Conversion.hs

-- | Getting a 'Fold' from some other type of fold
module Fold.Pure.Conversion where

import Fold.Pure.Type

import Data.Function (($))
import Data.Functor ((<$>))
import Data.Functor.Identity (Identity, runIdentity)
import Data.Maybe (Maybe)
import Fold.Effectful.Type (EffectfulFold (EffectfulFold))
import Fold.Nonempty.Type (NonemptyFold (NonemptyFold))

import qualified Fold.Effectful.Type as Effectful
import qualified Fold.Nonempty.Type as Nonempty
import qualified Strict

{-| Turn an effectful fold into a pure fold -}
effectfulFold :: EffectfulFold Identity a b -> Fold a b
effectfulFold
  EffectfulFold{ Effectful.initial, Effectful.step, Effectful.extract } =
    Fold
      { initial =         runIdentity ( initial   )
      , step    = \x a -> runIdentity ( step x a  )
      , extract = \x   -> runIdentity ( extract x )
      }

{-| Turn a fold that requires at least one input into a fold that returns
'Data.Maybe.Nothing' when there are no inputs -}
nonemptyFold :: NonemptyFold a b -> Fold a (Maybe b)
nonemptyFold
  NonemptyFold{ Nonempty.initial, Nonempty.step, Nonempty.extract } =
    Fold
      { initial = Strict.Nothing
      , step = \xm a -> Strict.Just $ case xm of
            Strict.Nothing -> initial a
            Strict.Just x -> step x a
      , extract = \xm -> extract <$> Strict.lazy xm
      }