On Mon, 15 Jul 2013, MightyByte wrote:
I'll just throw out a data point related to this discussion. In one of my previous commercial Haskell projects we ended up writing a similar Num instance for Maybe a. I think I asked about it in #haskell back then and got pretty vocal distaste about the idea from several people who spoke up. "Maybe is not a number!" But we did end up using the instance in our project because it greatly simplified a section of our code.
I also had this analogy in mind. Looks like syntactic heroin. Like (IsString (Maybe String)) does not fulfill intuitive laws, (Num (Maybe a)) does not, too. E.g. if in your code you see 'a+b-b' you would like to simplify that to 'a'. With a Maybe instance this is not true in case 'b' is Nothing.
I'm certainly not going to recommend that we also add the Num instance for Maybe. But I think it's a useful data point that this happened in real-world production code at a company where writing Haskell code was my full-time job.
This confirms that real world code contains a lot of mess.