Proposed: Language.Haskell.TH.Internal

[Alan & Kim Zimmerman]

This can serve as a starting point for the Trees that Grow implementation
too, I understand Shayan Najd is intending to eventually harmonise the
hsSyn AST and the TH one.

Alan

On 18 July 2017 at 14:27, Richard Eisenberg <rae at cs.brynmawr.edu> wrote:

> Hi devs,
>
> This is a brief proposal for an internal, non-user-facing restructuring of
> the implementation of Template Haskell. It's here (instead of at
> ghc-proposals) because it's not user-facing.
>
> When a user writes a TH quote [| foo bar baz |], GHC must desugar that
> into some Core code that builds a Q Exp. This is currently done by calling
> functions in Language.Haskell.TH.Lib, which is capable of building all the
> TH abstract syntax. However, many end-users of TH *also* import
> Language.Haskell.TH.Lib and use its functions. This means that, as the
> Haskell AST evolves, we can't reasonably evolve this Lib module, as it
> breaks user code. (There is a requisite amount of churn in TH, but we try
> to avoid gratuitous changes in Lib.) This has bitten in the past when we
> have, say, changed the representation of type families to accommodate
> closed type families, and it's biting now as we (ahem, Ryan Scott) are
> trying to merge types and kinds in TH.
>
> I thus propose a new module Language.Haskell.TH.Internal. This will start
> out as a slimmed-down copy of Lib (slimmed-down because Lib already has a
> bunch of now-disused functions) but may evolve independently. Desugaring
> quotes will go via this new Internal module, and we will have no qualms
> about changing its interface, as it's Internal.
>
> What do we think? Ryan has already implemented this idea in D3751.
>
> Pros:
>  - Flexibility
>  - Separation between user-facing interface and internal interface
>
> Cons:
>  - More stuff
>  - We already break the TH API with every release; maybe it's not so bad
> to break more of it.
>
> Thanks,
> Richard
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