Re: [Haskell-cafe] How to pretty print code efficiently

5 Jul 2009

      Hi all,

Thanks everyone for the help.  The HughesPJ module works well for me.

Cheers,

-John

On Mon, Jul 6, 2009 at 3:49 AM, Chris Eidhof  wrote:
...
On 4 jul 2009, at 05:13, Alexander Dunlap wrote:
On Fri, Jul 3, 2009 at 6:45 PM, John Ky wrote:
...
...
Hi,
Currently I'm pretty printing code by building arrays of strings and
calling
indent.  For example:
instance JavaPrintableNamed AST.EnumeratedType where
  javaLinesNamed parentName (AST.EnumeratedType memberDefinitions) =
     [ "public enum " ++ asJavaId(parentName)
     , "{"
     ] ++ memberCodeLines ++
     [ "}"
     , ""
     ]
     where
        memberCodeLines = indent $ javaLines memberDefinitions
The indent function takes a list of strings and adds an indent to the
beginning of every line.
I can imagine this to be very inefficient as it builds many strings and
concatenates them.
In Ruby, I might do the same thing like this:
class EnumeratedType < JavaPrintableNamed
  def writeTo(writer)
     writer.print "public enum "
     writer.puts self.asJavaId
     writer.puts "{"
     writer.indent do
        self.memberDefinitions.writeTo(writer)
        writer.puts
     end
where above, the writer.indent takes care of the indent, and everything
is
appended to a stream, which doesn't seem so bad in terms of efficiency.
I'm looking for a way to do something similar in Haskell.
Anyone can give me a hand?
Thanks
-John
_______________________________________________
You may want to investigate the standard module
Text.PrettyPrint.HughesPJ, which contains a number of (I assume fairly
efficient) combinators for pretty printing.
I second that. Also, there is uulib which has a pretty printing module
that's quite similar:
http://hackage.haskell.org/packages/archive/uulib/0.9.10/doc/html/UU-PPrint....
I think both packages are based on the paper "The Design of a
Pretty-printing Library" which can be found at
http://www.cs.chalmers.se/~rjmh/Papers/pretty.pshttp://www.cs.chalmers.se/%7Erjmh/Papers/pretty.ps
Not only do they provide abstractions for things like indentation,
concatenation in different forms, etc., but they also are  more efficient
than a naive implementation using lists.
-chris
-chris