On 30/11/2009 23:05, Duncan Coutts wrote:
On Mon, 2009-11-30 at 17:26 +0000, Ian Lynagh wrote:
On Mon, Nov 30, 2009 at 11:12:36AM +0000, Duncan Coutts wrote:
Hmm, 'where' is a keyword though, so can't that be done at the lexical
level without having to tangle it with the parsing?
I think it would be possible to have a special rule for this case, but
it would be an odd rule in my opinion. I'm not sure it would be possible
to explain why we had the rule, other than "because Haskell 98 behaved
that way".
Yep, I'm happy with the H98 way :-)
Out of interest, without trying it, what do you think this program
should print (the only difference between the 3 functions is the
indentation of the "where" line)?:
main = do print $ f1 1
print $ f2 1
print $ f3 1
10
10
6
f1 x = x + case () of
() -> x
where x = 5
10 because, the where clause belongs to the top level of the function
and names defined in a where clause mask the function parameters.
f2 x = x + case () of
() -> x
where x = 5
10, same reason.
No, the only way to understand this example is by the mechanics of the
layout translation. Layout inserts a ';' before the 'where', because
the 'where' lines up with the current layout context started by the
'()'. Now, 'where' is not a valid way to start a pattern, so the
parse-error rule kicks in, and inserts a '}', after which the 'where' is
valid in the context of the outer function.
Clearly this is not ideal at all. We don't want users to have to reason
in this way in order to understand how their code is parsed.
Cheers,
Simon
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