Commits on Jun 26, 2024

1. Record local variables' kinds during lambda lifting …

   Previously, `singletons-th` made no effort to track the kinds of local
   variables when generating lambda-lifted code, instead generating local variable
   binders with no kind annotations. As a result, GHC would generalize the kinds
   of these lambda-lifted definitions to things that are way more polymorphic than
   intended. While this technically works in today's GHC, it won't in a future
   version of GHC that implements
   [GHC#23515](https://gitlab.haskell.org/ghc/ghc/-/issues/23515).
   
   In general, generating kinds for every local variable would require
   `singletons-th` to implement something akin to full-blown type inference over
   the Template Haskell AST, which is not something I am eager to implement.
   
   Fortunately, there is a relatively simple approach we can do to alleviate this
   problem that doesn't require full type inference. In situations where we know
   the kind of a local variable (e.g., when there is a top-level signature or
   there is a pattern signature), we record the variable's kind and use it when
   generating binders for any lambda-lifted definitions that close over the
   variable. For the full story on how this works, see `Note [Local variables and
   kind information]` `D.S.TH.Promote.Syntax.LocalVar`.
   
   This is not a perfect solution, as there will still be examples of the original
   problem that won't be covered by this simple approach (see the Note). This
   approach is still much better than what `singletons-th` was doing before, and I
   think it's worth using this simple approach even if it doesn't fix 100% of all
   cases.
   
   This patch mostly resolves the "Overly polymorphic lambda-lifting, part 2"
   section of #601.

   

   RyanGlScott committed Jun 26, 2024
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