    {-# LANGUAGE CPP #-}
    {-# LANGUAGE DeriveFunctor #-}
    -- Note: This module was copied from cabal-install.

    -- | A progress monad, which we use to report failure and logging from
    -- otherwise pure code.
    module Distribution.Utils.Progress
        ( Progress
        , stepProgress
        , failProgress
        , foldProgress
        ) where

    import Prelude ()
    import Distribution.Compat.Prelude

    import qualified Data.Monoid as Mon

    -- | A type to represent the unfolding of an expensive long running
    -- calculation that may fail (or maybe not expensive, but complicated!)
    -- We may get intermediate steps before the final
    -- result which may be used to indicate progress and\/or logging messages.
    --
    -- TODO: Apply Codensity to avoid left-associativity problem.
    -- See http://comonad.com/reader/2011/free-monads-for-less/ and
    -- http://blog.ezyang.com/2012/01/problem-set-the-codensity-transformation/
    --
    data Progress step fail done = Step step (Progress step fail done)
                                 | Fail fail
                                 | Done done
      deriving (forall a b. a -> Progress step fail b -> Progress step fail a
    forall a b.
    (a -> b) -> Progress step fail a -> Progress step fail b
    forall step fail a b.
    a -> Progress step fail b -> Progress step fail a
    forall step fail a b.
    (a -> b) -> Progress step fail a -> Progress step fail b
    forall (f :: * -> *).
    (forall a b. (a -> b) -> f a -> f b)
    -> (forall a b. a -> f b -> f a) -> Functor f
    <$ :: forall a b. a -> Progress step fail b -> Progress step fail a
    $c<$ :: forall step fail a b.
    a -> Progress step fail b -> Progress step fail a
    fmap :: forall a b.
    (a -> b) -> Progress step fail a -> Progress step fail b
    $cfmap :: forall step fail a b.
    (a -> b) -> Progress step fail a -> Progress step fail b
    Functor)

    -- | Emit a step and then continue.
    --
    stepProgress :: step -> Progress step fail ()
    stepProgress :: forall step fail. step -> Progress step fail ()
    stepProgress step
    step = forall step fail done.
    step -> Progress step fail done -> Progress step fail done
    Step step
    step (forall step fail done. done -> Progress step fail done
    Done ())

    -- | Fail the computation.
    failProgress :: fail -> Progress step fail done
    failProgress :: forall fail step done. fail -> Progress step fail done
    failProgress fail
    err = forall step fail done. fail -> Progress step fail done
    Fail fail
    err

    -- | Consume a 'Progress' calculation. Much like 'foldr' for lists but with two
    -- base cases, one for a final result and one for failure.
    --
    -- Eg to convert into a simple 'Either' result use:
    --
    -- > foldProgress (flip const) Left Right
    --
    foldProgress :: (step -> a -> a) -> (fail -> a) -> (done -> a)
                 -> Progress step fail done -> a
    foldProgress :: forall step a fail done.
    (step -> a -> a)
    -> (fail -> a) -> (done -> a) -> Progress step fail done -> a
    foldProgress step -> a -> a
    step fail -> a
    err done -> a
    done = Progress step fail done -> a
    fold
      where fold :: Progress step fail done -> a
    fold (Step step
    s Progress step fail done
    p) = step -> a -> a
    step step
    s (Progress step fail done -> a
    fold Progress step fail done
    p)
            fold (Fail fail
    f)   = fail -> a
    err fail
    f
            fold (Done done
    r)   = done -> a
    done done
    r

    instance Monad (Progress step fail) where
      return :: forall a. a -> Progress step fail a
    return   = forall (f :: * -> *) a. Applicative f => a -> f a
    pure
      Progress step fail a
    p >>= :: forall a b.
    Progress step fail a
    -> (a -> Progress step fail b) -> Progress step fail b
    >>= a -> Progress step fail b
    f  = forall step a fail done.
    (step -> a -> a)
    -> (fail -> a) -> (done -> a) -> Progress step fail done -> a
    foldProgress forall step fail done.
    step -> Progress step fail done -> Progress step fail done
    Step forall step fail done. fail -> Progress step fail done
    Fail a -> Progress step fail b
    f Progress step fail a
    p

    instance Applicative (Progress step fail) where
      pure :: forall a. a -> Progress step fail a
    pure a
    a  = forall step fail done. done -> Progress step fail done
    Done a
    a
      Progress step fail (a -> b)
    p <*> :: forall a b.
    Progress step fail (a -> b)
    -> Progress step fail a -> Progress step fail b
    <*> Progress step fail a
    x = forall step a fail done.
    (step -> a -> a)
    -> (fail -> a) -> (done -> a) -> Progress step fail done -> a
    foldProgress forall step fail done.
    step -> Progress step fail done -> Progress step fail done
    Step forall step fail done. fail -> Progress step fail done
    Fail (forall a b c. (a -> b -> c) -> b -> a -> c
    flip forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    fmap Progress step fail a
    x) Progress step fail (a -> b)
    p

    instance Monoid fail => Alternative (Progress step fail) where
      empty :: forall a. Progress step fail a
    empty   = forall step fail done. fail -> Progress step fail done
    Fail forall a. Monoid a => a
    Mon.mempty
      Progress step fail a
    p <|> :: forall a.
    Progress step fail a
    -> Progress step fail a -> Progress step fail a
    <|> Progress step fail a
    q = forall step a fail done.
    (step -> a -> a)
    -> (fail -> a) -> (done -> a) -> Progress step fail done -> a
    foldProgress forall step fail done.
    step -> Progress step fail done -> Progress step fail done
    Step (forall a b. a -> b -> a
    const Progress step fail a
    q) forall step fail done. done -> Progress step fail done
    Done Progress step fail a
    p