Use correct precedence in Complex's Read1/Show1 instances

Fixes #19719.

4 files changed, 50 insertions, 3 deletions

diff --git a/libraries/base/Data/Functor/Classes.hs b/libraries/base/Data/Functor/Classes.hs
index 6a0d008982..d672c340d7 100644
--- a/libraries/base/Data/Functor/Classes.hs
+++ b/libraries/base/Data/Functor/Classes.hs
@@ -848,11 +848,13 @@ instance Eq1 Complex where
 -- [(2 % 3 :+ 3 % 4,"")]
 --
 instance Read1 Complex where
-    liftReadPrec rp _  = parens $ prec 9 $ do
+    liftReadPrec rp _  = parens $ prec complexPrec $ do
         x <- step rp
         expectP (Symbol ":+")
         y <- step rp
         return (x :+ y)
+      where
+        complexPrec = 6
 
     liftReadListPrec = liftReadListPrecDefault
     liftReadList     = liftReadListDefault
@@ -863,8 +865,10 @@ instance Read1 Complex where
 -- "2 :+ 3"
 --
 instance Show1 Complex where
-    liftShowsPrec sp _ d (x :+ y) = showParen (d >= 10) $
-        sp 10 x . showString " :+ " . sp 10 y
+    liftShowsPrec sp _ d (x :+ y) = showParen (d > complexPrec) $
+        sp (complexPrec+1) x . showString " :+ " . sp (complexPrec+1) y
+      where
+        complexPrec = 6
 
 -- Building blocks
 
diff --git a/libraries/base/tests/T19719.hs b/libraries/base/tests/T19719.hs
new file mode 100644
index 0000000000..613f92ad6c
--- /dev/null
+++ b/libraries/base/tests/T19719.hs
@@ -0,0 +1,26 @@
+module Main (main) where
+
+import Data.Complex (Complex(..))
+import Data.Functor.Classes (readPrec1, showsPrec1)
+import Text.ParserCombinators.ReadPrec (readPrec_to_S)
+import Text.Read (Read(..))
+
+comp :: Complex Int
+comp = 1 :+ 1
+
+compareInstances :: Int -> IO ()
+compareInstances p = do
+  let precBanner = " (at precedence " ++ show p ++ ")"
+  putStrLn $ "Read vs. Read1" ++ precBanner
+  print (readPrec_to_S readPrec  p "1 :+ 1" :: [(Complex Int, String)])
+  print (readPrec_to_S readPrec1 p "1 :+ 1" :: [(Complex Int, String)])
+  putStrLn ""
+  putStrLn $ "Show vs. Show1" ++ precBanner
+  putStrLn $ showsPrec  p comp ""
+  putStrLn $ showsPrec1 p comp ""
+  putStrLn ""
+
+main :: IO ()
+main = do
+  compareInstances 6
+  compareInstances 7
diff --git a/libraries/base/tests/T19719.stdout b/libraries/base/tests/T19719.stdout
new file mode 100644
index 0000000000..8f59bd40f0
--- /dev/null
+++ b/libraries/base/tests/T19719.stdout
@@ -0,0 +1,16 @@
+Read vs. Read1 (at precedence 6)
+[(1 :+ 1,"")]
+[(1 :+ 1,"")]
+
+Show vs. Show1 (at precedence 6)
+1 :+ 1
+1 :+ 1
+
+Read vs. Read1 (at precedence 7)
+[]
+[]
+
+Show vs. Show1 (at precedence 7)
+(1 :+ 1)
+(1 :+ 1)
+
diff --git a/libraries/base/tests/all.T b/libraries/base/tests/all.T
index b02d77ef11..7ea69949e2 100644
--- a/libraries/base/tests/all.T
+++ b/libraries/base/tests/all.T
@@ -263,3 +263,4 @@ test('T16643', normal, compile_and_run, [''])
 test('clamp', normal, compile_and_run, [''])
 test('T18642', extra_run_opts('+RTS -T -RTS'), compile_and_run, ['-O2'])
 test('T19288', exit_code(1), compile_and_run, [''])
+test('T19719', normal, compile_and_run, [''])