Fix: make type pretty printer less confusing for boxed types (#906)

Fixes #659, but not #667. I propose deferring fixing #667 to a later
point in time, as it requires more design.

What this PR does, for now, is to parenthesize boxed types when
occurring in parameter or return type positions. For example, `(Int =>
Int at {}) => Int` instead of `Int => Int at {} => Int`, which, in my
opinion, is utterly unreadable.

Author: dvdvgt

effekt/shared/src/main/scala/effekt/symbols/TypePrinter.scala

@@ -66,13 +66,17 @@ object TypePrinter extends ParenPrettyPrinter {
       val tps = if (tparams.isEmpty) emptyDoc else typeParams(tparams)
       val ps: Doc = (vparams, bparams) match {
         case (Nil, Nil)       => "()"
+        case (List(tpe: BoxedType), Nil) => parens(toDoc(tpe))
         case (List(tpe), Nil) => if (tparams.isEmpty) toDoc(tpe) else parens(toDoc(tpe))
         case (_, _) =>
           val vps = if (vparams.isEmpty) emptyDoc else parens(hsep(vparams.map(toDoc), comma))
           val bps = if (bparams.isEmpty) emptyDoc else hcat(bparams.map(toDoc).map(braces))
           vps <> bps
       }
-      val ret = toDoc(result)
+      val ret = result match {
+        case _: BoxedType => parens(toDoc(result))
+        case _ => toDoc(result)
+      }
       val eff = if (effects.isEmpty) emptyDoc else space <> "/" <+> toDoc(effects)
       tps <> ps <+> "=>" <+> ret <> eff
 

examples/neg/lambdas/inference.check

@@ -1,22 +1,22 @@
 [error] Expected type
-  Int => Bool at {} => String
+  (Int => Bool at {}) => String
 but got type
-  Int => Unit at {} => String
+  (Int => Unit at {}) => String
 
 Type mismatch between Bool and Unit.
   comparing the argument types of
-    Int => Unit at {} => String (given)
-    Int => Bool at {} => String (expected)
+    (Int => Unit at {}) => String (given)
+    (Int => Bool at {}) => String (expected)
   when comparing the return type of the function.
 [error] examples/neg/lambdas/inference.effekt:13:8: Expected type
-  Int => Bool at {} => String at {}
+  (Int => Bool at {}) => String at {}
 but got type
-  Int => Unit at {} => String at ?C
+  (Int => Unit at {}) => String at ?C
 
 Type mismatch between Bool and Unit.
   comparing the argument types of
-    Int => Unit at {} => String (given)
-    Int => Bool at {} => String (expected)
+    (Int => Unit at {}) => String (given)
+    (Int => Bool at {}) => String (expected)
   when comparing the return type of the function.
   hof2(fun(f: (Int) => Unit at {}) { "" })
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^