Re: [Haskell-cafe] ByteString typechecking issues....

27 Dec 2008


      Hi Luke,

        joinhttp://cvs.haskell.org/Hugs/pages/libraries/base/Data-ByteString.html#v%3Ajo...::
ByteStringhttp://cvs.haskell.org/Hugs/pages/libraries/base/Data-ByteString.html#t%3ABy...->
[
ByteStringhttp://cvs.haskell.org/Hugs/pages/libraries/base/Data-ByteString.html#t%3ABy...]
-> ByteStringhttp://cvs.haskell.org/Hugs/pages/libraries/base/Data-ByteString.html#t%3ABy...???

Vasili

On Sat, Dec 27, 2008 at 1:58 AM, Luke Palmer  wrote:
...
2008/12/26 Galchin, Vasili 
Hello,
...
I have a ByteString -> [ByteString] -> ByteString situation, i.e.
concatenation .....
-- marshall into ByteString representation
                   join
                      (encode (buildHeader
ss))                             -- ByteString
                      (map encode (buildEntries (sequenceListExtract
ss)))  -- [ByteString]
I get the following typecheck error which is vexing me ....
Couldn't match expected type `t -> t -> B.ByteString'
           against inferred type `B.ByteString'
???
join is not a function in Data.ByteString.  By the error I'm guessing
you're getting the join from Control.Monad, instantiated to (->).
You are looking for "concat"; i.e.
concat $
                      encode (buildHeader ss) :
-- ByteString
                      map encode (buildEntries (sequenceListExtract ss))
 -- [ByteString]
(Control.Monad.join does end up meaning"concat" when working on lists of
lists, but it does not generalize to lists of other things).
Luke