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th-dict-discovery
| Hackage
Overview
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 Dict
s
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:
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
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.