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

Thanks Arnaud! With my "proposal co-author" hat on, I'd like to make a few points inline... On 09/01/2025 06:34, Arnaud Spiwack wrote:

On Thu, 9 Jan 2025 at 15:31, Arnaud Spiwack mailto:arnaud.spiwack@tweag.io> wrote:

[...]

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.

Regarding typed TH, the proposal currently grants a bit of flexibility to the implementation in suggesting that TTH might not be supported at all, primarily because TTH has some existing unresolved issues around constraints. We could alternately say that TTH remains available (but also remains somewhat broken, because fixing it is out of scope of the implementation of this proposal).

For these situations, the proposal kind of advertises using `-XImplicitStagePersistence`. Which does seem like a fork-like situation to me. Not cool.

Rather than seeing ImplicitStagePersistence as creating a language fork, I see it as necessary for backwards compatibility, but with the intention that in the long term NoImplicitStagePersistence is the way to go. This may still be difficult in some cases (e.g. codebases that make heavy use of Lift), but the idea is to start with a simple, restrictive baseline (NoImplicitStagePersistence) and then gradually add features relaxing this as needed (ExplicitLevelImports being the first of these, but perhaps later something for multiple levels within a single file). Cheers, Adam -- Adam Gundry, Haskell Consultant Well-Typed LLP, https://www.well-typed.com/ Registered in England & Wales, OC335890 27 Old Gloucester Street, London WC1N 3AX, England