Like the previous no experience with Data.Binary, but my (rusty) monad experience is enough to see the source of the problem: bbrown wrote:
The issue stems from here, it says I didn't define an instance, but I did:
instance Binary URLSet where put _ = do BinaryPut.putWord8 0 get = do remainingByteData <- BinaryGet.getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData k :: DescrInfo <- decode remainingByteData x :: KeywordsInfo <- decode remainingByteData return (URLSet {urlinfo=i, titleinfo=j, descrinfo=k, keywordsinfo=x})
Data.Binary seems to use the Get monad which looks to be a garden variety parsing monad. For line in the do block: i :: URLInfo <- decode remainingByteData Because of the way do notation works x::a <- is expecting a value of M a for a monad M, above Get URLInfo, inplying a type of ByteString -> (Get URLInfo) for decode and therefore the comiler is looking for the corresponding Binary instance (and of course, not finding it since, quite properly, your binary instance is URLInfo not Get URLInfo). If you can't follow this, find a monad tutorial and look at how do notation expands to >>= and >>. The code you have almost certainly isn't doing what you want/expect (even if you fix the bad monad nesting you are trying to repeatedly decode the same data as different types). Not knowing exactly how your data is encoded it is hard to be certain of the correct code but something like this seems more likely (untried): instance Binary URLSet where put _ = do BinaryPut.putWord8 0 get = do i :: URLInfo <- get j :: TitleInfo <- get k :: DescrInfo <- get x :: KeywordsInfo <- get return (URLSet {urlinfo=i, titleinfo=j, descrinfo=k, keywordsinfo=x}) This assumes that the data contains the structures serialized in order. In this case for i the type of get is inferred to Get URLInfo - which will work since URLInfo has a Binary instance. You also have a similar issue in the SpiderDatabase instance. Clive