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Constrain the resolvers with MonadIO

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This replaces the most usages of MonadPlus, which is not appropriate for
the resolvers, since a resolver is unambiguously chosen by the name (no
need for 'mplus'), and the resolvers are often doing IO.
This commit is contained in:
Eugen Wissner
2019-07-08 10:15:47 +02:00
parent 22d4a4e583
commit 61879fb124
6 changed files with 63 additions and 59 deletions
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26
tests/Test/StarWars/Data.hs
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@ -2,8 +2,10 @@
module Test.StarWars.Data where
import Data.Monoid (mempty)
import Control.Applicative (liftA2)
import Control.Monad (MonadPlus(..))
import Control.Applicative ( Alternative(..)
, liftA2
)
import Control.Monad.IO.Class (MonadIO(..))
import Control.Monad.Trans.Except (throwE)
import Data.Maybe (catMaybes)
import Data.Text (Text)
@ -52,7 +54,7 @@ appearsIn :: Character -> [Int]
appearsIn (Left x) = _appearsIn . _droidChar $ x
appearsIn (Right x) = _appearsIn . _humanChar $ x
secretBackstory :: MonadPlus m => Character -> ActionT m Text
secretBackstory :: MonadIO m => Character -> ActionT m Text
secretBackstory = const $ ActionT $ throwE "secretBackstory is secret."
typeName :: Character -> Text
@ -150,30 +152,30 @@ getHero _ = artoo
getHeroIO :: Int -> IO Character
getHeroIO = pure . getHero
getHuman :: MonadPlus m => ID -> m Character
getHuman :: Alternative f => ID -> f Character
getHuman = fmap Right . getHuman'
getHuman' :: MonadPlus m => ID -> m Human
getHuman' :: Alternative f => ID -> f Human
getHuman' "1000" = pure luke'
getHuman' "1001" = pure vader
getHuman' "1002" = pure han
getHuman' "1003" = pure leia
getHuman' "1004" = pure tarkin
getHuman' _ = mzero
getHuman' _ = empty
getDroid :: MonadPlus m => ID -> m Character
getDroid :: Alternative f => ID -> f Character
getDroid = fmap Left . getDroid'
getDroid' :: MonadPlus m => ID -> m Droid
getDroid' :: Alternative f => ID -> f Droid
getDroid' "2000" = pure threepio
getDroid' "2001" = pure artoo'
getDroid' _ = mzero
getDroid' _ = empty
getFriends :: Character -> [Character]
getFriends char = catMaybes $ liftA2 mplus getDroid getHuman <$> friends char
getFriends char = catMaybes $ liftA2 (<|>) getDroid getHuman <$> friends char
getEpisode :: MonadPlus m => Int -> m Text
getEpisode :: Alternative f => Int -> f Text
getEpisode 4 = pure "NEWHOPE"
getEpisode 5 = pure "EMPIRE"
getEpisode 6 = pure "JEDI"
getEpisode _ = mzero
getEpisode _ = empty