Make benign non-determinism in pretty-printing more obvious

This change takes us one step closer to being able to remove
`varSetElemsWellScoped`. The end goal is to make every source
of non-determinism obvious at the source level, so that when
we achieve determinism it doesn't get broken accidentally.

Test Plan: compile GHC

Reviewers: simonmar, goldfire, simonpj, austin, bgamari

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2123

GHC Trac Issues: #4012

1 files changed, 19 insertions, 1 deletions

diff --git a/compiler/utils/UniqFM.hs b/compiler/utils/UniqFM.hs
index 969e1dc10a..3632926d91 100644
--- a/compiler/utils/UniqFM.hs
+++ b/compiler/utils/UniqFM.hs
@@ -67,7 +67,7 @@ module UniqFM (
         eltsUFM, keysUFM, splitUFM,
         ufmToSet_Directly,
         ufmToList,
-        joinUFM, pprUniqFM
+        joinUFM, pprUniqFM, pprUFM, pluralUFM
     ) where
 
 import Unique           ( Uniquable(..), Unique, getKey )
@@ -324,3 +324,21 @@ pprUniqFM ppr_elt ufm
   = brackets $ fsep $ punctuate comma $
     [ ppr uq <+> text ":->" <+> ppr_elt elt
     | (uq, elt) <- ufmToList ufm ]
+
+-- | Pretty-print a non-deterministic set.
+-- The order of variables is non-deterministic and for pretty-printing that
+-- shouldn't be a problem.
+-- Having this function helps contain the non-determinism created with
+-- eltsUFM.
+pprUFM :: ([a] -> SDoc) -- ^ The pretty printing function to use on the elements
+       -> UniqFM a      -- ^ The things to be pretty printed
+       -> SDoc          -- ^ 'SDoc' where the things have been pretty
+                        -- printed
+pprUFM pp ufm = pp (eltsUFM ufm)
+
+-- | Determines the pluralisation suffix appropriate for the length of a set
+-- in the same way that plural from Outputable does for lists.
+pluralUFM :: UniqFM a -> SDoc
+pluralUFM ufm
+  | sizeUFM ufm == 1 = empty
+  | otherwise = char 's'