More transformers instances #366
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robrix
commented
Mar 13, 2020
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| instance Algebra sig m => Algebra (Empty :+: sig) (Maybe.MaybeT m) where | ||
| alg hdl sig ctx = case sig of | ||
| L Empty -> Maybe.MaybeT (pure Nothing) | ||
| R other -> Maybe.MaybeT $ thread (maybe (pure Nothing) Maybe.runMaybeT) hdl other (Just ctx) | ||
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I honestly don’t know why I didn’t implement this before.
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| newtype Swap s a = Swap { getSwap :: (a, s) } | ||
| deriving (Functor) |
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This is handy for giving us a Functor instance for the backwards pairs transformers is so fond of.
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| #if MIN_VERSION_transformers(0,5,6) | ||
| instance (Algebra sig m, Monoid w) => Algebra (Writer w :+: sig) (Writer.CPS.WriterT w m) where | ||
| alg hdl sig ctx = case sig of | ||
| L (Tell w) -> ctx <$ Writer.CPS.tell w | ||
| L (Listen m) -> swapAndLift <$> Writer.CPS.listen (hdl (m <$ ctx)) | ||
| L (Censor f m) -> Writer.CPS.censor f (hdl (m <$ ctx)) | ||
| R other -> Writer.CPS.writerT $ getSwap <$> thread (\ (Swap (x, s)) -> Swap . fmap (mappend s) <$> Writer.CPS.runWriterT x) hdl other (Swap (ctx, mempty)) | ||
| #endif |
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I had implemented this before, but removed it because it was unavailable on some of our ghcs. But since we added CPP to support the Ap instance I figured I might as well add it back in.
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This PR adds
Algebrainstances for a few more types fromtransformers, namely including two only available in 0.5.6+.