Thanks a lot! This makes clear. I haven't noticed before that OverlappingInstances don't look at constraint! John Lato-2 wrote
This line
instance Monad m => Applicative m where
tells the compiler "Every type (of the appropriate kind) is an instance of Applicative. And it needs to have a Monad instance as well."
That's what Edward means when he said that it means "every Applicative is a Monad". Theoretically the statement makes no sense, but that's what this instance head means. Everything is Applicative, and it also needs a Monad instance to use that Applicative.
Consider what happens for something that isn't a Monad, e.g. ZipList. Since it's not a Monad, it would need its own instance
instance Applicative ZipList where ...
But now you'd need to enable OverlappingInstances, because ZipList matches both this instance and the general one you've defined above (GHC doesn't consider constraints when matching instance heads). OverlappingInstances is much more problematic than the other extensions because it could (and almost certainly would in this case) give rise to incoherence (see the warning under http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extension... ).
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