mtl-style class for YesodExample

[chris-martin]

Yesod testing assumes that you're going to write your tests in the YesodExample context. If you're working in a different monad (e.g. YesodExample augmented with a ReaderT or StateT layer that helps manage test fixture data), you're constantly having to lift YesodExample actions. I'd like to propose adding the following polymorphic lifting function:

class (MonadIO m, Yesod site) => MonadYesodExample site m | m -> site where
  liftYesodExample :: YesodExample site a -> m a

Many of the utilities in the Yesod.Test module could then be generalized to a MonadYesodExample constraint.

I've already done this internally. If there is interest from maintainers, I'll start work on a PR to upstream it.

[parsonsmatt]

One downside to polymorphic test helpers is that it can make inference worse - you may have to specify type annotations or aliases to get hspec to pick the right instance.

This seems like a useful library - yesod-test-lifted - any thoughts on packaging that, linking to it from the yesod-test readme, and see what adoption/real-world use looks like before making a change to upstream?

[chris-martin]

Fair concern. Our own test suite required very little extra type annotation, but I suppose this change would break the examples in the yesod-test readme. I have some other things I'd like to contribute to yesod-test as well, and I'd really prefer not to try to keep a separate package in sync. What about doing the work directly in yesod-test but in a separate Yesod.Test.Lifted module so that the change is non-breaking?

[parsonsmatt]

That works for me!

I'm curious what else you're intending to do - I have a rewrite/new library planned with yesod-hspec #1763 and it may be nice to join forces there.

[chris-martin]

Not anything too exciting, I think. We've just been collecting an assortment of convenient assertions that might be nice to share.

[chris-martin]

Rather than expanding yesod-test, I wonder if instead I could persuade you to make some adjustments to yesod-hspec, because you've already got a polymorphic API there that looks very close to what I'm after. (And now that I see you've already made your own variant of the Yesod.Test module, the thought of making yet another one of my own is making me uneasy.)

Your utilities in yesod-hspec use a MonadState (YesodExampleData site) constraint, which is not quite general enough for me due to the MonadState class's functional dependency. Would it be reasonable to introduce a new class (akin to my MonadYesodExample) instead?

[parsonsmatt]

Yeah, I'd be happy to introduce that. The generalization from YesodExample to MonadState _ m, MonadIO m would be nicer as a MonadYesodExample, though I do think it'd be nice to use MonadIO where feasible (ie assertEq).