th-dict-discovery

Automatically discover available dictionaries at compile time.

th-dict-discovery is a library to get your hands on the in-scope, concrete instances of a class. The motivating use-case is to generate property tests to automatically prove new instances of a class follow the laws.

This library uses Template Haskell to provide a list of existentialized Dicts for each concrete (entirely monomorphic) instance in scope.

defInstances :: [SomeDict1 Default]
defInstances = $(someDicts ''Default)

These lists can be consumed via withSomeDict1 :: (forall a. c a => Proxy a -> r) -> SomeDict1 c -> r, for example, we can generate hspec tests:

spec :: Spec
spec = describe "each default instance" .
  forM_ defInstances $ withSomeDict1 $ \(_ :: Proxy a) ->
    it "shouldn't be undefined" $
      seq (def @a) True `shouldBe` True

The library also defines up to SomeDict8, with eliminators up to withSomeDict8. These correspond to multi-parameter typeclasses with first-order, potentially polykinded parameters.

The TH function someDicts is smart enough to generate the correct arity of SomeDictN for the class under inspection, but at this time, you're responsible for using the correct withSomeDictN to consume it.