+-- |@'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
+
+instance ConvertSuccess Headers Ascii where
+ {-# INLINE convertSuccess #-}
+ convertSuccess = convertSuccessVia ((⊥) ∷ AsciiBuilder)
+
+instance ConvertSuccess Headers AsciiBuilder where
+ {-# INLINEABLE convertSuccess #-}
+ convertSuccess (Headers m)
+ = mconcat (header <$> fromFoldable m) ⊕ cs ("\x0D\x0A" ∷ Ascii)
+ where
+ header ∷ (CIAscii, Ascii) → AsciiBuilder
+ {-# INLINE header #-}
+ header (name, value)
+ = cs name ⊕
+ cs (": " ∷ Ascii) ⊕
+ cs value ⊕
+ cs ("\x0D\x0A" ∷ Ascii)
+
+deriveAttempts [ ([t| Headers |], [t| Ascii |])
+ , ([t| Headers |], [t| AsciiBuilder |])
+ ]