Add MonadFix instance

[treeowl]

I believe this is equivalent to the instance for []. Writing
QuickCheck properties for mfix seems pretty tricky, so I just
added a small unit test.

Fixes#178

[treeowl]

This implementation doesn't make any attempt to be fusion-friendly. That would seem to require digging into some internals. Unfortunately, I tend to get rather lost in Bundle.

[dolio]

I don't think being fused is a realistic goal for this.

I'll try to look it over soon.

[treeowl]

In case this helps: I just tested this implementation against the two list monad examples in Levent Erkök's thesis, and it seems to check out.

…

On Wed, Jul 5, 2017 at 9:31 PM, dolio ***@***.***> wrote: I don't think being fused is a realistic goal for this. I'll try to look it over soon. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#179 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABzi_TDwwE9EQDTfLsqov-jkN4JOCtJ8ks5sLDjpgaJpZM4OO3UN> .

[dolio]

Do the properties of mfix here mean that boostrapping with f ⊥ is just as good as fix (f . head) (because we just want to know the 'statically determined' length)? And then the generate is less complicated, and we don't need ST?

[treeowl]

I believe so. The only trouble is that we end up with an extra call to the passed function. I don't *think* we save much complexity over a slightly simplified version. I'll post one or two options later if I have time. Please poke me if I haven't done so by Monday. On Jul 15, 2017 11:47 AM, "dolio" <notifications@github.com> wrote: Do the properties of mfix here mean that boostrapping with f ⊥ is just as good as fix (head . f) (because we just want to know the 'statically determined' length)? And then the generate is less complicated? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#179 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABzi_ebHMJBGrf8Heoh5-gjHLSVCd5uKks5sON8GgaJpZM4OO3UN> .

[Shimuuar]

Superseded by #312