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The Monad Transformer Library
http://www.haskell.org/haskellwiki/Monad_Transformers
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Switch to MonadReader based MonadState implementation for ContT #70

Closed Atry closed 4 years ago

Atry commented 4 years ago

The implementation is inspired by https://github.com/ekmett/kan-extensions/blob/f9f8717883d250c2067c13b1e867274c29d7095e/src/Control/Monad/Codensity.hs#L251

chessai commented 4 years ago

This is interesting, but I'm not sure if it's at all warranted. Especially considering that it will cause breakage. I'm leaning toward a no on this.

cartazio commented 4 years ago

Could you explain the motivation and context?

Either way thanks for sharing. But some exposition About context and such would be appreciated

On Sat, Jan 18, 2020 at 11:47 AM chessai notifications@github.com wrote:

This is interesting, but I'm not sure if it's at all warranted. Especially considering that it will cause breakage. I'm leaning toward a no on this.

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Atry commented 4 years ago

The problem is that the existing instance MonadState s m => MonadState s (ContT r m) conflicts with reader based instance.

chessai commented 4 years ago

Could you elaborate?

Atry commented 4 years ago

@cartazio A ReaderT has a simpler implementation than StateT. Suppose we have the following type:

type X = ContT Text (ReaderT String (ReaderT Double)

With the help of this PR, X will support both MonadReader String X and MonadState String X. Moreover, with the help of https://github.com/haskell/mtl/pull/73, we can even have both MonadReader Double X and MonadState Double X. Therefore, monad transformers can be easily extended, used as a simple and fast alternative to Freer / Eff things.

Atry commented 4 years ago

The trick to implement MonadState from MonadReader is explored by @ekmett 9 years ago, this PR just applies the same idea to ConT

chessai commented 4 years ago

This would be a weird and confusing departure from the typical lifting witnessed by monad transformers. That is, MTL instances typically lift like:

instance C x m => C x (t m)

and not

instance C0 x m => C1 x (t m)

While clever and certainly used by Codensity, this would cause a lot of breakage and confusion. I'm a strong -1 on this. Feel free to re-open if you want to make a case for this being changed.

cartazio commented 4 years ago

agreed