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{-# language ConstraintKinds, FlexibleContexts, TypeFamilies,
UndecidableInstances, DeriveFunctor #-}
module T13320 where
import GHC.Exts (Constraint)
data QCGen
newtype Gen a = MkGen { unGen :: QCGen -> Int -> a }
deriving Functor
sized :: (Int -> Gen a) -> Gen a
sized f = MkGen (\r n -> let MkGen m = f n in m r n)
class Arbitrary a where
arbitrary :: Gen a
type family X_Var ξ
data TermX ξ = Var (X_Var ξ)
type ForallX (φ :: * -> Constraint) ξ = ( φ (X_Var ξ) )
-- This type signature used to be necessary to prevent the
-- type checker from looping.
-- genTerm :: ForallX Arbitrary ξ => Int -> Gen (TermX ξ)
genTerm 0 = Var <$> arbitrary
genTerm n = Var <$> genTerm (n - 1)
instance ForallX Arbitrary ξ => Arbitrary (TermX ξ) where
arbitrary = sized genTerm
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