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add substFreeT #18

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Merged
merged 2 commits into from
Aug 21, 2019
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add substFreeT #18

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2a692e8
add substFreeT
safareli Aug 14, 2019
8f05d0c
expose substFreeT
safareli Aug 14, 2019
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11 changes: 11 additions & 0 deletions src/Control/Monad/Free/Trans.purs
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Expand Up @@ -7,6 +7,7 @@ module Control.Monad.Free.Trans
, hoistFreeT
, interpret
, bimapFreeT
, substFreeT
, resume
, runFreeT
) where
Expand Down Expand Up @@ -119,6 +120,16 @@ bimapFreeT :: forall f g m n a. Functor f => Functor n => (f ~> g) -> (m ~> n) -
bimapFreeT nf nm (Bind e) = runExists (\(Bound a f) -> bound (bimapFreeT nf nm <<< a) (bimapFreeT nf nm <<< f)) e
bimapFreeT nf nm (FreeT m) = FreeT \_ -> map (nf <<< map (bimapFreeT nf nm)) <$> nm (m unit)


-- | Like `runFreeT`, but for running into some other FreeT without the
-- | overhead that `MonadRec` incurs.
substFreeT :: forall a m f g. Monad m => Functor g => (f ~> FreeT g m) -> FreeT f m a -> FreeT g m a
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NOTE: Monad m is needed because of pure val

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Not Applicative?

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Monad m => Applicative (FreeT f m) because Apply is implemented using ap

substFreeT fBind (Bind e) = runExists (\(Bound a f) -> bound (substFreeT fBind <<< a) (substFreeT fBind <<< f)) e
substFreeT fBind (FreeT m) = join $ FreeT \_ -> m unit <#> case _ of
Left val -> Left $ pure val
Right fFree -> Left $ bound (\_ -> fBind fFree) (substFreeT fBind)


-- | Run a `FreeT` computation to completion.
runFreeT :: forall f m a. Functor f => MonadRec m => (f (FreeT f m a) -> m (FreeT f m a)) -> FreeT f m a -> m a
runFreeT interp = tailRecM (go <=< resume)
Expand Down