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Add ((->) r') as a base monad for SelectT. #140

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Add ((->) r') as a base monad for SelectT. #140

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3 changes: 3 additions & 0 deletions Control/Monad/Select.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -125,6 +125,9 @@ class (Monad m) => MonadSelect r m | m -> r where
instance MonadSelect r (SelectT r Identity) where
select = Select.select

instance MonadSelect r (SelectT r ((->) r')) where
select f = Select.SelectT $ \k r' -> f $ \a -> k a r'

-- | \'Extends\' the possibilities considered by @m@ to include 'Nothing'; this
-- means that 'Nothing' gains a \'rank\' (namely, a value of @r@), and the
-- potential result could also be 'Nothing'.
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