Proposed: Language.Haskell.TH.Internal

Simon Peyton Jones simonpj at
Tue Jul 18 21:40:53 UTC 2017

OK with me, provided we document the thinking so that users understand the architecture.


| -----Original Message-----
| From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Richard
| Eisenberg
| Sent: 18 July 2017 13:27
| To: GHC developers <ghc-devs at>
| Cc: Ryan Scott < at>
| Subject: Proposed: Language.Haskell.TH.Internal
| 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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