I'm trying to add a parser for infix operators to a simple expressions parser. I have already looked at the documentation and at this question, but it seems like I am missing something.
import qualified Text.Parsec.Expr as Expr
import qualified Text.Parsec.Token as Tokens
import Text.ParserCombinators.Parsec
import Text.Parsec
data Expr = Number Integer
| Op Expr Expr
| Boolean Bool
instance Show Expr where
show (Op l r) = "(+ " ++ (show l) ++ " " ++ (show r) ++ ")"
show (Number r) = show r
show (Boolean b) = show b
parens = Tokens.parens haskell
reserved = Tokens.reservedOp haskell
infix_ operator func =
Expr.Infix (spaces >> reserved operator >> spaces >> return func) Expr.AssocLeft
infixOp =
Expr.buildExpressionParser table parser
where
table = [[infix_ "+" Op]]
number :: Parser Expr
number =
do num <- many1 digit
return $ Number $ read num
bool :: Parser Expr
bool = (string "true" >> return (Boolean True)) <|> (string "false" >> return (Boolean False))
parser = parens infixOp <|> number <|> bool
run = Text.Parsec.runParser parser () ""
This parser is able to parse expressions like 1, false, (1 + 2), (1 + false), but not 1 + 2 (it's parsed as 1). If I try to change the parser to parens infixOp <|> infixOp <|> number <|> bool, it get stuck.
What should i change in order to parse expressions like 1 + 2 without parenthesis?
You have to run the infixOp parser at the top level like this:
run = Text.Parsec.runParser infixOp () ""
Otherwise the your infix expressions can only be parsed when occuring in parentheses.
The attempt to use parens infixOp <|> infixOp <|> number <|> bool most likely gets stuck because it loops: parser tries to parse using infixOp, which tries to parse using parse and so on...
These tutorial might help you getting started with parsec (they did for me):
https://wiki.haskell.org/Parsing_a_simple_imperative_language