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Monoid instance for FreeT #8

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Merged
merged 6 commits into from
Jan 31, 2017
Merged

Monoid instance for FreeT #8

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1e8751a
interesting error
athanclark Jan 20, 2017
a22e537
monoid instance
athanclark Jan 22, 2017
0380e84
cleaning up
athanclark Jan 28, 2017
94675a8
only needs semigroup
athanclark Jan 29, 2017
50fa4b8
purescript-eff not used
athanclark Jan 29, 2017
4a2cbc4
redundant module
athanclark Jan 30, 2017
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8 changes: 8 additions & 0 deletions src/Control/Monad/Free/Trans.purs
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Expand Up @@ -16,7 +16,9 @@ import Prelude
import Data.Bifunctor (bimap)
import Data.Either (Either(..))
import Data.Exists (Exists, mkExists, runExists)
import Data.Monoid (class Monoid, mempty)

import Control.Apply (lift2)
import Control.Monad.Rec.Class (class MonadRec, Step(..), tailRecM)
import Control.Monad.Trans.Class (class MonadTrans)

Expand Down Expand Up @@ -76,6 +78,12 @@ instance monadRecFreeT :: (Functor f, Monad m) => MonadRec (FreeT f m) where
Loop s1 -> go s1
Done a -> pure a

instance semigroupFreeT :: (Functor f, Monad m, Semigroup w) => Semigroup (FreeT f m w) where
append = lift2 append

instance monoidFreeT :: (Functor f, Monad m, Monoid w) => Monoid (FreeT f m w) where
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Same here.

mempty = pure mempty
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Likewise you only need Applicative m here.

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Actually I'm not sure about that - FreeT's Apply instance depends on Monad due to the use of ap - I'm not sure if there's a way around that, I'm not too familiar with FreeT's design :x


-- | Lift an action from the functor `f` to a `FreeT` action.
liftFreeT :: forall f m a. (Functor f, Monad m) => f a -> FreeT f m a
liftFreeT fa = FreeT \_ -> pure (Right (map pure fa))
Expand Down