brian wrote:
From https://secure.wikimedia.org/wikipedia/en/wiki/Bencoding , I think I should code
data BValue = BString String | BInteger Integer | BList [BValue] | BDictionary (M.Map BString BValue)
but: Not in scope: type constructor or class `BString'
The implementations I've found just implemented BDictionary's map key as String, but I think that's kind of wrong.
Data Types a la Carte[1] seems like a good fit:
newtype BString e = BString String newtype BInteger e = BInteger Integer newtype BList e = BList [Bvalue] newtype BDictionary e = BDictionary (M.Map (BString e) BValue)
newtype Y f = Y { unY :: f (Y f) } data (f :+: g) e = Inl (f e) | Inr (g e)
type BValue = Y (BString :+: BInteger :+: BList :+: BDictionary )
...with the necessary Functor instances. Or, since BString is the only member of the coproduct that needs special treatment, you could squash things a bit more:
newtype BString e = BString String data BValue_ e = BInteger Integer | BList [Bvalue] | BDictionary (M.Map (BString e) BValue)
type BValue = Y (BString :+: BValue_)
The former is more homogeneous and so would probably look prettier in code, but the latter is a bit more efficient since it needs less coproduct tagging and function indirection. Of course, another approach is to just go with your first approach and doubly wrap String. So there's a newtype BString_ = BString_ String, and BValue has a constructor (BString BString_) and the dictionary uses the BString_ data type. [1] With added discussion: http://wadler.blogspot.com/2008/02/data-types-la-carte.html -- Live well, ~wren