[Haskell-cafe] Pretty Printing Libraries with an Emphasis on Consistency

[Alan & Kim Zimmerman]

I'm not sure if it is relevant, but in HaRe (or rather
haskell-token-utils), I use the dual-tree structure from diagrams.  This
allows you to push constraints down from the top, but then build the
formatting up from the bottom, and you only need to look at one level at a
time. So effectively you format the leaves, then move up a level and format
there, and so on.

On Tue, Oct 14, 2014 at 12:51 AM, Christopher Done <chrisdone at gmail.com>
wrote:

> On 11 October 2014 18:00, Oliver Charles <ollie at ocharles.org.uk> wrote:
> > Yes, hindent is exponential in its pretty printing and that's exactly the
> > program I'm trying to speed up ;) To be specific, hindent has `sandbox`
> > which runs a printer and then lets me make decisions on what the
> resulting
> > state might be. I'm curious if there are other approaches.
>
> Indeed, it's slow to backtrack like that. For what it's worth, one
> trick to speed that up is if the resulting state is OK, then just
> `put` that state and continue with it, rather than throwing away a
> rendered result. I doubled the speed of my style like that. But yeah,
> it's not much consolation.
>
> If you compare with the "Fundamental" style: that can pretty print a
> 60-line function I have here instantly, it takes no time, but my
> "ChrisDone" style takes over 50 seconds and counting. So there's a
> massive cost to this, but I haven't profiled it yet or attempted any
> real optimizations yet (been too busy). Perhaps that can be next on my
> todo list.
>
> Another thing to consider is to turn off this branching of "this style
> or that style" after a certain depth. Sort of "prune" the results to
> be smaller. The deeper down the tree, the fewer branches it could
> make, for example. I haven't tried this, but it could be nice.
>
> Good luck! If you find a similarly expressive and faster way to do
> this I'm all ears. :-)
>
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