On Mon, Jul 6, 2009 at 3:49 AM, Chris Eidhof <chris@eidhof.nl> wrote:

On 4 jul 2009, at 05:13, Alexander Dunlap wrote:

On Fri, Jul 3, 2009 at 6:45 PM, John Ky<newhoggy@gmail.com> 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.html

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.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