add error message related to missing yield/do in for-comprehensions

1 files changed, 35 insertions, 0 deletions

diff --git a/src/dotty/tools/dotc/reporting/diagnostic/messages.scala b/src/dotty/tools/dotc/reporting/diagnostic/messages.scala
index c4e84aaee..264479467 100644
--- a/src/dotty/tools/dotc/reporting/diagnostic/messages.scala
+++ b/src/dotty/tools/dotc/reporting/diagnostic/messages.scala
@@ -512,4 +512,39 @@ object messages {
         |}""".stripMargin
   }
 
+  case class YieldOrDoExpectedInForComprehension()(implicit ctx: Context) extends Message(19) {
+    val kind = "Syntax"
+    val msg = hl"${"yield"} or ${"do"} expected"
+    
+    val code1 = "val numbers = for i <- 1 to 3 yield i"
+    val code2 = "val numbers = for (i <- 1 to 3) yield i"
+    val code3 = "for (i <- 1 to 3) println(i)"
+    val code4 = "for i <- 1 to 3 do println(i) // notice the 'do' keyword"
+    
+    val explanation =
+      hl"""When the enumerators in a for comprehension are not placed in parentheses or braces, a ${"do"} or ${"yield"} statement
+           |is required after the enumerators section of the comprehension.
+           |
+           |You can save some keystrokes by omitting the parentheses and writing
+           |
+           |$code1
+           |
+           |  instead of
+           |
+           |$code2
+           |
+           |but the ${"yield"} keyword is still required.
+           |
+           |For comprehensions that simply perform a side effect without yielding anything can also be written without parentheses
+           |but a ${"do"} keyword has to be included. For example,
+           |
+           |$code3
+           |
+           |  can be written as
+           |
+           |$code4
+           |
+           |""".stripMargin
+  }
+ 
 }