[Haskell-cafe] Is there a handy refl or refl generator for converting GADT types?

[Roman]

You can define a view

    viewGlobalRef :: GlobalRef s -> Either InstanceName (GlobalRef s')
    viewGlobalRef FromIntegerGlobal     = Right FromIntegerGlobal
    viewGlobalRef FromDecimalGlobal     = Right FromDecimalGlobal
    viewGlobalRef (InstanceGlobal name) = Left name

but that loses the `s ~ Resolved` information in the `InstanceGlobal`
case, so I'd define and use

    matchGlobalRef
        :: GlobalRef s
        -> (s ~ Resolved => InstanceName -> r)
        -> (forall s'. GlobalRef s' -> r)
        -> r
    matchGlobalRef globalRef handleInstanceGlobal handleOther =
        case globalRef of
            FromIntegerGlobal   -> handleOther FromIntegerGlobal
            FromDecimalGlobal   -> handleOther FromDecimalGlobal
            InstanceGlobal name -> handleInstanceGlobal name