Tommy Thorn wrote:
> In an attempt to gather experience with Philip Wadlers pretty printing
> combinators, I converted a semi big pretty printer to his pretty
> printing.
>
> > punctuate sep [] = nil
> > punctuate sep [x] = x
> > punctuate sep (x:xs) = x <> sep <> punctuate sep xs
>
> > vcat = punctuate line
> > sep x = group (vcat x)
>
Incidentally, the above def of `sep' doesn't match Hughes' in an important
way. Consider:
sep [ text "hi", text "bye" $$ text "bye" ]
If we lay this out using Hughes' combinators with a large width, we get
hi
bye
bye
while with the above def, we get:
hi bye bye
The problem is that, while Hughes' `sep' only flattens the newlines inserted
between each element, `group' flattens everything in its scope. Further,
Hughes evidently recognizes that any break from a flat layout should result in
a vertical layout. We can partially fix the def of `sep':
sep x = punctuate (group line) x
This gives us the questionable:
hi bye
bye
Hughes `sep' seems pretty well-behaved in this circumstance, and I'm not sure
how to recapture that using Wadler-style combinators.
--Jeff
