>   Somebody mentioned that opApply would be better if we could eliminate
> the int return code.
> So I thought, "well, you could make the unusual situations (return,
> break, etc.) exceptions, but that would be too slow..."
> Then I thought, "wouldn't it be cool if the compiler could recognize
> that we had a void return value, and somehow convert the exception into
> a simple return code..."
> Then I thought, "what if we could delcare a function's return type to be
> 'exception'?"
> The idea here is that if you had return type 'exception', then
> exceptions coming out of that function would be returned as return
> values rather than thrown with the ordinary mechanism.  The calling
> function would be responsible for checking that return code and handling
> it properly if an exception were returned.
> So, inside this type of function, "throw" statements would be converted
> (by the compiler) into "return"s.  And when you called a function of
> this type, it would check the return code and throw the proper exception
> if one was returned.  In something like opApply, where you have this
> type of function calling this type of delegate, the compiler-generated
> code would look like opApply does now.
> PROPOSED SYNTAX
> 	exception opApply(exception delegate(ref MyType) dg)
> 	{
> 		<do some looping stuff>
> 			dg(<thing>)
> 			// compiler checks the return code for us!
> 	}
> END SYNTAX
> Thoughts?

I like the back-end compiler optimization part of this for void return types. However, I don't like the idea of making the programmer responsible for checking the error code.  The nice thing about exceptions is that if you don't handle an exception because you believe that it can't happen in your case, then you've basically got an assert, and if you were wrong and it CAN happen, you'll know about it fast.  They're also nice when you're writing a quick and dirty prototype, because the default behavior given bad input (such as a file that doesn't exist) is to fail in a reasonable way.  Realistically, in a throwaway prototype I would never bother to check error codes, and this would probably lead to some frustrating "bugs" that were really caused by bad input.

dsimcha wrote:
> I like the back-end compiler optimization part of this for void return types.
> However, I don't like the idea of making the programmer responsible for checking
> the error code.  The nice thing about exceptions is that if you don't handle an
> exception because you believe that it can't happen in your case, then you've
> basically got an assert, and if you were wrong and it CAN happen, you'll know
> about it fast.  They're also nice when you're writing a quick and dirty prototype,
> because the default behavior given bad input (such as a file that doesn't exist)
> is to fail in a reasonable way.  Realistically, in a throwaway prototype I would
> never bother to check error codes, and this would probably lead to some
> frustrating "bugs" that were really caused by bad input.

I think you're missing the point of what I'm saying.  What I'm suggesting is that when you call a function with return type "exception," the compiler would automatically check the return code for you, and auto-throw the exception if one is returned.  It's the syntax of exceptions, with the execution speed of return codes.