MonadReader instance for Codensity

[jonsterling]

Is there a particular reason that Codensity does not implement MonadReader? If not, I'd be happy to submit a patch. Thanks!

[ekmett]

I'm somewhat torn.

On one hand, yes, it can be implemented.

On the other hand, it is effectively saying "hey I have a state, which is large enough to implement the MonadReader API as well".

It seems analogous to me to wanting MonadReader s (State s) or Monoid s => MonadWriter s (State s) which would be fine if we had more of a sub-typing style of relationship between State and Reader (with ask/get being unified) or State and Writer (with put/tell being unified), but we culturally haven't chosen to embrace that notion.

[jonsterling]

@ekmett that's a fair point. The main reason I meant to add MonadReader to Codensity was to enable the composition of code which already uses MonadReader into a Codensity computation, without having to either lift or write a conversion.

In any case, the solution in the absence of this instance is trivial enough that I'm not going to argue strongly for it, though it seems like a nice thing to have.

[ekmett]

Merged. I'd rather have the instance than not, and it's not like there is a viable alternative instance.