[EMAIL PROTECTED] (Carl R. Witty) wrote,
> "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?
Seems plausible to me.
> Are there any implementations that actually follow the
> standard here?
GHC (4.02) and Hugs 98 do not recognise
do a == b == c
They produce an error messge complaining about the mismatch
of associativity.
Manuel
