[Haskell-cafe] types for parsing a tree
S. Doaitse Swierstra
doaitse at swierstra.net
Fri Sep 17 16:39:01 EDT 2010
On 16 sep 2010, at 05:42, Jared Jennings wrote:
> On Fri, Sep 10, 2010 at 2:00 PM, S. Doaitse Swierstra
> <doaitse at swierstra.net> wrote:
>> I show how this can be done using uu-parsinglib. Note that we have sevral parsers, each having its own type:
> Thanks for such a complete example, Doaitse! Unfortunately I have a
> requirement I didn't disclose: the simple tags like <TRNUID>, <NAME>,
> <AMOUNT> could come in any order; and some are optional. I tried to
> fix that by making every field in my Transaction record a Maybe, and
> keeping a Transaction as state for my parser. But after so many Maybes
> I began to think this was not the right way. And I had to run a parser
> as part of another parser. And after all that, it wouldn't build
> because it was badly typed.
The good news is that the library has combinators for that too ;-} Just change a few lines. If they are optional use the pOpt combinator instead of the pOne.
module Transactions where
pTagged tag (pAttr, pPayload) = pToken ("<" ++ tag ++ ">") *> pAttr *> spaces *> pPayload <* spaces <*
pToken ("</" ++ tag ++ ">")
pTag tag pPayload = pToken ("<" ++ tag ++ ">") *> pPayload
data OFX = OFX Response deriving Show
data Response = Response [Transaction] deriving Show
data Transaction = Transaction String String Amount deriving Show
data Amount = Amount Int Int deriving Show
pAmount = "TRNAMT" `pTag` (Amount <$> pNatural <* pSym '.' <*> pNatural)
pTransaction = "STMTTRN" `pTagged` (pAttr, Transaction `pMerge` ( pOne ("TRNUID" `pTag` pLine)
<||> pOne ("NAME" `pTag` pLine)
<||> pOne pAmount
pResponse = "STMTRS" `pTagged` (pAttr, Response <$> pList (pTransaction <* spaces))
pOFX = "OFX" `pTagged` (pAttr, OFX <$> pResponse )
pAttr :: Parser String
pAttr = pToken "[...]"
spaces = pMunch (`elem` " \n\t")
pDigitAsInt = digit2Int <$> pDigit
pNatural = foldl (\a b -> a * 10 + b ) 0 <$> pList1 pDigitAsInt
digit2Int a = ord a - ord '0'
pDigit :: Parser Char
pDigit = pSym ('0', '9')
pLine = pMunch (/='\n') <* spaces
main = do input <- readFile "TrInput"
run (pOFX <* spaces) input
> But in any case, thanks for turning me on to
> Text.ParserCombinators.UU; I'd only tried Parsec before.
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