At Mon, 4 Jul 2011 20:36:33 +1000,
John Ky wrote:
>
> Hi Haskell Cafe,
>
> enum |$ inumLines .| inumReverse .| inumUnlines .| iter
> ...
>
> iterLines :: (Monad m) => Iter L.ByteString m [L.ByteString]
> iterLines = do
> line <- lineI
> return [line]
>
> iterUnlines :: (Monad m) => Iter [L.ByteString] m L.ByteString
> iterUnlines = (L.concat . (++ [C.pack "\n"])) `liftM` dataI
>
> iterReverse :: (Monad m) => Iter [L.ByteString] m [L.ByteString]
> iterReverse = do
> lines <- dataI
> return (map L.reverse lines)
>
> inumLines = mkInum iterLines
> inumUnlines = mkInum iterUnlines
> inumReverse = mkInum iterReverse
>
> It all works fine.
>
> My question is: Is it possible to rewrite inumReverse to be this:
>
> iterReverse :: (Monad m) => Iter L.ByteString m L.ByteString
> iterReverse = do
> line <- dataI
> return (L.reverse line)
>
> inumReverse = mkInum iterReverse
>
> And still be able to use it in the line:
>
> enum |$ inumLines .| {-- inumReverse goes in here somehow --} .|
> inumUnlines .| iter
>
> The reason I ask is that the Haskell function reverse has the type [a] -> [a],
> not [[a]] -> [[a]].
>
> I thought perhaps the alternative inumReverse is cleaner than the original as
> it behaves more similarly to Haskell's own reverse function.
I'm not sure what you are trying to achieve. If you want an iter that
works on L.ByteStrings, then you can say:
iterReverse :: (Monad m) => Iter L.ByteString m L.ByteString
iterReverse = do
line <- lineI
return (L.reverse line)
In that case you don't need inumLines and inumUnlines. If, however,
you want the type to be [L.ByteString], and you would rather do this
one line at a time, instead of calling map, then you could do
something like the following:
iterReverse :: (Monad m) => Iter [L.ByteString] m [L.ByteString]
iterReverse = do
line <- headI
return [L.reverse line]
But the code you have above should also work, so it all depends on
what you are trying to achieve.
David
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