Associative prefix operators in Parsec

7 Mar 2012


      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

Troels Henriksen

Christian Maeder

Troels Henriksen

Christian Maeder

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