Thank you Edward for your reply. It took some time for me to understand a formal description (translation function L) of the Haskell layout rule in Section 10.3 of Haskell 2010 language report. :)
I myself tried to write an example with the Haskell layout rule for me to understand it. I am sharing this for those who are curious about the Haskell layout rules. - http://lazyswamp.blogspot.com/2021/10/on-haskell-layout-rule.html Kwanghoon PS. To be honest, however, I still don't know how Haskell parser (Parser.y) and lexer (Lexer.x) interact with each other to implement the following rule on the insertion of a closing brace on a parse error. [Sec. 10.3] - L (t : ts) (m : ms) = } : (L (t : ts) ms) if m /= 0 and parse-error(t) It seems that Happy's error recover is relevant to the interaction via a production rule: [ Parser.y ] - close :: { () } : vccurly { () } -- context popped in lexer. | error {% popContext } Is it enough for a Haskell lexer just to pop the top context? It is not clear to me how vccurly is inserted just by having such a production rule and an error recovery. On Wed, 18 Aug 2021 at 17:05, Edward Kmett <ekm...@gmail.com> wrote: > Unfortunately, the current parsing rules for Haskell aren't fully > phase-separable like this. > > If you look at the rules for Layout token insertion in the Haskell report > the 9th rule requires that in the event the parser encounters a parse > error it should insert a virtual close brace and continue on! > > Otherwise you couldn't parse things like *let **{** foo = bar **}** in > baz *where the {}'s are virtual without reframing *let* and *in* as a > different kind of paired opening and closing brace or using other hacks in > the grammar. It is quite difficult to hack around all the ways parses can > go wrong. > > The main downside this has from a language standpoint is you simply can't > properly lex Haskell without more or less fully parsing Haskell. > > -Edward > > On Wed, Aug 18, 2021 at 7:22 AM Kwanghoon Choi <lazysw...@gmail.com> > wrote: > >> >> Hi, >> >> I have recently been playing with GHC's Lexer.lexer in the ghc-parser-lib >> package. >> >> Given >> >> module HelloWorld where >> >> main = putStrLn "Hello World!\n" >> >> it produces >> >> stack exec -- lexer-exe ./examples/HelloWorld.hs >> Lexing&Parsing: ./examples/HelloWorld.hs >> module at (1, 1): module >> CONID at (1, 8): CONID >> where at (1, 19): where >> vocurly at (3, 1): vocurly <==== { is inserted automatically!! >> VARID at (3, 1): VARID >> = at (3, 6): = >> VARID at (3, 8): VARID >> STRING at (3, 17): STRING >> ; at (4, 1): ; >> >> By the example above, the lexer automatically inserts an opening brace >> (i.e. vocurly) right after 'where'. But it does not insert a matching >> closing brace (i.e., vccurly), which would lead to a failure in parsing a >> list of tokens produced by the lexer. >> >> My question is how to use the GHC lexer to produce closing braces as >> well. >> >> All my code is available >> - https://github.com/kwanghoon/hslexer >> >> To save your time, the relevant part of the code is as follows: >> >> In app/HaskellLexer.hs, >> >> singleHaskellToken :: P (Located Token) >> singleHaskellToken = >> Lexer.lexer False >> (\locatedToken -> P (\pstate -> POk pstate locatedToken)) >> >> tokInfos :: [Terminal Token] -> P (Line, Column, [Terminal Token]) >> tokInfos s = do >> locatedToken <- singleHaskellToken >> case locatedToken of >> L srcspan ITeof -> >> let (start_line, start_col, end_line, end_col) = >> srcSpanToLineCol srcspan in >> return (end_line, end_col, s) >> >> L srcspan tok -> >> let (start_line, start_col, end_line, end_col) = >> srcSpanToLineCol srcspan in >> tokInfos (Terminal (fromToken tok) start_line start_col (Just >> tok) : s) >> >> Thanks in advance >> >> Best regards, >> >> Kwanghoon >> >> >> >> >> _______________________________________________ >> ghc-devs mailing list >> ghc-devs@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >> >
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