+-- |Convert an 'ETag' to 'Ascii'.
+printETag ∷ ETag → Ascii
+printETag et
+ = A.fromAsciiBuilder $
+ ( ( if etagIsWeak et then
+ A.toAsciiBuilder "W/"
+ else
+ (∅)
+ )
+ ⊕
+ quoteStr (etagToken et) )
+
+-- |Parse 'Etag' from an 'Ascii'. This functions throws an exception
+-- for parse error.
+parseETag ∷ Ascii → ETag
+parseETag str
+ = let p = do et ← eTagP
+ endOfInput
+ return et
+ bs = A.toByteString str
+ in
+ case parseOnly p bs of
+ Right et → et
+ Left err → error ("unparsable ETag: " ⧺ A.toString str ⧺ ": " ⧺ err)