WIP: Implement 'shrink' for Docs by hand

[sjakobi]

No description provided.

[sjakobi]

If I cause the fusion tests to fail, for example with

--- a/prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs
+++ b/prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs
@@ -551,7 +551,7 @@ changesUponFlattening :: Doc ann -> Maybe (Doc ann)
 changesUponFlattening = \doc -> case doc of
     FlatAlt _ y     -> Just (flatten y)
     Line            -> Just Fail
-    Union x _       -> changesUponFlattening x <|> Just x
+    Union x _       -> Nothing
     Nest i x        -> fmap (Nest i) (changesUponFlattening x)
     Annotated ann x -> fmap (Annotated ann) (changesUponFlattening x)

… shrink still produces nonsensical Docs like Cat Empty Fail:

Tests
  Fusion
    Deep fusion does not change rendering:                          FAIL (0.03s)
      *** Failed! (after 61 tests and 44 shrinks):
      Exception:
        »SFail« must not appear in a rendered »SimpleDocStream«. This is a bug in the layout algorithm! Please report this as a bug
        CallStack (from HasCallStack):
          error, called at src/Data/Text/Prettyprint/Doc/Render/Util/Panic.hs:13:21 in prettyprinter-1.5.1-inplace:Data.Text.Prettyprint.Doc.Render.Util.Panic
      Cat Empty Fail
      LayoutPretty (LayoutOptions {layoutPageWidth = AvailablePerLine 55 56.4237144509971})
      Exception thrown while showing test case:
        »SFail« must not appear in a rendered »SimpleDocStream«. This is a bug in the layout algorithm! Please report this as a bug
        CallStack (from HasCallStack):
          error, called at src/Data/Text/Prettyprint/Doc/Render/Util/Panic.hs:13:21 in prettyprinter-1.5.1-inplace:Data.Text.Prettyprint.Doc.Render.Util.Panic
      
      Use --quickcheck-replay=824164 to reproduce.

I think what could help is to filter the raw shrink results with a valid function that checks certain invariants such as the absence of Fail (except in a first Union alternative?!).

Such a valid function could be good documentation too…

[sjakobi]

Is 0 a valid ribbon width at all? Not sure how to interpret the docs there:

https://github.com/quchen/prettyprinter/blob/ff555e19b7b17a74f16e6fb062256a57dabe4d92/prettyprinter/src/Data/Text/Prettyprint/Doc/Internal.hs#L1609-L1618

Suggested change

	WithPageWidth f -> all (go mayFail) (map f (Unbounded : [AvailablePerLine c r | c <- [1..80], r <- [0, 0.1 .. 1]]))
	WithPageWidth f -> all (go mayFail) (map f (Unbounded : [AvailablePerLine c r | c <- [1..80], r <- [0.1, 0.2 .. 1]]))

[sjakobi]

Not sure how to best handle these functions. Checking them exhaustively would take too long. Even the current check with "typical" values 0..80 seems excessive.

[quchen]

Might be one of the few places where a list comprehension is nicer to read! :-)