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mstksg / auto

Haskell DSL and platform providing denotational, compositional api for discrete-step, locally stateful, interactive programs, games & automations. http://hackage.haskell.org/package/auto
MIT License
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Remove MonadFix constraint from ArrowLoop instance #6

Closed k0001 closed 9 years ago

k0001 commented 9 years ago

@mstksg I am not really sure about this change, maybe MonadFix brings something to the table which I'm not realising. Anyway, here I go.

This change removes the MonadFix m constraint on ArrowLoop (Auto m), thus avoiding the propagation of the MonadFix constraint to all other Autos whenever you use proc notation's rec orControl.Arrow.loop on one of them.

Monads “out there in the wild” are often missing their MonadFix instances, and others such as as ContT can't get one due to the way they are definied, so this change will prevent the introduction of orphan instances or newtype wrappers just to use Autos recursively.

I haven't really thought this through extensively, I don't fully understand the implications of this change, but I wanted to share it with you in case you want to consider that at some point.

mstksg commented 9 years ago

huh. I really thought there'd be knot tying issues but after testing them out with the programs I had already written, everything seems to still work out. I'm a bit apprehensive but I'll play around with this some more and see the situations where this would break. Thanks :)

mstksg commented 9 years ago

Nevermind, was running my tests against the the MonadFix version by mistake, heh. Yeah, definitely immediate knot-tying issues with most usages of rec with the Non-MonadFix verison. That's because mfix is needed to lift knot-tying semantics from fix into a Monadic context...to let fix "know" what can be tied for a given Monad instance (which often times isn't just the constructor). If Cont doesn't have a MonadFix instance, it's because you can't write recursive bindings in Auto with it.

Still appreciate the effort though...because seeing MonadFix can have a "scary" factor when first seeing it, heh...which might throw people off at first. I mostly ignore it myself these days...but I did have someone did bring it up as a comment when they first read it.

k0001 commented 9 years ago

@mstksg thanks for the lesson! I'll sit down and learn a bit more about what makes mfix different from fix.