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