[Hs-Generics] Re: Traversible Functor Data, or: X marks the spot (Re: Haddock version during build)

Claus Reinke claus.reinke at talk21.com
Sun Jun 29 15:23:00 EDT 2008

>> Does anyone out there *want* functions and IO in Data?
> I have a feeling you sometimes need those instances to make the
> typechecker happy, even though you aren't actually doing anything
> generic on those parts of the types.
> syb-with-class has also defined IO and (->) instances, but no comment as
> to why.

I found this remark in "Scrap More Boilerplate: .." (section 5.3,
non-representable data types):

    Lastly, it is convenient to give Data instances even for types that are
    not strictly data types, such as function types or monadic IO types.
    Otherwise deriving ( Data ) would fail for a data type that had
    even one constructor with a functional argument type, so the user
    would instead have to write the Data instance by hand. Instead,
    we make all such types into vacuous instances of Data. Traversal
    will safely cease for values of such types. However, values of these
    types can not be read and shown.

So the convenience hypothesis seems correct. The problem
with this is that it assumes that the vacuous instances do not
involve any types that the generic traversals are operating on.
Also, while I could add such a vacuous instance Data (a->b),
and then use deriving Data over the remaining types, I cannot
remove an existing instance Data (a->b), nor can I easily 
override its effects.

I suggest to separate the vacuous from the proper instances,
and to expose only the former via Data.Generics. That way,
the convenience is only an import away, but doesn't get in
the way of non-standard applications.


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