[ghc-steering-committee] #682: Explicit Level Imports, recommendation: accept

[Arnaud Spiwack]

Mathew Pickering, Rodrigo Mesquita, and our own Adam Gundry put forward a
new proposal for the perenial problem of dependencies and Template Haskell
https://github.com/ghc-proposals/ghc-proposals/pull/682

I've got to be honest, I'm not fully convinced by the proposal. More on
that in a minute, but it learns a lesson from previous attempts at the same
problem by solving the absolute minimal problem, but this leads to a
somewhat fork-like situation for which it isn't clear whether it will be
resolved in the future. That being said, it solves a real problem which has
plagued GHC compilation forever. And I'm inclined to believe that we can't
really do much better.

But I'm getting ahead of myself. The problem is that when you have
-XTemplateHaskell in a file, all the dependencies' compiled code must
suddenly be available for typechecking. This breaks `-fno-code` and wounds
recompilation avoidance. This is probably the main reason why it's a widely
held belief that Template Haskell is slow: you use Template Haskell in a
few modules, and suddenly your IDE is much less responsive and you
recompile more files. Yay?

Anyway, the general gist of the solution is clear: we must be able to
specify that we don't want to import a module for Template Haskell (there
is subtleties in this too as you will want a little more control than that
for cross-compilation reasons which I'm not competent about to comment on).
But the devil is in the many details. There's this thing called implicit
cross-stage persistence which says that anything you import
not-for-template-haskell is going to be available in quotes and splices
anyway. Sigh… So you have to turn this off. This is what the proposal does.
And pretty much only.

They introduce a new extension-XNoImplicitStagePersistence which disables
that, and a little bit of syntax to specify the stage of imports. That's it.

But it comes with severe limitations, most importantly: you can't ever use
a symbol defined in the current module in a quote or splice of this current
module, typed template Haskell is turned off.

For these situations, the proposal kind of advertises using
`-XImplicitStagePersistence`. Which does seem like a fork-like situation to
me. Not cool. Yet… yet Template Haskell is a big messy ball of yarn, and I
don't think it's fair to ask of any proposal to entangle it completely. The
failure of past attempts seem to support this case. And I believe the
authors are correct when they claim that this proposal, in practice, covers
a vast majority of the uses of Template Haskell out there. So maybe we can
see that as a new foundation for Template Haskell. I'm not thrilled about
it, but it's probably the most reasonable way forward.

The real problem with this sort of proposal is that then I get to write way
too long an email to the committee. Hopefully this didn't deter you. Read
the proposal, and let's vote.

-- 
Arnaud Spiwack
Director, Research at https://moduscreate.com and https://tweag.io.
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