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