# [Haskell-cafe] parsing currency amounts with parsec

Eric Rasmussen ericrasmussen at gmail.com
Tue May 10 00:07:57 CEST 2011

```Hi everyone,

I am relatively new to Haskell and Parsec, and I couldn't find any articles
on parsing numbers in the following format:

Positive: \$115.33
Negative: (\$1,323.42)

I'm working on the parser for practical purposes (to convert a 3rd-party
generated, most unhelpful format into one I can use), and I'd really
appreciate any insight into a better way to do this, or if there are any
built-in functions/established libraries that would be better suited to the
task. My code below works, but doesn't seem terribly efficient.

Thanks!
Eric

-------------------------------------------------
{- parses positive and negative dollar amounts -}

integer :: CharParser st Integer
integer = PT.integer lexer

float :: CharParser st Double
float = PT.float lexer

currencyAmount = try negAmount <|> posAmount

negAmount = do char '('
char '\$'
a <- currency
char ')'
return (negate a)

posAmount = do char '\$'
a <- currency
return a

currency = do parts <- many floatOrSep
let result = combine orderedParts where
combine = sumWithFactor 1
orderedParts = reverse parts
return result

floatOrSep = try float <|> beforeSep

beforeSep = do a <- integer
char ','
return (fromIntegral a :: Double)

sumWithFactor n []     = 0
sumWithFactor n (x:xs) = n * x + next
where next = sumWithFactor (n*1000) xs
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