Hi all,

I'm generally in support of this proposal. As many of you know, I strongly believe TemplateHaskell is a major wart that Haskell has, for many

reasons. This proposal tries to address at least one of those: adding more clarity and explicitness about dependencies. It may help with

cross compilation in that we have a clearer idea of what we exactly need to load in iserv (alternatives where we implicit link a runnable for

target evaluation, can rely on dead code elimination for this, but having this from the start would already be helpful).

I've recently been looking a lot at Zig's comptime, as they seem to have gone down almost the same route. Maybe there's some inspiration to

be drawn from Zig's solution in the future. It is, however, WAY more restrictive than what we currently have in the form of TemplateHaskell.

+1 on this one.

Best,

 Moritz

On Fri, 10 Jan 2025 at 18:19, Adam Gundry <adam@well-typed.com> wrote:

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@tweag.io
> <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/

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