fischer at nii.ac.jp
Tue Aug 30 04:48:33 CEST 2011
On Mon, Aug 29, 2011 at 6:21 AM, Bas van Dijk <v.dijk.bas at gmail.com> wrote:
> Won't option 1 "Reject this as a duplicate instance declaration, which
> indeed it is." conflict with design goal 1: "a class C can be
> re-factored into a class C with a superclass S, without disturbing any
If yes, I prefer option 3:
> "Allow the explicit to supersede the intrinsic default silently".
The argument against this option is:
> I might notice
> that Foo is a monad and add a Monad Foo instance in my own code,
> expecting the Applicative Foo instance to be generated in concert; to
> my horror, I find my code has subtle bugs because the package
> introduced a different, non-monadic, Applicative Foo instance which
> I'm accidentally using instead.
This seems rare enough that it's feasible to issue a warning if a
default instance is overwritten by an explicit instance in a different
module which would prevent the described perplexity. This wouldn't,
for example, disturb the transformers example mentioned by Bas because
(I think) all instances are defined in the same module.
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