Commits on Jun 16, 2024

1. Partially revert TypeAbstractions-related changes in #596 …

   The changes in #596 cause `singletons-th` to use `TypeAbstractions` to
   explicitly quantify the kind variables of promoted class methods whenever the
   parent class has a standalone kind signature, thereby ensuring that the order
   of kind variables matches the order in which the user wrote them. At least,
   that was the intention. Unfortunately, as #605 reveals, this approach sometimes
   causes `singletons-th` to generate ill-kinded code for promoted class methods,
   and there isn't an obvious way to work around this limitation.
   
   As such, this patch reverts the `TypeAbstractions`-related changes from #596.
   Once again, `singletons-th` now does not make any guarantees about the order of
   kind variables for promoted class methods or their defunctionalization symbols.
   
   On the other hand, this patch _does_ keep the changes from #596 that cause
   `singletons-th` to propagate kind information from the parent class's
   standalone kind signature through to the promoted class methods'
   defunctionalization symbols, as this feature is useful independent of the
   `TypeAbstractions`-related changes. I have taken the opportunity to document
   why we do this in the new `Note [Propagating kind information from class
   standalone kind signatures]` in `D.S.TH.Promote`.
   
   I've removed the `T589` test case, as the functionality it was testing no
   longer exists after reverting the `TypeAbstractions`-related changes. I have
   also added a new `T605` test case that ensures that the regression from #605
   stays fixed. In a subsequent commit, I will add another test case that
   demonstrates that the kind propagation works as intended.
   
   Fixes #605.

   

   RyanGlScott committed Jun 16, 2024
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2. README.md: Document constraint limitations more thoroughly …

   In particular, make note of the circumstances under which issues like #604.
   There is no way for `singletons-th` to generate code that avoids the problems
   in #604 in all cases, so we instead advise users to be wary when promoting code
   involving classes that are parameterized over higher-kinded type variables
   (e.g., `Alternative`).

   

   RyanGlScott committed Jun 16, 2024
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3. Give PAlternative (and friends) the correct kinds …

   Previously, `singletons-th` would generalize the kinds of `PAlternative` and
   related classes (e.g., `PMonadPlus`), as well of the kinds of the
   defunctionalization symbols for various classes that are parameterized over a
   higher-kinded type variable. As described in the "Class constraints" section of
   the `README.md`, the recommended workaround for this issue is to give the
   classes in question explicit kinds, so this patch does just that by giving
   `Alternative`, `MonadPlus`, etc. standalone kind signatures.
   
   This causes the code in `singletons-base` to deviate a bit from the original
   code in the `base` library. I have written a new `Note [Using standalone kind
   signatures not present in the base library]` and cited it in all of the places
   where such a deviation occurs.
   
   Fixes #604.

   

   RyanGlScott committed Jun 16, 2024
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