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

[Adam Gundry]

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 <arnaud.spiwack at tweag.io 
> <mailto:arnaud.spiwack at 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
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