conditional-from-a-where-clause-into-declarations.hs

{-# LANGUAGE CPP #-}

-- A conditional that begins in a where clause and goes on into the
-- declarations after it stays one conditional.
module Data.Witness where

withWitness :: Witness a -> ((Known a) => r) -> r
withWitness w k = go k
  where
    go :: ((Known a) => r) -> r
#if MIN_VERSION_base(4,17,0)
    go = withDict @(Known a) w
#else
    go r = unsafeCoerce (Wrap r) w

-- the dictionary of a class with one method is that method
#if __GLASGOW_HASKELL__ >= 810
type Wrap :: Type -> Type -> Type
#endif
newtype Wrap a r = Wrap ((Known a) => r)
#endif

-- Without an #else.
render :: Witness a -> String
render w = label w
  where
    label _ = "witness"
#if WITNESS_DEBUG
    debug = show . Debugged

#if __GLASGOW_HASKELL__ >= 810
type Debugged :: Type -> Type
#endif
newtype Debugged a = Debugged (Witness a)
  deriving (Show)
#endif

-- Bindings lined up after a let leave the conditional in two.
trace :: Witness a -> IO ()
trace w = do
  let
    name = "witness"
#if WITNESS_DEBUG
    shown = show (Debugged w)
#if __GLASGOW_HASKELL__ >= 810
  putStrLn shown
#endif
#endif
  putStrLn name