On Mon, Feb 25, 2002 at 04:27:56PM -0000, Simon Marlow wrote:
This is a known problem with the Haskell grammar. Another example in a similar vein is
let x = 3 in x == 4 == True
which should parse as
(let x = 3 in x == 4) == True
according to the "extends as far to the right as possible" rule, because '==' is nonfix. In order to do this, the parser must have access to the fixity information for '==', which is unreasonable (fixity declarations can even be local to a binding group).
I'm not sure I agree with you here. By this point we have lexed and applied the layout rule so all the braces and semi-colons are in place. Extracting the fixity information is fairly easy Noticing when operators are redefined (thus dropping fixity information) is slightly trickier but still doable, and then all you have to do is annotate your tree appropriately. Further, if you first construct a tree of blocks then this can all be done in linear time I believe. I don't see that this is a forced change; it seems to be purely to make implementors lives easier (maybe I am just bitter from spending a while thinking about and implementing this myself!) and makes some previously legal scripts illegal. Thanks Ian