On Thu, Nov 26, 2015 at 8:36 AM, Herbert Valerio Riedel
On 2015-11-26 at 09:28:32 +0100, Akio Takano wrote:
[...]
I understand this, but perhaps there is a way to achieve this without slowing down existing code. How about introducing a new warning (enabled with -Wall) that is triggered when a type satisfies the following 3 conditions?
1. The type has a Monad instance and an Applicative instance declared in the same module, with the same set of constraints. 2. (*>) is not defined as (*>) = (>>). i.e. either it has a non-trivial definition or its definition is left out. 3. (>>) is not defined as (>>) = (*>). i.e. either it has a non-trivial definition or its definition is left out.
This way, people can be warned when (*>) and (>>) can share an implementation but they don't.
Coincidentally, I've recently implemented something similar to that end (but it is *not* enabled via -Wall/-Wcompat by default yet), see
https://git.haskell.org/ghc.git/commitdiff/f09f2470a76bb08b7f51d2f5663daa672...
for details.
Thank you for the information. If I understand correctly, this -fwarn-noncanonical-monad-instances warning doesn't address my concern, because it won't warn about a monad that defines neither (>>) nor (*>) explicitly, which I expect to be a common case. - Akio
Cheers, hvr