Hi Peter, | I´m progarmming a parser for functional programs. Now | I want to implement the infixL and infixR feature to increase | the readability of the code. I would be very glad if anyone | can send me some information about the implementation of | this feature of the Haskell parser or where I can find | something about it. The comment shown below is taken from the Hugs sources (the file static.c) and might help to answer your questions. Hope this helps, Mark /* -------------------------------------------------------------------------- * Dealing with infix operators: * * Expressions involving infix operators or unary minus are parsed as * elements of the following type: * * data InfixExp = Only Exp | Neg InfixExp | Infix InfixExp Op Exp * * (The algorithms here do not assume that negation can be applied only once, * i.e., that - - x is a syntax error, as required by the Haskell report. * Instead, that restriction is captured by the grammar itself, given above.) * * There are rules of precedence and grouping, expressed by two functions: * * prec :: Op -> Int; assoc :: Op -> Assoc (Assoc = {L, N, R}) * * InfixExp values are rearranged accordingly when a complete expression * has been read using a simple shift-reduce parser whose result may be taken * to be a value of the following type: * * data Exp = Atom Int | Negate Exp | Apply Op Exp Exp | Error String * * The machine on which this parser is based can be defined as follows: * * tidy :: InfixExp -> [(Op,Exp)] -> Exp * tidy (Only a) [] = a * tidy (Only a) ((o,b):ss) = tidy (Only (Apply o a b)) ss * tidy (Infix a o b) [] = tidy a [(o,b)] * tidy (Infix a o b) ((p,c):ss) * | shift o p = tidy a ((o,b):(p,c):ss) * | red o p = tidy (Infix a o (Apply p b c)) ss * | ambig o p = Error "ambiguous use of operators" * tidy (Neg e) [] = tidy (tidyNeg e) [] * tidy (Neg e) ((o,b):ss) * | nshift o = tidy (Neg (underNeg o b e)) ss * | nred o = tidy (tidyNeg e) ((o,b):ss) * | nambig o = Error "illegal use of negation" * * At each stage, the parser can either shift, reduce, accept, or error. * The transitions when dealing with juxtaposed operators o and p are * determined by the following rules: * * shift o p = (prec o > prec p) * || (prec o == prec p && assoc o == L && assoc p == L) * * red o p = (prec o < prec p) * || (prec o == prec p && assoc o == R && assoc p == R) * * ambig o p = (prec o == prec p) * && (assoc o == N || assoc p == N || assoc o /= assoc p) * * The transitions when dealing with juxtaposed unary minus and infix * operators are as follows. The precedence of unary minus (infixl 6) is * hardwired in to these definitions, as it is to the definitions of the * Haskell grammar in the official report. * * nshift o = (prec o > 6) * nred o = (prec o < 6) || (prec o == 6 && assoc o == L) * nambig o = prec o == 6 && (assoc o == R || assoc o == N) * * An InfixExp of the form (Neg e) means negate the last thing in * the InfixExp e; we can force this negation using: * * tidyNeg :: OpExp -> OpExp * tidyNeg (Only e) = Only (Negate e) * tidyNeg (Infix a o b) = Infix a o (Negate b) * tidyNeg (Neg e) = tidyNeg (tidyNeg e) * * On the other hand, if we want to sneak application of an infix operator * under a negation, then we use: * * underNeg :: Op -> Exp -> OpExp -> OpExp * underNeg o b (Only e) = Only (Apply o e b) * underNeg o b (Neg e) = Neg (underNeg o b e) * underNeg o b (Infix e p f) = Infix e p (Apply o f b) * * As a concession to efficiency, we lower the number of calls to syntaxOf * by keeping track of the values of sye, sys throughout the process. The * value APPLIC is used to indicate that the syntax value is unknown. * ------------------------------------------------------------------------*/