Parsing expressions with combinators
1
I'm very new to Haskell's ReadP library, and the entire concept of parser combinators, so I was wondering whether there are better ways to do some things in this program:
import Text.ParserCombinators.ReadP
import Control.Applicative
import Data.List
data Operator = Add | Subtract
instance Show Operator where
show Add = "+"
show Subtract = "-"
instance Read Operator where
readsPrec _ "+" = [(Add, "")]
readsPrec _ "-" = [(Subtract, "")]
readsPrec _ _ =
data Expression = Number Int
| Infix { left :: Expression, op :: Operator, right :: Expression }
instance Show Expression where
show (Number x) = show x
show (Infix left op right) = "(" ++ (show left) ++ " " ++ (show op) ++ " " ++ (show right) ++ ")"
digit :: ReadP Char
digit = satisfy $ char -> char >= '0' && char <= '9'
number :: ReadP Expression
number = fmap (Number . read) (many1 digit)
operator :: ReadP Operator
operator = fmap read (string "+" <|> string "-")
expression :: ReadP Expression
expression = do
skipSpaces
left <- number
skipSpaces
op <- Control.Applicative.optional operator
case op of
Nothing -> return left
Just op -> do
skipSpaces
right <- expression
return (Infix left op right)
parseExpression :: String -> Maybe Expression
parseExpression input = case readP_to_S expression input of
-> Nothing
xs -> (Just . fst . last) xs
The main area where I'm looking for improvements is the expression function, and specifically the things which seem improvable are the repeated calls to skipSpaces and the case expression to check whether an operator was parsed, but of course if you notice anything else that would be helpful too!
parsing haskell
asked Jan 3 at 18:01
Zac
1061
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1
I'm very new to Haskell's ReadP library, and the entire concept of parser combinators, so I was wondering whether there are better ways to do some things in this program:
import Text.ParserCombinators.ReadP
import Control.Applicative
import Data.List
data Operator = Add | Subtract
instance Show Operator where
show Add = "+"
show Subtract = "-"
instance Read Operator where
readsPrec _ "+" = [(Add, "")]
readsPrec _ "-" = [(Subtract, "")]
readsPrec _ _ =
data Expression = Number Int
| Infix { left :: Expression, op :: Operator, right :: Expression }
instance Show Expression where
show (Number x) = show x
show (Infix left op right) = "(" ++ (show left) ++ " " ++ (show op) ++ " " ++ (show right) ++ ")"
digit :: ReadP Char
digit = satisfy $ char -> char >= '0' && char <= '9'
number :: ReadP Expression
number = fmap (Number . read) (many1 digit)
operator :: ReadP Operator
operator = fmap read (string "+" <|> string "-")
expression :: ReadP Expression
expression = do
skipSpaces
left <- number
skipSpaces
op <- Control.Applicative.optional operator
case op of
Nothing -> return left
Just op -> do
skipSpaces
right <- expression
return (Infix left op right)
parseExpression :: String -> Maybe Expression
parseExpression input = case readP_to_S expression input of
-> Nothing
xs -> (Just . fst . last) xs
The main area where I'm looking for improvements is the expression function, and specifically the things which seem improvable are the repeated calls to skipSpaces and the case expression to check whether an operator was parsed, but of course if you notice anything else that would be helpful too!
parsing haskell
asked Jan 3 at 18:01
Zac
1061
New contributor
Zac is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
1
1
1
I'm very new to Haskell's ReadP library, and the entire concept of parser combinators, so I was wondering whether there are better ways to do some things in this program:
import Text.ParserCombinators.ReadP
import Control.Applicative
import Data.List
data Operator = Add | Subtract
instance Show Operator where
show Add = "+"
show Subtract = "-"
instance Read Operator where
readsPrec _ "+" = [(Add, "")]
readsPrec _ "-" = [(Subtract, "")]
readsPrec _ _ =
data Expression = Number Int
| Infix { left :: Expression, op :: Operator, right :: Expression }
instance Show Expression where
show (Number x) = show x
show (Infix left op right) = "(" ++ (show left) ++ " " ++ (show op) ++ " " ++ (show right) ++ ")"
digit :: ReadP Char
digit = satisfy $ char -> char >= '0' && char <= '9'
number :: ReadP Expression
number = fmap (Number . read) (many1 digit)
operator :: ReadP Operator
operator = fmap read (string "+" <|> string "-")
expression :: ReadP Expression
expression = do
skipSpaces
left <- number
skipSpaces
op <- Control.Applicative.optional operator
case op of
Nothing -> return left
Just op -> do
skipSpaces
right <- expression
return (Infix left op right)
parseExpression :: String -> Maybe Expression
parseExpression input = case readP_to_S expression input of
-> Nothing
xs -> (Just . fst . last) xs
The main area where I'm looking for improvements is the expression function, and specifically the things which seem improvable are the repeated calls to skipSpaces and the case expression to check whether an operator was parsed, but of course if you notice anything else that would be helpful too!
parsing haskell
asked Jan 3 at 18:01
Zac
1061
New contributor
Zac is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
I'm very new to Haskell's ReadP library, and the entire concept of parser combinators, so I was wondering whether there are better ways to do some things in this program:
import Text.ParserCombinators.ReadP
import Control.Applicative
import Data.List
data Operator = Add | Subtract
instance Show Operator where
show Add = "+"
show Subtract = "-"
instance Read Operator where
readsPrec _ "+" = [(Add, "")]
readsPrec _ "-" = [(Subtract, "")]
readsPrec _ _ =
data Expression = Number Int
| Infix { left :: Expression, op :: Operator, right :: Expression }
instance Show Expression where
show (Number x) = show x
show (Infix left op right) = "(" ++ (show left) ++ " " ++ (show op) ++ " " ++ (show right) ++ ")"
digit :: ReadP Char
digit = satisfy $ char -> char >= '0' && char <= '9'
number :: ReadP Expression
number = fmap (Number . read) (many1 digit)
operator :: ReadP Operator
operator = fmap read (string "+" <|> string "-")
expression :: ReadP Expression
expression = do
skipSpaces
left <- number
skipSpaces
op <- Control.Applicative.optional operator
case op of
Nothing -> return left
Just op -> do
skipSpaces
right <- expression
return (Infix left op right)
parseExpression :: String -> Maybe Expression
parseExpression input = case readP_to_S expression input of
-> Nothing
xs -> (Just . fst . last) xs
The main area where I'm looking for improvements is the expression function, and specifically the things which seem improvable are the repeated calls to skipSpaces and the case expression to check whether an operator was parsed, but of course if you notice anything else that would be helpful too!
parsing haskell
parsing haskell
asked Jan 3 at 18:01
Zac
1061
New contributor
Zac is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked Jan 3 at 18:01
Zac
1061
New contributor
Zac is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked Jan 3 at 18:01
Zac
1061
New contributor
Zac is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked Jan 3 at 18:01
Zac
1061
asked Jan 3 at 18:01
Zac
1061
1061
New contributor
Zac is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Zac is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
Zac is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
add a comment |
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