+ getHeaders ∷ a → Headers
+ setHeaders ∷ a → Headers → a
+
+ modifyHeaders ∷ (Headers → Headers) → a → a
+ {-# INLINE modifyHeaders #-}
+ modifyHeaders = (setHeaders ⊛) ∘ (∘ getHeaders)
+
+ getHeader ∷ CIAscii → a → Maybe Ascii
+ {-# INLINE getHeader #-}
+ getHeader = (∘ getHeaders) ∘ lookup
+
+ hasHeader ∷ CIAscii → a → Bool
+ {-# INLINE hasHeader #-}
+ hasHeader = (∘ getHeaders) ∘ member
+
+ getCIHeader ∷ CIAscii → a → Maybe CIAscii
+ {-# INLINE getCIHeader #-}
+ getCIHeader = ((cs <$>) ∘) ∘ getHeader
+
+ deleteHeader ∷ CIAscii → a → a
+ {-# INLINE deleteHeader #-}
+ deleteHeader = modifyHeaders ∘ delete
+
+ setHeader ∷ CIAscii → Ascii → a → a
+ {-# INLINE setHeader #-}
+ setHeader = (modifyHeaders ∘) ∘ insertWith const
+
+instance HasHeaders Headers where
+ getHeaders = id
+ setHeaders _ = id
+
+C.derive [d| instance Foldable Headers (CIAscii, Ascii)
+ instance Collection Headers (CIAscii, Ascii)
+ instance Indexed Headers CIAscii Ascii
+ instance Map Headers CIAscii Ascii
+ instance SortingCollection Headers (CIAscii, Ascii)
+ |]
+
+-- |@'insert' (key, val)@ merges @val@ with an existing one if any.
+instance Unfoldable Headers (CIAscii, Ascii) where
+ {-# INLINE insert #-}
+ insert (key, val) (Headers m)
+ = Headers $ insertWith merge key val m
+ {-# INLINE empty #-}
+ empty = Headers empty
+ {-# INLINE singleton #-}
+ singleton = Headers ∘ singleton
+
+instance Monoid Headers where
+ {-# INLINE mempty #-}
+ mempty = empty
+ {-# INLINE mappend #-}
+ mappend = insertMany
+
+merge ∷ Ascii → Ascii → Ascii
+{-# INLINE merge #-}
+merge a b
+ | nullA a ∧ nullA b = (∅)
+ | nullA a = b
+ | nullA b = a
+ | otherwise = a ⊕ ", " ⊕ b
+ where
+ nullA ∷ Ascii → Bool
+ {-# INLINE nullA #-}
+ nullA = null ∘ A.toByteString