[Haskell-cafe] Parsec operator with letter problem

Reto Kramer kramer at acm.org
Thu Mar 30 18:15:20 EST 2006

I ran into the following problem with parsec's handling of operators.  
I hope someone on the list can show me a trick that resolve my  
current issue. At the end of this message is the full code for the  

The language I'm parsing has infix operators of two forms. Some are  
special characters (e.g. a dot) and some are LaTeX like (e.g. \in).  
The letters that appear in the e.g. \in lead to problems with  
expression parsing.

The following is what I expect:
   $ cat input1
   $ ./test input1
   InfixExpr OP_Dot (Ident "a") (Ident "b")

Then I try (change the b to an n)
   $ cat input2
   $ ./test input2
   Ident "a"
OUTSCH!  Changing the name of an identifier changed the expression!

Then I try (add a space right after the dot)
   $ cat input3
   a. n
   $ ./test input2
   InfixExpr OP_Dot (Ident "a") (Ident "n")

What is going on?

The 'n' is part of the "\in" operator (see reservedOpNames in test.hs  
program at the end of message) and confuses the parsec expression  
parser's ability to determine the end of expressions/identifiers.   
Adding a space right after the '.' operator resolves the issue, but  
that's not a suitable option for my users.

Unfortunately I cannot remove the "\in" operator from the  
reservedOpNames list since otherwise "\in" is not recognized as an  
infix operator itself anymore.

how can I get parsec's expression parser to work with infix operators  
of the form "\in"?

- Reto

-- complete reproducer (test.hs)
-- compile with: ghc -o test -package parsec test.hs

module Main where

import List (nub)
import System (getArgs)
import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Expr
import Text.ParserCombinators.Parsec.Language( emptyDef )
import qualified Text.ParserCombinators.Parsec.Token as P

main :: IO ()
main = do{ args <- getArgs
	 ; let fname = args !! 0
	 ; input <- readFile fname
	 ; case parse spec fname input of
              Left err -> do{ putStr "parse error at "
                            ; print err
              Right x  -> print x

data Op = OP_Dot | OP_In deriving (Show, Eq, Ord)
data Expr = InfixExpr Op Expr Expr
	  | Ident String
	  | Number Integer deriving (Show, Eq, Ord)

spec :: Parser Expr
spec = do { whiteSpace
	  ; e <- expression
	  ; return e

expression :: Parser Expr
expression = buildExpressionParser table basicExpr <?> "expression"

op_infix :: Op -> Expr  -> Expr -> Expr
op_infix op a b = InfixExpr op a b

table :: OperatorTable Char () Expr
table = [ [binary "."  (op_infix OP_Dot) AssocLeft]
         , [binary "\\in" (op_infix OP_In) AssocLeft] ]
binary  name fun assoc = Infix (do{ reservedOp name; return fun }) assoc

basicExpr :: Parser Expr
basicExpr = choice [
               do{ i <- identifier
		; return $ Ident i

lexer = P.makeTokenParser testdef

testdef = emptyDef {
   P.identStart      = letter <|> char '_'
, P.identLetter     = alphaNum <|> char '_'
, P.opStart         = oneOf $ nub $
                         map (\s -> head s) $ P.reservedOpNames testdef
, P.opLetter        = oneOf (concat (P.reservedOpNames testdef))
, P.reservedOpNames = [ ".", "\\in" ] }

dot             = P.dot lexer
parens          = P.parens lexer
braces          = P.braces lexer
squares         = P.squares lexer
semiSep         = P.semiSep lexer
semiSep1        = P.semiSep1 lexer
commaSep        = P.commaSep lexer
commaSep1       = P.commaSep1 lexer
brackets        = P.brackets lexer
whiteSpace      = P.whiteSpace lexer
symbol          = P.symbol lexer
identifier      = P.identifier lexer
reserved        = P.reserved lexer
reservedOp      = P.reservedOp lexer
integer         = P.integer lexer
natural         = P.natural lexer
charLiteral     = P.charLiteral lexer
stringLiteral   = P.stringLiteral lexer

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