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
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