How does GHC implement layout?
Hi all, I’m wondering if there are any resources that discuss the design of GHC’s implementation of layout. (I haven’t been able to find any.) From looking at the code, here’s what I’ve gathered so far: * Layout is implemented in the lexer (compiler/GHC/Parser/Lexer.x). * The implementation is similar in some respects to the approach described in the Haskell Report, but still fairly different. Virtual braces and semicolons are inserted during the lexing process itself with the assistance of Alex lexer states (aka “start codes”). * In order to handle particularly tricky cases like if e then do x; y else z where the virtual close brace must be inserted in the middle of a line, tokens such as in and else are given special context-sensitive treatment. This appears to be quite subtle. Overall, I can mostly follow the code, but I still have a few unanswered questions: * The layout-related code consistently uses the phrase “alternative layout rule”—what does “alternative” mean here? Does it refer to GHC’s implementation of layout? Or maybe it refers to NondecreasingIndentation? It isn’t clear. * The implementation of layout seems quite complex, in large part because it has to worry about parsing concerns in the lexer in order to handle tricky cases like the one I provided above. Is there are reason all this is done in the lexer, rather than deferring some more of the work to the parser? I’ve found remarkably little information about implementing layout in general, so perhaps I’m missing some resources or keywords to search for, but any information or perspectives would be appreciated! Thanks, Alexis
Hi Alexis,
I wasn't sure what the "alternative layout" is either and did some
googling, and it appears that it is something that was never really
documented properly. The following link contains pointers to the commit
that introduced it (in 2009!) (not the main ticket but some of the
comments)
https://ghc-tickets.haskell.narkive.com/htgwkF80/13087-alternativelayoutrule...
Overall, I do think that Haskell's layout rule is more complicated than it
needs to be, and this is mostly because of the rule that requires the
insertion of a "virtual close curly" on a parse error. This means that the
parser and lexer have to communicate. I've implemented a few
languages with layout, and usually use a simpler version of layout that
just omits that case. The benefit is that layout can be implemented as a
simple pre-processor pass on the stream of tokens, which is much simpler
to specify and implement. The drawback is that sometimes you have to
write programs in a slightly different way, but nothing that can't be
easily worked around.
My feeling is that it'd be pretty tricky to do layout in the parser with
grammar rules, but you may be able to do something with the parser state.
I wonder how different it would end up looking though, as in a way that's
exactly what we are doing now, it is just that some of the state is the
lexer.
-Iavor
On Sat, Apr 3, 2021 at 5:05 PM Alexis King
Hi all,
I’m wondering if there are any resources that discuss the design of GHC’s implementation of layout. (I haven’t been able to find any.) From looking at the code, here’s what I’ve gathered so far:
- Layout is implemented in the lexer (compiler/GHC/Parser/Lexer.x).
- The implementation is similar in some respects to the approach described in the Haskell Report, but still fairly different. Virtual braces and semicolons are inserted during the lexing process itself with the assistance of Alex lexer states (aka “start codes”).
- In order to handle particularly tricky cases like
if e then do x; y else z
where the virtual close brace must be inserted in the middle of a line, tokens such as in and else are given special context-sensitive treatment. This appears to be quite subtle.
Overall, I can mostly follow the code, but I still have a few unanswered questions:
- The layout-related code consistently uses the phrase “alternative layout rule”—what does “alternative” mean here? Does it refer to GHC’s implementation of layout? Or maybe it refers to NondecreasingIndentation? It isn’t clear.
- The implementation of layout seems quite complex, in large part because it has to worry about parsing concerns in the lexer in order to handle tricky cases like the one I provided above. Is there are reason all this is done in the lexer, rather than deferring some more of the work to the parser?
I’ve found remarkably little information about implementing layout in general, so perhaps I’m missing some resources or keywords to search for, but any information or perspectives would be appreciated!
Thanks, Alexis _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
On 4/4/21 1:52 PM, Iavor Diatchki wrote:
Hi Alexis,
I wasn't sure what the "alternative layout" is either and did some googling, and it appears that it is something that was never really documented properly. The following link contains pointers to the commit that introduced it (in 2009!) (not the main ticket but some of the comments)
Thanks, that’s a helpful pointer, though of course it still doesn’t explain very much. I’m still interested in understanding what the purpose of “alternative layout” is and how it operates, if anyone else has any idea.
Overall, I do think that Haskell's layout rule is more complicated than it needs to be, and this is mostly because of the rule that requires the insertion of a "virtual close curly" on a parse error.
Yes, this does seem to be by far the trickiest bit. But I’d be sad not to have it, as without it, even simple things like let x = 3 in e would not be grammatically valid.
My feeling is that it'd be pretty tricky to do layout in the parser with grammar rules, but you may be able to do something with the parser state.
Yes, I have some vague ideas, but none of them are particularly fleshed out. It’s entirely possible that I just don’t understand the relationship between the lexer and the parser (which seems somewhat obscured by the “alternative layout” stuff), and the ideas I have are what’s already implemented today. I’ll have to study the implementation more closely. In any case, thank you for your response! The ALR-related pointer certainly clarifies at least a little. Alexis
Hi Alexis, Hi Iavor,
I'm afraid I'm not particularly acquainted with how GHC implements
indentation-sensitive parsing, but I really like the way in which this book
https://www.springer.com/gp/book/9783642175398 frames the problem. If you
look at the preview for the first chapter, you'll notice that (they call
the lexer scanner, and) they introduce an additional pass between lexer and
parser that handles the context-sensitive bits about indentation-sensitive
parsing, which doesn't fit well with the parser (which assumes a
context-free grammar to stay sane) or with the lexer (which should better
be just a simple DFA).
In particular, the short section 1.3 about the screener explicitly mentions
Haskell as use case:
[image: 2021-04-05-140820_543x163_scrot.png]
Although that doesn't really explain the how. Maybe section 2.5 (where impl
of screeners is covered) provides more insight into that.
Screeners are a bit like semantic analysis, but on the token stream instead
of the parse tree.
Reach out in private if you want more excerpts.
Cheers,
Sebastian
Am Mo., 5. Apr. 2021 um 00:19 Uhr schrieb Alexis King : On 4/4/21 1:52 PM, Iavor Diatchki wrote: Hi Alexis, I wasn't sure what the "alternative layout" is either and did some
googling, and it appears that it is something that was never really
documented properly. The following link contains pointers to the commit
that introduced it (in 2009!) (not the main ticket but some of the
comments) Thanks, that’s a helpful pointer, though of course it still doesn’t
explain very much. I’m still interested in understanding what the purpose
of “alternative layout” is and how it operates, if anyone else has any idea. Overall, I do think that Haskell's layout rule is more complicated than it
needs to be, and this is mostly because of the rule that requires the
insertion of a "virtual close curly" on a parse error. Yes, this does seem to be by far the trickiest bit. But I’d be sad not to
have it, as without it, even simple things like let x = 3 in e would not be grammatically valid. My feeling is that it'd be pretty tricky to do layout in the parser with
grammar rules, but you may be able to do something with the parser state. Yes, I have some vague ideas, but none of them are particularly fleshed
out. It’s entirely possible that I just don’t understand the relationship
between the lexer and the parser (which seems somewhat obscured by the
“alternative layout” stuff), and the ideas I have are what’s already
implemented today. I’ll have to study the implementation more closely. In any case, thank you for your response! The ALR-related pointer
certainly clarifies at least a little. Alexis
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while not totally perfectly related, i have wondered if something like the
the recent pika parsing paper https://arxiv.org/abs/2005.06444 could be
used to provide better error recovery from layout related errors in
haskell and similar languages.
On Sun, Apr 4, 2021 at 6:19 PM Alexis King
On 4/4/21 1:52 PM, Iavor Diatchki wrote:
Hi Alexis,
I wasn't sure what the "alternative layout" is either and did some googling, and it appears that it is something that was never really documented properly. The following link contains pointers to the commit that introduced it (in 2009!) (not the main ticket but some of the comments)
Thanks, that’s a helpful pointer, though of course it still doesn’t explain very much. I’m still interested in understanding what the purpose of “alternative layout” is and how it operates, if anyone else has any idea.
Overall, I do think that Haskell's layout rule is more complicated than it needs to be, and this is mostly because of the rule that requires the insertion of a "virtual close curly" on a parse error.
Yes, this does seem to be by far the trickiest bit. But I’d be sad not to have it, as without it, even simple things like
let x = 3 in e
would not be grammatically valid.
My feeling is that it'd be pretty tricky to do layout in the parser with grammar rules, but you may be able to do something with the parser state.
Yes, I have some vague ideas, but none of them are particularly fleshed out. It’s entirely possible that I just don’t understand the relationship between the lexer and the parser (which seems somewhat obscured by the “alternative layout” stuff), and the ideas I have are what’s already implemented today. I’ll have to study the implementation more closely.
In any case, thank you for your response! The ALR-related pointer certainly clarifies at least a little.
Alexis _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
On Sun, Apr 04, 2021 at 05:18:52PM -0500, Alexis King wrote:
On 4/4/21 1:52 PM, Iavor Diatchki wrote:
Overall, I do think that Haskell's layout rule is more complicated than it needs to be, and this is mostly because of the rule that requires the insertion of a "virtual close curly" on a parse error.
Yes, this does seem to be by far the trickiest bit. But I’d be sad not to have it, as without it, even simple things like
let x = 3 in e
would not be grammatically valid.
That is accepted by the AlternativeLayoutRule. Thanks Ian
participants (5)
-
Alexis King -
Carter Schonwald -
Ian Lynagh -
Iavor Diatchki -
Sebastian Graf