Consider a simple language of logical expressions:
import Control.Applicative import Text.Parsec hiding ((<|>), many) import Text.Parsec.String import Text.Parsec.Expr
data Expr = Truth | Falsity | And Expr Expr | Or Expr Expr | Not Expr deriving (Show, Eq)
I define a simple expression parser using Parsec:
expr :: Parser Expr expr = buildExpressionParser table (lexeme term) "expression"
term :: Parser Expr term = between (lexeme (char '(')) (lexeme (char ')')) expr <|> bool "simple expression"
bool :: Parser Expr bool = lexeme (string "true" *> pure Truth) <|> lexeme (string "false" *> pure Falsity)
lexeme :: Parser a -> Parser a lexeme p = p <* spaces
table = [ [prefix "!" Not ] , [binary "&" And AssocLeft ] , [binary "|" Or AssocLeft ] ]
binary name fun assoc = Infix (do{ lexeme (string name); return fun }) assoc prefix name fun = Prefix (do{ lexeme (string name); return fun })
Now this doesn't work:
test1 = parseTest expr "!!true"
But this does:
test2 = parseTest expr "!(!true)"
I have studied the code for buildExpressionParser, and I know why this happens (prefix operators are treated as nonassociative), but it seems like one would often want right-associative prefix operators (so test1 would work). Is there a common workaround or solution for this problem? I assume the nonassociativity in Parsec is by design and not a bug. -- \ Troels /\ Henriksen