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Add subclasses to enforce the type class laws #126

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Merged
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Oct 15, 2020
Merged

Add subclasses to enforce the type class laws #126

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4 changes: 2 additions & 2 deletions src/Control/Monad/Writer/Class.purs
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Original file line number Diff line number Diff line change
Expand Up @@ -16,7 +16,7 @@ import Data.Tuple (Tuple(..))
-- | Law:
-- |
-- | - `do { tell x ; tell y } = tell (x <> y)`
class Monad m <= MonadTell w m | m -> w where
class (Semigroup w, Monad m) <= MonadTell w m | m -> w where
tell :: w -> m Unit

-- | An extension of the `MonadTell` class that introduces some operations on
Expand All @@ -33,7 +33,7 @@ class Monad m <= MonadTell w m | m -> w where
-- | - `do { tell x ; tell y } = tell (x <> y)`
-- | - `listen (pure a) = pure (Tuple a mempty)`
-- | - `listen (writer a x) = tell x $> Tuple a x`
class MonadTell w m <= MonadWriter w m | m -> w where
class (Monoid w, MonadTell w m) <= MonadWriter w m | m -> w where
listen :: forall a. m a -> m (Tuple a w)
pass :: forall a. m (Tuple a (w -> w)) -> m a

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