I was unhappy with the use of `error` in my first solution, so I wrote a second solution that's more robust. It also demonstrates monadic style. The new solution is at the bottom. Dean On Thu, 23 Jan 2003, Dean Herington wrote:
On Sun, 19 Jan 2003, Nick Name wrote:
I got another trouble: I need to build a record type like
Package { name :: String, version :: Int , mantainer :: String ... other fields ... }
from a list of string of the form
["Package: ..." , "Mantainer: ..." , "Version: ..." , ... ]
where the fields are not bound to be in a particular order, except for Package wich is always the first of a record.
The natural solution in this case seems to be a mutable record, and an iteration over the list. Has someone got ideas? This appear to be a particularly difficult problem in haskell and it should not be.
V.
Here's one way to do it:
module ReadRecord where
import Maybe (isJust)
data Package = Package { name :: Maybe String, version :: Maybe Int, maintainer :: Maybe String } deriving Show
parsePackage :: [String] -> Package parsePackage = foldl (parseItem trials) (Package Nothing Nothing Nothing) where parseItem (f:fs) p s = maybe (parseItem fs p s) id (f p s) parseItem [] _ s = error $ "can't match " ++ show s
trials = [ try "Package" (ij.name) (\ p v -> p{name = Just v}), try "Version" (ij.version) (\ p v -> p{version = Just $ read v}), try "Maintainer" (ij.maintainer) (\ p v -> p{maintainer = Just v}) ]
try key tester setter pkg str = fmap parse $ match (key ++ ": ") str where parse text = if tester pkg then error $ "duplicate field: " ++ key else setter pkg text
match want str = if want == pre then Just post else Nothing where (pre, post) = splitAt (length want) str
ij = isJust
main = mapM_ try goods try = print . parsePackage
goods = [ [], ["Package: p1", "Version: 1", "Maintainer: m1"], ["Maintainer: m2", "Package: p2"] ]
bad1 = ["Version: v-b1", "Foo: foo-b1"] bad2 = ["Maintainer: me", "Maintainer: me"] bad3 = ["Version: foo"]
-- Dean
module ReadRecord where import Maybe (isJust) import Monad import Control.Monad import Control.Monad.Error data Package = Package { name :: Maybe String, version :: Maybe Int, maintainer :: Maybe String } deriving Show parsePackage :: [String] -> Either String Package parsePackage = foldM parseItem (Package Nothing Nothing Nothing) where parseItem p s = join $ msum [ trial p s | trial <- trials ] trials = [ try "Package" (isJust . name) (\ p t -> return p{name = Just t}), try "Version" (isJust . version) (\ p t -> do v <- int t; return p{version = Just v}), try "Maintainer" (isJust . maintainer) (\ p t -> return p{maintainer = Just t}), \ _ str -> return $ fail $ "can't match " ++ show str ] try fld full set p s = match (fld ++ ": ") s >>= return . parse where parse t = if full p then fail $ "duplicate field: " ++ fld else set p t match want str = do guard (want == pre); return post where (pre, post) = splitAt (length want) str int text = case reads text of [(val,"")] -> return val _ -> fail $ "invalid Int: " ++ text main = mapM_ try trials try = print . parsePackage trials = goods ++ bads goods = [ [], ["Package: p1", "Version: 1", "Maintainer: m1"], ["Maintainer: m2", "Package: p2"]] bads = [bad1, bad2, bad3] bad1 = ["Package: b1", "Foo: foo-b1"] bad2 = ["Maintainer: me1", "Maintainer: me2"] bad3 = ["Version: foo"]