group: Split top-level FlatAlt

[sjakobi]

This results in a significant speedup in the usual dhall benchmark.

Includes #123.

Closes #115.

[sjakobi]

Before merging this I would like to figure out how pipelines are impacted depending on whether they use layout{Pretty,Smart,Compact}.

[sjakobi]

I should add some documentation that explains why Union and FlatAlt are treated specially.

[sjakobi]

Here's a slightly different take that should be pretty fast with layoutCompact:

--- a/prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs
+++ b/prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs
@@ -518,11 +518,16 @@ hardline = Line
 -- See 'vcat', 'line', or 'flatAlt' for examples that are related, or make good
 -- use of it.
 group :: Doc ann -> Doc ann
--- See note [Group: special flattening]
-group x = case changesUponFlattening x of
-    Flattened x' -> Union x' x
-    AlreadyFlat  -> x
-    NeverFlat    -> x
+-- See note [Group: special flattening] -- TODO
+group x = case x of
+    Union{}     -> x
+    FlatAlt a b -> Union (flat b) a
+    _           -> Union (flat x) x
+  where
+    flat a = case changesUponFlattening a of
+        Flattened a' -> a'
+        AlreadyFlat  -> a
+        NeverFlat    -> Fail
 
 -- Note [Group: special flattening]
 --
@@ -563,6 +568,7 @@ changesUponFlattening :: Doc ann -> FlattenResult (Doc ann)
 changesUponFlattening = \doc -> case doc of
     FlatAlt _ y     -> Flattened (flatten y)
     Line            -> NeverFlat
+    Union Fail _    -> NeverFlat
     Union x _       -> Flattened x
     Nest i x        -> fmap (Nest i) (changesUponFlattening x)
     Annotated ann x -> fmap (Annotated ann) (changesUponFlattening x)
@@ -1823,6 +1829,7 @@ layoutWadlerLeijen
         -> SimpleDocStream ann -- ^ Choice A. Invariant: first lines should not be longer than B's.
         -> SimpleDocStream ann -- ^ Choice B.
         -> SimpleDocStream ann -- ^ Choice A if it fits, otherwise B.
+    selectNicer _ _ SFail y = y
     selectNicer lineIndent currentColumn x y = case pWidth of
         AvailablePerLine lineLength ribbonFraction
           | fits pWidth minNestingLevel availableWidth x -> x

That turns out to slow down prettyprinting in dhall (with layoutSmart) though.

Right now I believe it's better to optimize for layoutSmart and layoutPretty, mostly because I'm not sure whether anyone's using layoutCompact at all. This package is about pretty-printing after all, and layoutCompact's output isn't exactly pretty.

[sjakobi]

For the record, this is the new output for the motivating examples from #115 and #120:

> diag $ group (flatAlt "Default" "Fallback")
Union ( Text 8 "Fallback" ) ( Text 7 "Default" )
> diag $ group . group . group $ "x" <> line
Union 
    ( Cat ( Char 'x' ) ( Char ' ' ) ) 
    ( Cat ( Char 'x' ) 
        ( FlatAlt Line ( Char ' ' ) )
    )

For the second example, we might still want to try pushing the Union behind the Char 'x' and removing the inner FlatAlt, if that makes things faster.