group: Add shortcut for previously grouped documents

[sjakobi]

Fixes #120.

[sjakobi]

This doesn't seem to speed up my dhall test case, but it prevents documents from needlessly blowing up, so I think it still makes sense.

[sjakobi]

I noticed that this shortcut only works for documents that, in a previous group iteration, triggered the Flattened case – AlreadyFlat and NeverFlat don't get a Union constructor, so the result will be checked again by the next group. :(

This makes me wonder whether we should wrap the result of the AlreadyFlat and NeverFlat cases with e.g. Union Fail ... or a new Flat constructor to mark them as "grouped before".

Maybe this would be a useless optimization though – benchmarking necessary!

We should probably also document that group can be expensive, and should not be over-used.

[sjakobi]

This makes me wonder whether we should wrap the result of the AlreadyFlat and NeverFlat cases with e.g. Union Fail ... or a new Flat constructor to mark them as "grouped before".

I gave this a try:

--- a/prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs
+++ b/prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs
@@ -527,8 +527,8 @@ group x = case x of
     Union{} -> x
     _ -> case changesUponFlattening x of
         Flattened x' -> Union x' x
-        AlreadyFlat  -> x
-        NeverFlat    -> x
+        AlreadyFlat  -> Union Fail x
+        NeverFlat    -> Union Fail x
 
 -- Note [Group: special flattening]
 --

dhall's testsuite still works, but I couldn't observe any performance gains either…

[sjakobi]

Superseded by #140.