Why doesn't group "split" FlatAlts?

[sjakobi]

This is what group currently does:

> diag $ group (flatAlt "Default" "Fallback")
Union ( Text 8 "Fallback" ) 
    ( FlatAlt ( Text 7 "Default" ) ( Text 8 "Fallback" ) )

I wonder why the second Union alternative still contains the full FlatAlt, and not just the default:

Union ( Text 8 "Fallback" ) 
    ( Text 7 "Default" )

[sjakobi]

One reason could be performance / allocations: Both branches have to be preserved anyway, so there's no point in rewriting the original.

[sjakobi]

This seems to be unproblematic (tested in dhall too):

--- a/prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs
+++ b/prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs
@@ -523,6 +523,7 @@ hardline = Line
 -- use of it.
 group :: Doc ann -> Doc ann
 -- See note [Group: special flattening]
+group (FlatAlt x y) = Union (flatten y) x
 group x = case changesUponFlattening x of
     Nothing -> x
     Just x' -> Union x' x
@@ -569,24 +570,24 @@ changesUponFlattening = \doc -> case doc of
     Char{} -> Nothing
     Text{} -> Nothing
     Fail   -> Nothing
-  where
-    -- Flatten, but don’t report whether anything changes.
-    flatten :: Doc ann -> Doc ann
-    flatten = \doc -> case doc of
-        FlatAlt _ y     -> flatten y
-        Cat x y         -> Cat (flatten x) (flatten y)
-        Nest i x        -> Nest i (flatten x)
-        Line            -> Fail
-        Union x _       -> flatten x
-        Column f        -> Column (flatten . f)
-        WithPageWidth f -> WithPageWidth (flatten . f)
-        Nesting f       -> Nesting (flatten . f)
-        Annotated ann x -> Annotated ann (flatten x)
-
-        x@Fail   -> x
-        x@Empty  -> x
-        x@Char{} -> x
-        x@Text{} -> x
+
+-- Flatten, but don’t report whether anything changes.
+flatten :: Doc ann -> Doc ann
+flatten = \doc -> case doc of
+    FlatAlt _ y     -> flatten y
+    Cat x y         -> Cat (flatten x) (flatten y)
+    Nest i x        -> Nest i (flatten x)
+    Line            -> Fail
+    Union x _       -> flatten x
+    Column f        -> Column (flatten . f)
+    WithPageWidth f -> WithPageWidth (flatten . f)
+    Nesting f       -> Nesting (flatten . f)
+    Annotated ann x -> Annotated ann (flatten x)
+
+    x@Fail   -> x
+    x@Empty  -> x
+    x@Char{} -> x
+    x@Text{} -> x
 
 
 

[sjakobi]

#124 (comment) explains why it's safe to keep only the left FlatAlt branch on a right Union branch.