Overly complicated parser combinator

Question

I recently started to learn Haskell in order to write some small tools for me. Currently I'm working on a program which calculates how much steam a (railcraft) boiler of a given size outputs per tick.

In the console the user inputs a string of the following format: S | L | BL-Size | BH-Size, example BH-3x3x4

I then use ParserCombinators from the Text.ParserCombinators.ReadP library to parse this string into a custom type. However I think my parser is overly complicated. Any suggestions how to simplify the following code?

module Boilers (Boiler(..), Tank(..)) where
data Boiler = Hobbyist | Locomotive | Boiler (Int, Int, Int) Tank deriving (Show)
data Tank = HighPressure | LowPressure deriving (Eq, Show)
module BoilerParser where
import Data.Char
import Text.ParserCombinators.ReadP
import Control.Applicative
import Boilers
import Data.Maybe
boiler :: ReadP Boiler
boiler = do
 b <- standard <++ hobbyist <++ locomotive
 maybe pfail return b
standard :: ReadP (Maybe Boiler)
standard = do
 satisfy (\c -> toUpper c == 'B')
 t <- fmap (\t -> case toUpper t of
 'H' -> Just HighPressure
 'L' -> Just LowPressure
 _ -> Nothing) get
 satisfy (== '-')
 s <- BoilerParser.size
 case t of
 Nothing -> pfail
 (Just a) -> return (Just (Boiler s a))
hobbyist :: ReadP (Maybe Boiler)
hobbyist = fmap (\c -> if toUpper c == 'H' then Just Hobbyist else Nothing) get
locomotive :: ReadP (Maybe Boiler)
locomotive = fmap (\c -> if toUpper c == 'L' then Just Locomotive else Nothing) get
size :: ReadP (Int, Int, Int)
size = do
 x <- size' 1
 satisfy (== 'x')
 y <- size' 1
 satisfy (== 'x')
 z <- size' 1
 return (x,y,z)
size':: Int -> ReadP Int
size' ds = fmap read (count ds (satisfy isDigit))

Answer 1

boiler :: ReadP Boiler
boiler
 = Hobbyist <$ char 'H'
 <|> Locomotive <$ char 'L'
 <|> flip Boiler <$ char 'B'
 <*> (HighPressure <$ char 'H' <|> LowPressure <$ char 'L')
 <*> ((,,) <$ char '-'
 <*> (read <$> munch1 isDigit) <* char 'x'
 <*> (read <$> munch1 isDigit) <* char 'x'
 <*> (read <$> munch1 isDigit))

By changing the internal definition of Boiler you can clean up that flip.

By also changing the user interface you can use the Read instance deriver:

data Boiler = H | L | BL Int Int Int | BH Int Int Int deriving (Read, Show)

Your example would then look like BH 3 3 4.

By the way, whereever you're using the Eq instance of Tank, consider pattern matching instead.

• Ruhrpottpatriot: Tanks for the answer. I'll look into changing the definitions.But I'm a bit puzzled about the <$ operator ? What does it do? Is it simply a left biased $ or does it do something else?
• Gurkenglas: It is the thing with the characteristic property of having the type signature Functor f => a -> f b -> f a :P. a <$ b = fmap (const a) b. Note the existence of hayoo.fh-wedel.de .
• Ruhrpottpatriot: Yeah, I searched Hoogle, but found nothing the first time. Second time it was there. Anyway: Additional question: You said I can change my data so I can use the Read instance deriver. Care to elaborate? I'm not sure how to do it.
• Gurkenglas: Using data Boiler = H | L | BL Int Int Int | BH Int Int Int deriving (Read, Show) instead of your definitions of Boiler and Tank will give you access to, for example, readLn :: IO Boiler, which turns an input line like "BH 3 3 4" into a Boiler value.