On Thu, May 19, 2011 at 8:05 PM, Adam C. Emerson
Good morning,
As my first "real" toy project (something trivial but with some pretense of usefulness at least to me) in Haskell, I had been trying to write an XDR encoder/decoder based on top of Data.Binary. As such, I have a typeclass:
class Encodable t where -- | Encode a value in the Put monad. put :: t -> Put -- | Decode a value in the Get monad get :: Get t
And I have various instances, all of which seem to work, except for two. My thought was to treat a list of Chars as a string and a list of "encodable" types as a counted array. Thus, I tried:
instance Encodable [Char] where put s = put $ runPut (putUTF8str s) get = do bs <- get return (runGet getUTF8str bs)
and
instance (Encodable e) => Encodable [e] where put l = if (length l) > xdrmaxlen then fail "Length of data exceeds XDR maximum for arrays." else (put (length l) >> putFixed (length l) l) get = do n <- get getFixed n
This failed, and the compiler suggested I try adding:
{-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE OverlappingInstances #-}
But even with these two statements, I get:
*Data.XDR.Encodable> encode [1, 2, 3]
<interactive>:1:0: Overlapping instances for Encodable [t] arising from a use of `encode' at <interactive>:1:0-15 Matching instances: instance [overlap ok] (Encodable e) => Encodable [e] -- Defined at /home/azure/programming/xdr/src/Data/XDR/Encodable.hs:226:9-38 instance [overlap ok] Encodable [Char] -- Defined at /home/azure/programming/xdr/src/Data/XDR/Encodable.hs:160:9-24 (The choice depends on the instantiation of `t' To pick the first instance above, use -XIncoherentInstances when compiling the other instance declarations) In the expression: encode [1, 2, 3]
So, the question I have, more than wondering how to get this to work (I suspect I shouldn't even be doing this, and instead I should newtype XDRInt and XDRString and so on), is why the two instances overlap. I have no instance for Char, so the first instance should apply to lists of Char (which isn't Encodable.) And the second should apply to lists of Encodable things (which Char isn't.)
A key point about instance resolution - when GHC tries to find an instance to useit only looks at what is on the left-hand side of the (=>) mark. So an instance of the form:
instance (Constraint a) => MyClass [a] where ...
can be read aloud as: "[a] is an instance of MyClass. Also, it is an error if Constraint a is not satisfied". One way to get around this is to do what the 'Show' class does:
class Show a where show :: a -> String
showList :: [a] -> String showList [] = "[]" showList (x:xs) = "[" ++ ... ++ "]"
and then:
instance Show a => Show [a] where show xs = showList xs
instance Show Char where show x = ... showList xs = "\"" + ... + "\"
This way, the special handling that strings need is put in the Char instance. The above instances are an approximation only, the Show class is a bit more complex, but I just wanted to show the trick it uses for strings. The other approach is to use the OverlappingInstances extension, which I'm less able to explain as readily. Antoine
Is there a fairly comprehensible source I should read to understand typeclasses and instances better? (I've read my way through Real World Haskell, the Wikibook, and A Gentle Introduction, though it's possible they covered this and I just missed it.)
Thank you.
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