Remove special-casing when singling error and friends
#597
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Previously, `singExp` had a number of _ad hoc_ special cases for handling functions that look like exceptions (e.g., `sError` and `sUndefined`). This was because their return types are `a` rather than, say, `Sing (Undefined @A)`, which impaired GHC's ability to typecheck `singletons-th`–generated code. As discussed in #588, however, there doesn't appear to be any good reason for these functions to have return types that are so polymorphic, which called into question why we are maintained these _ad hoc_ special cases in the first place. This patch removes the special casing and gives `sError`, `sErrorWithoutStackTrace`, and `sUndefined` less polymorphic return types. The upshot is that I was able to delete a fair bit of ugly code from `D.S.TH.Single`, which is a nice payoff. Fixes #588.
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Previously,
singExphad a number of ad hoc special cases for handling functions that look like exceptions (e.g.,sErrorandsUndefined). This was because their return types arearather than, say,Sing (Undefined @a), which impaired GHC's ability to typechecksingletons-th–generated code. As discussed in #588, however, there doesn't appear to be any good reason for these functions to have return types that are so polymorphic, which called into question why we are maintained these ad hoc special cases in the first place.This patch removes the special casing and gives
sError,sErrorWithoutStackTrace, andsUndefinedless polymorphic return types. The upshot is that I was able to delete a fair bit of ugly code fromD.S.TH.Single, which is a nice payoff.Fixes #588.