Hi,
On Thu, Oct 23, 2008 at 3:37 PM, Larry Evans
Does BNFC calculate the lookahead sets for a grammar? I'm attempting to do this with haskell, but I would hate to "reinvent the wheel". I would think if haskell can tranform a syntax tree, then haskell could transform a grammar tree to calculate the lookahead sets. My purpose it not so much create a compiler as illustrate how it's done. I found the Dragon's book description hard to follow and thought redoing it using functors (to transform the tree into another, with different operators) then simplify the transformed trees (one tree for each lookahead set) then find the fixpoint of those trees (really forest: 1 tree for each production) would be a good way to demonstrate the method.
As stated at the BNFC web page, "BNFC is a compiler compiler compiler", i.e. it generates specifications for other lexers and parsers based on the given grammar (expressed in Backus Nour Form). In Haskell mode it outputs specifications for Alex[1] and Happy[2] which in turn can be used to generate haskell code for lexing and parsing. So if I understood you correctly, I think the answer to your question is no. But maybe you wanna have a look the haskell parser code generated by Happy? Unfortunately I really don't know anything about the implementations of either BNFC or Happy, so I'm sure others may provide you with more insightful explanations. - Joel [1] http://www.haskell.org/alex/ [2] http://www.haskell.org/happy/