On 30/06/2011 14:52, Yitzchak Gale wrote:
It was pointed out by Ben Millwood on the Cafe that there is an undocumented way to escape the closing oxford bracket of a quasi-quote using a backslash:
[s|This quasi-quote contains this, \|], an escaped closing oxford bracket.|]
The backslash itself cannot be escaped in this way:
[s|Also contains an escaped bracket \\|] |]
Thus there is a fairly strong limitation on the contents of a quasi-quote: it can never end in a backslash.
This behavior is not mentioned in the GHC docs. Is it a mistake, or is it meant to be a supported feature?
This behavior is a bit surprising to me. Since the whole point of a quasi-quoter is to allow the user to define syntax, you would think that the syntax for the quasi-quote itself would be as quiet as possible and stay out of the way. People who need to be able to escape the closing bracket can easily define their own syntax to do so.
In any case, if this is indeed a feature, it certainly should be documented.
It looks intentional to me: lex_quasiquote :: String -> P String lex_quasiquote s = do i <- getInput case alexGetChar' i of Nothing -> lit_error i Just ('\\',i) | Just ('|',i) <- next -> do setInput i; lex_quasiquote ('|' : s) | Just (']',i) <- next -> do setInput i; lex_quasiquote (']' : s) where next = alexGetChar' i Just ('|',i) | Just (']',i) <- next -> do setInput i; return s where next = alexGetChar' i Just (c, i) -> do setInput i; lex_quasiquote (c : s) Indeed, it also seems strange that "\\]" is interpreted as "]". That's all I know. I agree we should either document or remove the feature. Cheers, Simon