Re: Re[2]: patch applied (packages/HaXml): Use ByteString for output (contributed by Jason Dagit, with modifications)

14 Nov 2006

      On Tue, 2006-11-14 at 14:53 -0800, Jason Dagit wrote:
...
On 11/14/06, Bulat Ziganshin  wrote:
...
Hello Malcolm,
Tuesday, November 14, 2006, 8:49:31 PM, you wrote:
...
involved, which is why I choose a different route.  I am open to further
ideas about how to best achieve a similar effect (i.e. ByteString ops
available, but minimal code duplication).
For minimal code duplication make it generate ByteString internally and
have the String version just call pack/unpack. If I remember correctly,
unpack is lazy (and if it isn't, it should be).
...
It would also be nice if the pretty printer came with a version that
runs in instances of MonadIO so that they write their output eagerly
instead of lazily.  My experience with haskell tells me that laziness
is wonderful, especially when you can separate the concerns of
computation and IO.  But, sometimes it seems that when you move to the
optimization phase of a project that, that same approach becomes a
problem and sometimes you end up needing to do your IO as soon/often
as you can to keep from having poor space behavior.
The Text.PrettyPrint.HughesPJ module can output to various targets using
the fullRender function:

fullRender
:: Mode
Rendering mode
-> Int
Line length
-> Float
Ribbons per line
-> (TextDetails -> a -> a)
What to do with text
-> a
What to do at the end
-> Doc
The document
-> a

So you can produce String, ByteString, do IO directly etc. Note how it
doesn't need MonadIO or anything, it's a pure function.

So for example:

hRender :: Handle -> Doc -> IO ()
hRender h =
  fullRender blah blah blah hPut
  where hPut (Chr c) next = hPutChar h c
        hPut (Str c) next = hPutStr  h s

Note that Text.PrettyPrint.HughesPJ originally envisaged optionally
using a packed string type: the TextDetails has a PStr constructor for a
packed string type. However at the moment that's also just String,
perhaps we should change that to Data.PackedString once the new
implementation of PackedString is available.

Duncan

Re: Re[2]: patch applied (packages/HaXml): Use ByteString for output (contributed by Jason Dagit, with modifications)

Duncan Coutts