    module Distribution.Solver.Types.Progress
        ( Progress(..)
        , foldProgress
        ) where

    import Prelude ()
    import Distribution.Solver.Compat.Prelude hiding (fail)

    -- | A type to represent the unfolding of an expensive long running
    -- calculation that may fail. We may get intermediate steps before the final
    -- result which may be used to indicate progress and\/or logging messages.
    --
    data Progress step fail done = Step step (Progress step fail done)
                                 | Fail fail
                                 | Done done

    -- This Functor instance works around a bug in GHC 7.6.3.
    -- See https://gitlab.haskell.org/ghc/ghc/-/issues/7436#note_66637.
    -- The derived functor instance caused a space leak in the solver.
    instance Functor (Progress step fail) where
      fmap :: forall a b.
    (a -> b) -> Progress step fail a -> Progress step fail b
    fmap a -> b
    f (Step step
    s Progress step fail a
    p) = step -> Progress step fail b -> Progress step fail b
    forall step fail done.
    step -> Progress step fail done -> Progress step fail done
    Step step
    s ((a -> b) -> Progress step fail a -> Progress step fail b
    forall a b.
    (a -> b) -> Progress step fail a -> Progress step fail b
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    fmap a -> b
    f Progress step fail a
    p)
      fmap a -> b
    _ (Fail fail
    x)   = fail -> Progress step fail b
    forall step fail done. fail -> Progress step fail done
    Fail fail
    x
      fmap a -> b
    f (Done a
    r)   = b -> Progress step fail b
    forall step fail done. done -> Progress step fail done
    Done (a -> b
    f a
    r)

    -- | 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
    fail 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
    fail 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   = a -> Progress step fail a
    forall a. a -> Progress step fail a
    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  = (step -> Progress step fail b -> Progress step fail b)
    -> (fail -> Progress step fail b)
    -> (a -> Progress step fail b)
    -> Progress step fail a
    -> Progress step fail b
    forall step a fail done.
    (step -> a -> a)
    -> (fail -> a) -> (done -> a) -> Progress step fail done -> a
    foldProgress step -> Progress step fail b -> Progress step fail b
    forall step fail done.
    step -> Progress step fail done -> Progress step fail done
    Step fail -> Progress step fail b
    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  = a -> Progress step fail 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 = (step -> Progress step fail b -> Progress step fail b)
    -> (fail -> Progress step fail b)
    -> ((a -> b) -> Progress step fail b)
    -> Progress step fail (a -> b)
    -> Progress step fail b
    forall step a fail done.
    (step -> a -> a)
    -> (fail -> a) -> (done -> a) -> Progress step fail done -> a
    foldProgress step -> Progress step fail b -> Progress step fail b
    forall step fail done.
    step -> Progress step fail done -> Progress step fail done
    Step fail -> Progress step fail b
    forall step fail done. fail -> Progress step fail done
    Fail (((a -> b) -> Progress step fail a -> Progress step fail b)
    -> Progress step fail a -> (a -> b) -> Progress step fail b
    forall a b c. (a -> b -> c) -> b -> a -> c
    flip (a -> b) -> Progress step fail a -> Progress step fail b
    forall a b.
    (a -> b) -> Progress step fail a -> Progress step fail b
    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   = fail -> Progress step fail a
    forall step fail done. fail -> Progress step fail done
    Fail fail
    forall a. Monoid a => a
    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 = (step -> Progress step fail a -> Progress step fail a)
    -> (fail -> Progress step fail a)
    -> (a -> Progress step fail a)
    -> Progress step fail a
    -> Progress step fail a
    forall step a fail done.
    (step -> a -> a)
    -> (fail -> a) -> (done -> a) -> Progress step fail done -> a
    foldProgress step -> Progress step fail a -> Progress step fail a
    forall step fail done.
    step -> Progress step fail done -> Progress step fail done
    Step (Progress step fail a -> fail -> Progress step fail a
    forall a b. a -> b -> a
    const Progress step fail a
    q) a -> Progress step fail a
    forall step fail done. done -> Progress step fail done
    Done Progress step fail a
    p