Hi Malcolm,
Thank you for the thoughtful reply.
I understand you to make the [reasonable] assertion that the presence of "{n}"
does not constitute a "nested context" because the existence of a "nested
context" is determined by the state of the "context stack", and thus by L
(rather than the mere presence of "{n}" in the input to L).
However, if you accept this interpretation of "nested context", then the second
sentence and example of Note 1 do not seem to make sense. That is, there does
not seem to be a case where the test for the "layout error" described in Note 1
would be met. Indeed, that test does not seem to even apply to the example
that is given in Note 1 (the example code would produce a syntax error, but not
be an example of the "layout error" defined in Note 1).
(Justification: the rules for pushing to the context stack seem to preclude
pushing a value that is less than or equal to the top of the stack, except in
the case of explicit braces which are allowed and clearly not the subject of
Note 1.)
So, put as a question, given your reasonable interpretation of nested context,
what is the purpose of the second sentence of Note 1 ("If not, ... layout
error.")?
I.e., can we conclude that the second sentence of Note 1 is a NOOP and the
example misleading or incorrect?
(To be clear, I very much appreciate the inclusion of notes and examples in the
spec, especially where they might reveal a misunderstanding -- this one is just
a minor sticking point for someone trying to write a correct parser.)
Thanks,
Seth
________________________________
From: Malcolm Wallace <malcolm.wall...@me.com>
To: Seth Lastname <i_don.t_check_t...@yahoo.com>
Cc: "haskell-cafe@haskell.org" <haskell-cafe@haskell.org>
Sent: Tuesday, April 2, 2013 5:43 AM
Subject: Re: [Haskell-cafe] Layout section of Haskell 2010 Language Report --
Notes 1&2
On 1 Apr 2013, at 01:21, Seth Lastname wrote:
> Note 2 says, "If the first token after a 'where' (say) is not indented more
> than the enclosing layout context, then the block must be empty, so empty
> braces are inserted."
>
> It seems that, in Note 2, the "first token" necessarily refers to a lexeme
> other than '{' (else it would not make sense),
Correct.
> in which case a '{n}' token will have been inserted after 'where' (in the
> example given in the note), yielding a nested context which is "not indented
> more than the enclosing layout context",
Yes, a "{n}" token has been inserted after the "where".
No, it does not yield an incorrectly nested context, because L is the function
that decides whether to add to the context.
Looking only at the three equations for L that deal with the pseudo-token
"{n}", including their side conditions, we see:
L ({n} : ts) (m : ms) = { : (L ts (n : m : ms)) if n > m (Note 1)
L ({n} : ts) [] = { : (L ts [n]) if n > 0 (Note 1)
L ({n} : ts) ms = { : } : (L (< n >: ts) ms) (Note 2)
So, the third clause is triggered either when the nested-context stack (ms) is
empty and n is zero or negative; or when the context stack is non-empty and
n<m. That is, we avoid creating a context stack that would be incorrect
(according to Note 1).
Regards,
Malcolm_______________________________________________
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