Is MonadPlus instance lawful?

[no-longer-on-githu-b]

let f = if _ then throwError e else pure unit

Then (pure true <|> pure false) >>= f is not the same as (pure true >>= f) <|> (pure false >>= f). Because in the first example, the exception thrown by f is not caught.

[natefaubion]

I don't think your example is great, because it's observably the same. The first exception is caught, but the second one throws, which is identical to an outside observer. But in general, distributivity doesn't really apply to effects.

(log "foo" <|> log "bar") >>= \_ -> log "baz"
(log "foo" >>= \_ -> log "baz") <|> (log "foo" >>= \_ -> log "baz")

Are observably different.

[natefaubion]

Actually I take that back, I'm getting this mixed up with ParAff. That above is lawful.

[natefaubion]

And again you are right in your original post 😆 Long day...

[natefaubion]

I'll remove this instance in my branch.

[natefaubion]

Does this mean that Alternative is also bad, since it relies on similar distributivity?

[safareli]

IO like structures can't be MonadPlus (slamdata/purescript-io#3)
But I think it can be lawful MonadOr (purescript/purescript-control#35)