"Manuel M. T. Chakravarty" <[EMAIL PROTECTED]> writes:
> One of our students just pointed out an IMHO rather
> problematic clause in the layout rule. In Section 2.7 of
> the Haskell 98 Report it says,
>
> A close brace is also inserted whenever the syntactic
> category containing the layout list ends; that is, if an
> illegal lexeme is encountered at a point where a close
> brace would be legal, a close brace is inserted.
>
> And in B.3, we have in the first equation of the definition
> of `L',
>
> L (t:ts) (m:ms) = } : (L (t:ts) ms) if parse-error(t) (Note 1)
>
> where Note 1 says,
>
> The side condition parse-error(t) is to be interpreted as
> follows: if the tokens generated so far by L together with
> the next token t represent an invalid prefix of the
> Haskell grammar, and the tokens generated so far by L
> followed by the token } represent a valid prefix of the
> Haskell grammar, then parse-error(t) is true.
Consider the following to see the true horror of the parse-error
condition of the layout rule. As far as I can tell (assuming the
standard prelude), the phrase
do a == b == c
is legal and parses as
do {a == b} == c
but
do a `elem` b `elem` c
is a syntax error. (Both == and `elem` are "infix 4".)
Since == is non-associative,
do { a == b ==
is not a legal prefix of the Haskell grammar, and
do { a == b }
is. So the layout rule inserts the implicit close-brace before the
second ==.
However,
do { a `elem` b `
is a legal prefix of the Haskell grammar: it could be completed by
do { a `elem` b `seq` c }
for instance. So no implicit close-brace gets inserted; and
a `elem` b `elem` c
is a syntax error.
Does anybody disagree with my interpretation of the standard? Are
there any implementations that actually follow the standard here?
(Maybe the standard should be changed to follow the implementations in
this area.)
Carl Witty
[EMAIL PROTECTED]
