Add Foldable & Traversable Field instances

Cabal/Distribution/Fields/Field.hs

 {-# LANGUAGE DeriveFunctor #-}
+{-# LANGUAGE DeriveFoldable #-}
+{-# LANGUAGE DeriveTraversable #-}
 -- | Cabal-like file AST types: 'Field', 'Section' etc
 --
 -- These types are parametrized by an annotation.
@@ -33,7 +35,7 @@ import qualified Data.Char                   as Char
 data Field ann
     = Field   !(Name ann) [FieldLine ann]
     | Section !(Name ann) [SectionArg ann] [Field ann]
-  deriving (Eq, Show, Functor)
+  deriving (Eq, Show, Functor, Foldable, Traversable)
 
 -- | Section of field name
 fieldName :: Field ann -> Name ann
@@ -56,7 +58,7 @@ fieldUniverse f@(Field _ _)      = [f]
 --
 -- /Invariant:/ 'ByteString' has no newlines.
 data FieldLine ann  = FieldLine  !ann !ByteString
-  deriving (Eq, Show, Functor)
+  deriving (Eq, Show, Functor, Foldable, Traversable)
 
 -- | Section arguments, e.g. name of the library
 data SectionArg ann
@@ -66,7 +68,7 @@ data SectionArg ann
       -- ^ quoted string
     | SecArgOther !ann !ByteString
       -- ^ everything else, mm. operators (e.g. in if-section conditionals)
-  deriving (Eq, Show, Functor)
+  deriving (Eq, Show, Functor, Foldable, Traversable)
 
 -- | Extract annotation from 'SectionArg'.
 sectionArgAnn :: SectionArg ann -> ann
@@ -84,7 +86,7 @@ type FieldName = ByteString
 --
 -- /Invariant/: 'ByteString' is lower-case ASCII.
 data Name ann  = Name       !ann !FieldName
-  deriving (Eq, Show, Functor)
+  deriving (Eq, Show, Functor, Foldable, Traversable)
 
 mkName :: ann -> FieldName -> Name ann
 mkName ann bs = Name ann (B.map Char.toLower bs)