Generic instance

[akegalj]

Is there a reason of not heaving default derived Generic instance ?

[rgrempel]

Generally speaking, having a default Generic instance is equivalent to exporting your constructors, so you might not want one for the same reasons you might not want to export your constructors.

For instance, in this case, the Int53 constructor takes a Number internally. However, we don't export it, because not all numbers are integers (we just use Number as a convenience to get all 53 bits). Since we want to limit Int53 values to actual integers, we can't export the constructor.

If we had a default Generic instance, you could do something like this:

badInt53 = fromSpine (SProd "Data.Int53.Int53" [\_ -> SNumber 27.3]) :: Maybe Int53
-- Just (Int53 27.3)

... which we wouldn't want to be possible.

Now, it's possible that we could write a Generic instance manually that would be fine ... it would probably use fromNumber at some point to take an SNumber and turn it into a Maybe Int53. But I haven't entirely thought that through.

[akegalj]

However, we don't export it, because not all numbers are integers
I agree, didn't think about it enough.

Yes, we are using custom Generic instance, but I guess I was slightly annoyed with writing additional newtype wrapper so that I can define Generic. But you are right, adding generic would be wrong in this case. Thank you for your explanation.

Note that you can derive Eq and Ord . I think their derived instance is the same as your manual instances.

[rgrempel]

Actually, I was thinking of adding a manual Generic instance to Int53 itself, so you wouldn't need the newtype. Something like this:

ea72550

Does that look like it might be helpful? I can merge it if so.

[akegalj]

We are using more simple instance of Generic (it might be wrong general case but it suits our needs). I don't have time now so will review it tomorrow (and read more about Generic, don't have much experience with it).

[rgrempel]

I've published version 2.1.0, with the manually-written Generic instance.

[akegalj]

Thank you. Sorry for not being able to review your code (I had it on my todo but can't allocate free time currently). Many thanks <3