Add missing Generic instances in base

Most primitive types (Int, Char, etc.) have a GHC.Generics.Generic instance, but some are missing, such as Integer and the various Word* data types.

We should consistently have instances for all primitive types offered by base, since they are required if you want to derive a Generic instance for any type that contains them.

I boldly set the milestone to 7.10 because I naively assume this should be straightforward.

Trac metadata

Trac field	Value
Version	7.8.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ekmett, hvr, mail@nh2.me
Operating system
Architecture

changed milestone to %7.10.1

changed weight to 5

added Tfeature request Trac import core libraries labels

Trac metadata

Trac field	Value
Blocking	→ #9527 (closed)

mentioned in issue #9527 (closed)

assigned to @trac-dreixel

Actually, I'm thinking that perhaps we should delete the existing Generic instances for base types. They don't really add much; the only thing I can think of is that you can find out the module and datatype name. Other than that, any generic function will still have to provide adhoc behaviour for types such as Int, because there is no generic behaviour for these base types.

Also, this doesn't really block #9527 (closed), as far as I can see. You can derive Generic for a datatype that contains Integer, for example, which doesn't have a Generic instance. Things only fail when you try to use a generic function that does not have an instance for Integer.

Moving over to new owning component 'Core Libraries'.

Trac metadata

Trac field	Value
Component	libraries/base → Core Libraries

Trac metadata

Trac field	Value
Blocking	#9527 (closed) →

removed milestone

Moving to 7.12.1 milestone; if you feel this is an error and should be addressed sooner, please move it back to the 7.10.1 milestone.

mentioned in issue #10512 (closed)

mentioned in issue #10512 (closed)

changed milestone to %8.0.1

Milestone renamed

closed

I'm going to close this for the same reasons that I closed #10512 (closed). Please re-open if you feel strongly otherwise.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedWon'tFix

I can't tell - I don't really understand the change; link in the ticket description of #9766 (closed) seems broken.

Can you explain a bit what this change does, and how it affects this issue? Thanks!

Sorry, my bad. I linked the wrong issue. It should be #10512 (closed), where Generic instances were proposed for Int32 and Word64. The resolution of that issue is wontfix, and in GHC 8.0, the Generic instances for Char, Float, Double, and Int will be removed.

Pedro outlines the reasons for not including Generic instances for primitive types here. Basically, Generic instances are intended for inspecting the structure of regular, structured algebraic data types until you get down to the "leaves" of a data structure. The leaves only require //ad hoc// instances of the base class, not of Generic.

Moreover, the previous Generic instances base types were lying to users. Previously, we had something like this:

type Rep Char = D1 D_Char (C1 C_Char (S1 NoSelector (Rec0 Char))

But according to the rules of how Generic operates, that would suggest that data Char = Char Char, which is absolutely not the case. To avoid this dishonesty, we decided to remove the offending instances altogether.

Trac metadata

Trac field	Value
Related	- → #10512 (closed)

I think you linked the right issue; I got to #9766 (closed) via the git commit you mentioned in #10512 (closed).

What you say makes sense, and it does look like a better solution.

I don't know if it addresses all issues that motivated this issue, but your comment at #10512 (closed)#ticket:9526#comment:94532 suggests that it does. I'll try that out when GHC 8 is released with this change, and reopen if there's any remaining problem.

Thanks!

See ticket:10512#comment:120834.

added Pnormal label