"packageName" with GHC.Generics

Question

I have a class that provides a globally unique identifier for types:

class Named a where
 nameOf :: a -> (String,String,String) -- (Package, Module, Identifier)
 default nameOf :: (Generic a, Named' (Rep a)) => a -> (String,String,String)
 nameOf = nameOf' . from

which almost works:

>>> data D = C
>>> instance Named D
>>> nameOf C
("","Main","D")

but I can't get the datatype's package with GHC.Generics:

class Named' f where  nameOf' :: f a -> (String,String,String)
instance (Datatype t) => Named' (M1 D t f) where nameOf' d = ("", moduleName d, datatypeName d)

Can I? The GUI's not really "globally" unique without the package.

btw, I know that with Data.Typeable I can write:

>>> import Data.Typeable
>>> :set -XDeriveDataTypeable
>>> let nameOf = (\t -> (tyConPackage t, tyConModule t, tyConName t)) . typeRepTyCon . typeRep
>>> data D = C deriving Typeable
>>> nameOf (Proxy :: Proxy D)
("interactive" "Ghci3" "D")

Which is what I may do. But I'm curious about GHC.Generics.

Fatma

AFAIK no. But it's impossible to have same module twice in the same program (even from different packages). – Irizarry

[1] even with PackageImports? (haven't used it, just heard about it) – Fatma

[2] what does "program" mean, in this context? what if you import two different modules from different packages, which each import the "same" module which are from different packages, neither of which packages (the latter two) you directly depend on. does that code not compile? And thanks! – Fatma

Seems that with PackageImports you can import the same module from different packages, even into the same module: gist.github.com/phadej/8b628d579ddf6958d937 Didn't ever tried to do anything like that! – Irizarry

I created a ghc ticket let's see what more wise people respond. – Irizarry

Answer 1

So far it's impossible to get package name using Generics. There is a GHC feature request -ticket now. It was straightforward to implement, but let's see when the patch lands release version.

Irizarry