In C, AFAIU you can (and probably should) defer `typedef` usage
recognition to a separate "renamer/ name resolution" pass. In
Haskell we are forced to do name resolution after parsing, as we
don't need to declare stuff before use. Even so, separate pass is
usually a good idea anyway, you are better equipped to produce
good error messages. In fact GHC does even more: it defers the
unbound names reporting to the type checking phase, so it can give
the types to unbound variables, like:
Prelude> x : "foo"
<interactive>:2:1: error: Variable not in scope: x ::
Char
- Oleg
Feedback between lexer and parser isn't exactly unusual. Consider that parsing a C `typedef` generally needs to feed back to the lexer so uses will be recognized properly.
On Wed, Nov 1, 2023 at 12:28 AM Oleg Grenrus <oleg.grenrus@iki.fi> wrote:
Yes, the "communication between lexer and parser" is exactly what GHC does._______________________________________________
Amelia has a nice post about it https://amelia.how/posts/parsing-layout.html which made it click it for me.
Note, you don't actually need to use alex and happy, you can do hand-written lexer and parsec (or alex and parsec, ...). The key insight is to have stateful lexer, and control it from the parser.
Amelia's post grammar is a bit too strict, e.g. GHC accepts real semis in virtual layout, and also empty "statements" in between, so we can write
\x y z -> case x of True -> y;;;;;; False -> z
but that's easy (at least in parsec) to adjust the parser grammar to accept those.
Or, you can *approximate* the parse-error rule with "alternative layout rule" [1], which can be implemented as a pass between lexing and parsing, or as a stateful lexer (but in this case parser won't need to adjust lexer's state). GHC has an undocumented AlternativeLayoutRule extension, so you can experiment with it to see what it accepts (look for tests in GHC source for examples). It handles let-in bindings well enough.
On 1.11.2023 0.31, Travis Athougies wrote:
[1] https://www.mail-archive.com/haskell-prime@haskell.org/msg01938.html which can be imp
- Oleg
According to the Haskell report [1] (See Note 5), a virtual `}` token> _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post.
is inserted if parsing the next token would cause a parse error and the
indentation stack is non-empty.
I'm trying to lex and parse Haskell source and this sort of interplay
(which requires two-way communication between lexer and parser) makes
it very difficult to write a conformant implementation.
I can't change the standard (obviously), but I'm wondering if this is
actually what GHC (de facto the only Haskell compiler) does, or if it
applies some other rule. If so, does anyone know the exact mechanism of
its implementation?
I've been programming Haskell for more than a decade, and while I have
an intuitive understanding of the indentation rules, I would have
assumed the source could be lexed without also having a parser. In
particular, the note seems to imply that the main purpose of this is to
properly lex `let`/`in` bindings. Perhaps there's an alternate
equivalent rule?
Curious to hear other's thoughts.
Travis
[1]
https://www.haskell.org/onlinereport/haskell2010/haskellch10.html#x17-17800010.3
Haskell-Cafe mailing list
To (un)subscribe, modify options or view archives go to:
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
Only members subscribed via the mailman list are allowed to post.
--
brandon s allbery kf8nh