On Thu, May 19, 2011 at 8:05 PM, Adam C. Emerson wrote:

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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On May 19, 2011, at 9:05 PM, Adam C.Emerson wrote:

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.

I have an unreleased XDR library[1] you're welcome to use or hack on. The source is not exactly beginner-friendly, but it's fairly easy to use (IMO, of course) and has been working very well for my purposes. It's more or less complete, but very unpolished so it may be more hassle than its worth for you to try to figure out how to use it. If you're interested, though, I wouldn't mind putting some time into polishing and documenting it. Or if you'd prefer to finish your own I absolutely understand and wouldn't be offended in the least. ;)

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.

Other people have responded with suggestions about how to achieve this, so I'll just mention that in my library I chose not to provide a class instance for strings, because there are so many ambiguities about how it could be encoded that I didn't feel comfortable making the choice once and for all. -- James [1] https://github.com/mokus0/xdr