Change Show-instances to use "#field := val"

The previous show instance would add another layer of quoting for each
nesting:

```
ghci> show (rcons (#foo := "bar") rnil)
"[(\"foo\",\"\\\"bar\\\"\")]"
```

Instead, what we want is to display a nicely readable variant of the
record, using the infix field syntax for both label/value pairs and
full records:

```
ghci> show (rcons (#hi := (rcons (#lea := "hi") rnil)) (rcons (#foo := "bar") rnil ))
"[#foo := \"bar\",#hi := [(#lea := \"hi\")]]"

ghci> show (#hi := "lea")
"#hi := \"lea\""
```

That’s better!

Note that we can’t have a roundtripping `Read` instance anyway, so we
might as well have `Show` be readable.

Fixes #36

src/SuperRecord.hs

@@ -101,6 +101,7 @@ import Data.Kind (Type)
 
 #if MIN_VERSION_aeson(2, 0, 0)
 import qualified Data.Aeson.Key as Key
+import Text.Show (showListWith)
 #else
 import qualified Data.Text as T
 #endif
@@ -160,7 +161,7 @@ class    ( c1 k a b, c2 k a b ) => Tuple222C c1 c2 k a b
 instance ( c1 k a b, c2 k a b ) => Tuple222C c1 c2 k a b
 
 instance (RecApply lts lts (ConstC Show)) => Show (Rec lts) where
-    show = show . showRec
+    showsPrec = showsPrecRec
 
 instance RecApply lts lts (Tuple22C (ConstC Eq) (Has lts)) => Eq (Rec lts) where
     r1 == r2 = recApply @lts @lts @(Tuple22C (ConstC Eq) (Has lts)) ( \lbl v b -> get lbl r2 == v && b ) r1 True
@@ -712,6 +713,13 @@ reflectRecFold f r =
 showRec :: forall lts. (RecApply lts lts (ConstC Show)) => Rec lts -> [(String, String)]
 showRec = reflectRec @(ConstC Show) (\(_ :: FldProxy lbl) v -> (symbolVal' (proxy# :: Proxy# lbl), show v))
 
+showsPrecRec :: forall lts. (RecApply lts lts (ConstC Show)) => Int -> Rec lts -> ShowS
+showsPrecRec d r =
+    showListWith id $
+        reflectRec
+        @(ConstC Show) (\(lbl :: FldProxy lbl) v -> showsPrec d (lbl := v))
+        r
+
 recToValue :: forall lts. (RecApply lts lts (ConstC ToJSON)) => Rec lts -> Value
 recToValue r = object $ reflectRec @(ConstC ToJSON) (\(_ :: FldProxy lbl) v -> (jsonKey $ symbolVal' (proxy# :: Proxy# lbl), toJSON v)) r
 

src/SuperRecord/Field.hs

@@ -29,8 +29,12 @@ instance (Ord value) => Ord (label := value) where
 
 instance (Show t) =>
          Show (l := t) where
-  showsPrec p (l := t) =
-      showParen (p > 10) (showString ("#" ++ symbolVal l ++ " := " ++ show t))
+  showsPrec d (l := t) =
+      showParen (d > labelPrec) $
+        showString ("#" ++ symbolVal l ++ " := ")
+        . showsPrec (labelPrec+1) t
+    where
+      labelPrec = 6
 
 -- | A proxy witness for a label. Very similar to 'Proxy', but needed to implement
 -- a non-orphan 'IsLabel' instance

test/Spec.hs

@@ -351,7 +351,7 @@ recordTests =
            do let vals = showRec r1
               vals `shouldBe` [("foo", "\"Hi\""), ("int", "213")]
        it "show works" $
-           show r1 `shouldBe` "[(\"foo\",\"\\\"Hi\\\"\"),(\"int\",\"213\")]"
+           show r1 `shouldBe` "[#foo := \"Hi\",#int := 213]"
        it "equality works" $
            do r1 == r1 `shouldBe` True
               r1 == set #foo "Hai" r1 `shouldBe` False