[ghc-steering-committee] A plea against "fancy types" in GHC2021

[Simon Marlow]

I would like to push back here: this seems to be suggesting a fork-like
situation, where we have two kinds of Haskell (fancy and non-fancy). I do
feel quite strongly that we should be converging on a single language
rather than creating splits. Or perhaps the suggestion is that we don't
want these extensions on by default *yet*?

Responding to Iavor's point:

> I think these extensions convey useful information about the mindset you
should use when working with a specific code base, which is quite different
from working with ordinary Haskell.

I personally have been working with these extensions enabled for all my
code for a long time now. I'm by no means a heavy user of "fancy types", I
make occasional use of type families and GADTs to solve specific problems
when they arise. But I'm not even sure what this "different mindset" is - I
certainly don't feel like I have to think differently. Of course it's
entirely possible that I'm just an unsophisticated user and if I understood
how to think about the type system with these extensions my life would be
better!

The one exception that does trip me up is MonoLocalBinds, I often have to
supply a type signature when intuitively I didn't think I needed one.

Cheers
Simon

On Tue, 8 Dec 2020 at 19:57, Richard Eisenberg <rae at richarde.dev> wrote:

> I agree with this. Fancy types are, well, fancy, and users should have to
> boldly declare that they're trying to be fancy.
>
> Richard
>
> > On Dec 8, 2020, at 11:46 AM, Iavor Diatchki <iavor.diatchki at gmail.com>
> wrote:
> >
> > Hello,
> >
> > I would like to advocate that things like `DataKinds`, `TypeFamilies`,
> and `GADTs` are not enabled by default in GHC2021.     The reason I ask for
> this is that unlike many others, I think these extensions convey useful
> information about the mindset you should use when working with a specific
> code base, which is quite different from working with ordinary Haskell.
> >
> > I do think it would be quite reasonable to have an umbrella extensions
> for FancyTypes too, which would enable all of those, I just don't think
> they should be enabled for every Haskell program.
> >
> > -Iavor
> >
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