[Haskell-beginners] parser for expressions

[Stephen Tetley]

Hi John

Whilst this won't have the learning value of working through parser
combinators yourself, here's code that uses Parsec 2 to do want you
want. Its the code from page 12 of Daan Leijen's Parsec manual [1]
except it builds a syntax tree of the expression rather than evaluates
it. I modified it for a query on Haskell cafe today, but only posted
it off list. Some formatting might get "lost in the mail" of course.

[1] http://research.microsoft.com/en-us/um/people/daan/download/parsec/parsec.pdf

Best wishes

Stephen


module ExprSyn where

import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Expr

runExpr :: String -> IO ()
runExpr str = case  runParser expr () "nofile"  str of
    Left err -> putStrLn "Error:" >> print err
    Right val -> print val

demo1 = runExpr "1+1"

data Expr = Mul Expr Expr
          | Div Expr Expr
          | Add Expr Expr
          | Sub Expr Expr
          | Val Integer
  deriving (Eq,Show)

expr :: Parser Expr
expr = buildExpressionParser table factor
    <?> "expression"

table :: [[Operator Char st Expr]]
table = [[op "*" Mul AssocLeft, op "/" Div AssocLeft]
        ,[op "+" Add AssocLeft, op "-" Sub AssocLeft]
        ]
  where
    op s f assoc
       = Infix (do{ string s; return f}) assoc

factor :: Parser Expr
factor = do{ char '('
           ; x <- expr
           ; char ')'
           ; return x
           }
      <|> number
      <?> "simple expression"

number :: Parser Expr
number = do{ ds <- many1 digit
           ; return (Val $ read ds)
           }
      <?> "number"