Re: [Caml-list] destructive local opens

[Goswin von Brederlow <goswin-v-b@web.de>]

On Tue, Aug 04, 2015 at 08:51:34AM +0200, Petter Urkedal wrote:
> On 2015-08-03, Daniel Bünzli wrote:
> > Le lundi, 3 août 2015 à 15:08, Nils Becker a écrit :
> > > It's possible that people actually want M.() to mean let open! more
> > > often than let open. For me I think that's the case.
> > 
> > If you are in the vector case, I don't think that's the case. With Gg [1] I often had the following kind of subtly wrong code (can't remember the exact details but something similar):  
> > 
> > let ox = V2.((dot v ox) * ox) in
> > V2.(3 * ox + oy)
> > 
> > The reality is that M.() is inherently dangerous, especially from an API evolution perspective where new identifiers with matching type may get introduced later leading to silent semantic changes in your code. So we should not encourage people to use M.!() as it's going to make the problem even more acute. Besides we should have that 44 warning by default so that we see the problems, but for now it's impossible to live with 44 and a Gg like library.
> 
> This suggests another option.  If type information is available at the
> point this warning is emitted, then the warning could be issued only in
> the case when the type of the shadowing identifier matched that of the
> shadowed identifier.
> 
> This assumes the common case for shadowing is to redefine operators or
> common functions at a custom type, the use of M.() being an alternative
> to overloading.  Loosely the warning should be emitted if the chosen
> identifier is not the one which would have been chosen by some sensible
> overloading scheme, but instead we make a simple estimate.
> 
> This could still go wrong, since the type required by the context may be
> general than the type of both the shadowed and shadowing terms, so a
> better rule might be to issue the warning if both are admissible in the
> given context, though my guess is that's harder to implement.

I like the idea but how feasable is it? Most of the time I figure the
type being infered from the operator being used. The use of M.(*)
makes it the custom type while simple (*) would make it int. The type
system would have to track the ambiguity until some other use of the
arguments or result decide the proper type. And if it doesn't resolve
then emit the warning.

MfG
	Goswin