> This program:
>
> module Main where
> f = map (++"a"++"b")
> g = map ("a"++"b"++)
> main = do print (f ["x","y","z"])
> print (g ["x","y","z"])
>
> gives the following error messages with ghc (versions 4.08.2
> and 5.02):
>
> plusplus.hs:3:
> The operator `++' [infixr 5] of a section
> must have lower precedence than the operand `++'
> [infixr 5]
> In the section: `(++ ("a" ++ "b"))'
>
> plusplus.hs:4:
> The operator `++' [infixr 5] of a section
> must have lower precedence than the operand `++'
> [infixr 5]
> In the section: `(("a" ++ "b") ++)'
>
> I believe this is the wrong behaviour - the first (right) section is
> perfectly valid Haskell'98 because the operator is right associative.
> Hugs, hbc, and nhc98 all accept it. Check also section 3.5 (p.17)
> of the Report.
>
> The second (left) section is indeed incorrect, again because the
> operator is right associative.
The text of section 3.5 seems to disagree with the syntax. The syntax
says that these sections are allowed:
'(' exp(i+1) op(a,i) ')'
'(' op(a,i) exp(i+1) ')'
Which looks like the rule that GHC is implementing: the precedence of
the operator must be strictly lower than that of the expression.
I think it would be better to change the syntax to match the text rather
than the other way around: clearly sections like (++a++b) are desirable.
If I understand the notation in the report correctly, I think adding
these two productions to aexp would do the trick:
'(' lexp(i) op(l,i) ')'
'(' op(r,i) rexp(i) ')'
And for the sake of fewer ambiguities, replace each op(a,i) with op(n,i)
in the existing two section productions.
Cheers,
Simon
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