+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
+ {-# INLINE convertSuccess #-}
+ convertSuccess (Headers m)
+ = mconcat (header <$> fromFoldable m) ⊕ cs ("\x0D\x0A" ∷ Ascii)
+ where
+ header ∷ (CIAscii, Ascii) → AsciiBuilder
+ header (name, value)
+ = cs name ⊕
+ cs (": " ∷ Ascii) ⊕
+ cs value ⊕
+ cs ("\x0D\x0A" ∷ Ascii)
+
+deriveAttempts [ ([t| Headers |], [t| Ascii |])
+ , ([t| Headers |], [t| AsciiBuilder |])
+ ]
+
+{-
+ message-header = field-name ":" [ field-value ]
+ field-name = token
+ field-value = *( field-content | LWS )
+ field-content = <field-value を構成し、*TEXT あるいは
+ token, separators, quoted-string を連結
+ したものから成る OCTET>
+
+ field-value の先頭および末尾にある LWS は全て削除され、それ以外の
+ LWS は單一の SP に變換される。
+-}
+headers ∷ Parser Headers
+{-# INLINEABLE headers #-}
+headers = do xs ← many header
+ crlf
+ return $ fromFoldable xs
+ where
+ header ∷ Parser (CIAscii, Ascii)
+ header = do name ← A.toCIAscii <$> token
+ void $ char ':'
+ skipMany lws
+ values ← content `sepBy` try lws
+ skipMany (try lws)
+ crlf
+ return (name, joinValues values)