How can I parse a float with a comma in place of the decimal point?

Question

I want to parse Float values from a file where they are stored using comma as the decimal separator. Thus i need a function myParse :: String -> Float such that, for instance, myParse "23,46" == 23.46.

I have some ideas about how to do this, but they all seem overcomplicated, for example:

• replace , with a . in the string and use read; or
• follow this FP Complete blogpost (entitled Parsing Floats With Parsec), and challenge the curse of the monomorphism restriction.

Is there a simpler way, or do I really need to use a parsing library? In the second case, could you please paste some suggestions in order to get me started? The monomorphism restriction scares me, and I believe that there has to be a way to do this without using language extensions.

Answer 1

Replacing , by . and then call read is straightforward enough; you just need to remember to use your own specialized function instead of plain old read:

readFloatWithComma :: String -> Float
readFloatWithComma = read . sanitize
  where
    sanitize = map (\c -> if c == ',' then '.' else c)

In GHCi:

λ> readFloatWithComma "23,46"
23.46

---

Regarding the parsec approach, despite what the article you link to suggest, the monomorphism restriction needs not be a worry, as long as you have type signatures for all your top-level bindings. In particular, the following code doesn't need any language extensions to compile properly (at least, in GHC 7.10.1):

import Text.Parsec
import Text.Parsec.String         ( Parser )
import Control.Applicative hiding ( (<|>) )

infixr 5 <++>
(<++>) :: Applicative f => f [a] -> f [a] -> f [a]
a <++> b = (++) <$> a <*> b

infixr 5 <:>
(<:>) :: Applicative f => f a -> f [a] -> f [a]
a <:>  b = (:) <$> a <*> b

number :: Parser String
number = many1 digit

plus :: Parser String
plus = char '+' *> number

minus :: Parser String
minus = char '-' <:> number

integer :: Parser String
integer = plus <|> minus <|> number

float :: Parser Float
float = fmap rd $ integer <++> decimal <++> exponent
    where rd       = read :: String -> Float
          decimal  = option "" $ ('.' <$ char ',') <:> number
          exponent = option "" $ oneOf "eE" <:> integer

In GHCi:

λ> parseTest float "23,46"
23.46