Re: The dreaded M-R

On Tuesday 31 January 2006 01:37, Andrew Pimlott wrote:

On Tue, Jan 31, 2006 at 12:57:18AM +0100, lennart@augustsson.net wrote:

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Quoting Andrew Pimlott :

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On Mon, Jan 30, 2006 at 11:06:29PM +0100, lennart@augustsson.net wrote:

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So I envisage that you'd turn off the warning in the same way as you turn off the M-R today: by a type signature.

But if people were happy adding type signatures for every polymorphic variable definition, they wouldn't be moving to eliminate the M-R, would they? Or do I misunderstand?

Well, my feeling is that the M-R is an ugly wart, and I want it gone. But I'm still happy to put a type signature when I want something to be polymorphic.

Ok, I understand your position now. But even given this view, I think the warning will be problematic. First, when will the warning be emitted? For all variable assignments without signatures, or only for those that the implementation fails to monomorphize (as an optimization)? The first is heavy-handed;

Agreed.

the second, obviously implementation-dependent (someone using another implementation, or another optimization level, will get the warning and will lose sharing).

And what, if I may ask, is the problem with that? I mean, I am used to different implementations having, sometimes drastic, differences in speed or memory use. Getting a warning about possible loss of efficiency due to loss of sharing might be /very/ helpful; and it can safely be ignored by a beginner, as long as it is unmistakenly flagged as such (i.e. an efficiency warning, nothing /wrong/ with the code). Furthermore, you would turn on such a warning only if you are expressly interested in your program's efficiency. And in this case I would be thankful to get as many (substantial) warnings as possible.

Second, a warning about "loss of sharing" may befuddle beginners (who are usually not taught to write type signatures at the start).

Well, maybe when someone implements this warning, we will find out I'm wrong and it doesn't cause trouble. And I agree with removing the M-R, with or without the warning.

I would make it a strong recommendation for implementations to provide a warning about loss of sharing due to 'accidental' (=inferred) polymorphism/overloading, whenever it /actually happens/. But I would demand that it is not turned on by default, so as not to unsettle the unsuspecting beginner. Ben