Re: [Caml-list] destructive local opens

[Petter Urkedal <paurkedal@gmail.com>]

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.