Deprecating fromIntegral

[Evan Laforge]

On Thu, Sep 21, 2017 at 4:19 PM, Niklas Hambüchen <mail at nh2.me> wrote:
> On 21/09/17 19:27, Evan Laforge wrote:
>> If I were to suddenly get 10,000 lines of warnings...
>
> If the built tool is set up sensibly (perhaps we should make it so if
> that is not already possible, but I think these days warnings of
> dependency packages are omitted anyway, as cabal and stack build them in
> parallel), you should get (and care about and fix) only warnings about
> those occurrences that are in the packages you maintain / build directly.

I mean dependencies within the project, not external packages.  If
modify something in the middle of the dependency tree, I'll be usually
recompiling (say) a couple of hundred modules, for my personal
medium-ish project.  I assume in a real company with a larger codebase
you could easily be recompiling thousands.  Maybe just the mono-repo
ones though.

Also I'm assuming most code is not in packages, and is not in hackage.
I have no real basis for that, other than experience with other
languages.  But, as shown below, my guesses can be pretty wrong:

> Most packages have < 3 occurrences of fromIntegral, a few have < 20
> occurrences. Very few have more than 20.
>
> Assuming we go for a naming improvement and add 3 functions ala
> maybeFromInteger / runtimeCheckedFromInteger / fromIntegerWrap, a
> package owner would have to step through their deprecation warnings and
> replace each with one of the new 3 functions to get their package
> warning-free.

I'm surprised about that, I assumed more.  Grepping for fromIntegral
in a local project, I get 259 occurrences over 693 modules... I
expected a lot more.

However, a lot of those I think would be false positives, because
they're converting across newtypes, where the underlying type is the
same.  I'm not sure what I would use for those... I guess it would be
fromIntegerWrapped, but it's kind of verbose and awkward and is
implying wrapping when it's not actually present.  Maybe they should
be using 'coerce'?  Most of them existed before coerce did, but that's
not an excuse any more.  I think I'd be more likely to try out
int-cast.

> I could probably already have fixed 1000 of the 7000 total occurrences
> on Hackage myself within the time it as taken me to debug integer
> conversion related problems in GHC/base in the last week.
>
> (In other words, these changes are cheap and have high payoff.)

Of course that's the bottom line, so if it's the case then it's hard
to argue against.

I've had similar bugs due to sloppy haskell -> C FFI conversions so
I'm sympathetic to the idea.  It's just a blanket deprecation on
fromIntegral seems like a broad brush.