Fixing type synonyms to Uniq(D)FM newtypes

[Spiwack, Arnaud]

In a way, if these types need to exist at all, they probably should be
newtypes. That being said, I'm pretty sure that the current APIs are
incomplete, so turning these into newtypes may be, in fact, quite a bit of
work.

But if we are starting this discussion, I'd like to understand first why
all these types exist at all? Why not use `UniqFM` everywhere?

/Arnaud

On Tue, Jan 14, 2020 at 7:16 AM Ömer Sinan Ağacan <omeragacan at gmail.com>
wrote:

> > but I still don't quite understand the motivation
>
> I give a concrete example (something that happened to me that I had to
> debug in
> runtime) in the issue I linked in my original post.
>
> > For example, delVarEnv will only work with a Var, not a Name.
>
> delVarEnv will happily accept a NameEnv in its first argument, which is the
> problem I was trying to describe.
>
> Ömer
>
> Richard Eisenberg <rae at richarde.dev>, 14 Oca 2020 Sal, 01:55 tarihinde
> şunu yazdı:
> >
> > I'd be fine with making these newtypes, but I still don't quite
> understand the motivation. Note that the specialized functions for the
> different instances of UniqFM all have type signatures. For example,
> delVarEnv will only work with a Var, not a Name.
> >
> > Was there a different scenario that you want to avoid?
> >
> > Thanks,
> > Richard
> >
> > > On Jan 13, 2020, at 9:10 AM, Ömer Sinan Ağacan <omeragacan at gmail.com>
> wrote:
> > >
> > > Hi,
> > >
> > > UniqFM and UniqDFM types are basically maps from Uniques to other
> stuff. Most of
> > > the time we don't actually map Uniques but instead map things like
> Vars or
> > > Names. For those we have types like VarEnv, NameEnv, FastStringEnv,
> ... which
> > > are defined as type synonyms to UniqFM or UniqDFM, and operations are
> defined
> > > like
> > >
> > >    extendFsEnv = addToUFM
> > >    extendNameEnv = addToUFM
> > >    extendVarEnv = addToUFM
> > >
> > > This causes problems when I have multiple Uniquables in scope and use
> the wrong
> > > one to update an environment because the program type checks and does
> the wrong
> > > thing in runtime. An example is #17667 where I did
> > >
> > >    delVarEnv env name
> > >
> > > where `name :: Name`. I shouldn't be able to remove a Name from a Var
> env but
> > > this currently type checks.
> > >
> > > Two solutions proposed:
> > >
> > > - Make these env types newtypes instead of type synonyms.
> > > - Add a phantom type parameter to UniqFM/UniqDFM.
> > >
> > > If you could share your opinion on how to fix this I'd like to fix
> this soon.
> > >
> > > Personally I prefer (1) because it looks simpler but I'd be happy with
> > > (2) as well.
> > >
> > > Ömer
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> >
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