gistfile1.txt

I am using generic lens to access this algebraic data type's elements by position. It works for position 1 and fails for position 2.

    putStrLn $ show $ ast ^. position @1
    putStrLn $ show $ ast ^. position @2

Here's the algebraic data type:

data Module l
    = Module l (Maybe (ModuleHead l)) [ModulePragma l] [ImportDecl l] [Decl l]
    -- ^ an ordinary Haskell module
    | XmlPage l (ModuleName l) [ModulePragma l] (XName l) [XAttr l] (Maybe (Exp l)) [Exp l]
    -- ^ a module consisting of a single XML document. The ModuleName never appears in the source
    --   but is needed for semantic purposes, it will be the same as the file name.
    | XmlHybrid l (Maybe (ModuleHead l)) [ModulePragma l] [ImportDecl l] [Decl l]
                (XName l) [XAttr l] (Maybe (Exp l)) [Exp l]
    -- ^ a hybrid module combining an XML document with an ordinary module
  deriving (Eq,Ord,Show,Typeable,Data,Foldable,Traversable,Functor,Generic)
  
  Here is the error I get when accessing position 2:
  
      • Couldn't match type ‘ModuleName SrcSpanInfo’
                     with ‘Maybe (ModuleHead SrcSpanInfo)’
        arising from a functional dependency between:
          constraint ‘generic-lens-1.1.0.0:Data.Generics.Product.Internal.GLens.GLens
                        (Data.Generics.Product.Positions.HasTotalPositionPSym 2)
                        (GHC.Generics.K1
                           (generic-lens-1.1.0.0:Data.Generics.Internal.Families.Has.Pos 2)
                           (ModuleName SrcSpanInfo))
                        (GHC.Generics.K1
                           (generic-lens-1.1.0.0:Data.Generics.Internal.Families.Has.Pos 2)
                           (ModuleName SrcSpanInfo))
                        (Maybe (ModuleHead SrcSpanInfo))
                        (Maybe (ModuleHead SrcSpanInfo))’
            arising from a use of ‘position’
          instance ‘generic-lens-1.1.0.0:Data.Generics.Product.Internal.GLens.GLens
                      pred (GHC.Generics.K1 r a) (GHC.Generics.K1 r b) a b’
            at <no location info>