+-- |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)