Fix: Splices with non-String type break if there is no splice (#901)

Fixes #892

Author: dvdvgt

effekt/shared/src/main/scala/effekt/RecursiveDescent.scala

@@ -1030,10 +1030,9 @@ class RecursiveDescent(positions: Positions, tokens: Seq[Token], source: Source)
   def templateString(): Term =
     nonterminal:
       backtrack(idRef()) ~ template() match {
-        // We do not need to apply any transformation if there are no splices
-        case _ ~ Template(str :: Nil, Nil) => StringLit(str)
-        case _ ~ Template(strs, Nil) => fail("Cannot occur")
-        // s"a${x}b${y}" ~> s { do literal("a"); do splice(x); do literal("b"); do splice(y) }
+        // We do not need to apply any transformation if there are no splices _and_ no custom handler id is given
+        case None ~ Template(str :: Nil, Nil) => StringLit(str)
+        // s"a${x}b${y}" ~> s { do literal("a"); do splice(x); do literal("b"); do splice(y); return () }
         case id ~ Template(strs, args) =>
           val target = id.getOrElse(IdRef(Nil, "s"))
           val doLits = strs.map { s =>

examples/pos/string_interpolation.check

@@ -1,2 +1,3 @@
 GET https://api.effekt-lang.org/users/effekt/resource/42
-Fix point combinator: \ f -> (\ x -> f x x) \ x -> f x x
+Fix point combinator: \ f -> (\ x -> f x x) \ x -> f x x
+12

examples/pos/string_interpolation.effekt

@@ -28,6 +28,15 @@ def pretty { prog: () => Unit / {literal, splice[Expr]} }: String = {
   }
 }
 
+def len { prog: () => Unit / {literal} }: Int = {
+  try {
+    prog()
+    0
+  } with literal { s =>
+    s.length
+  }
+}
+
 def main() = {
   val domain = "https://api.effekt-lang.org"
   val user = "effekt"
@@ -36,4 +45,6 @@ def main() = {
 
   val fixpoint = Abs("f", App(Abs("x", App(Var("f"), App(Var("x"), Var("x")))), Abs("x", App(Var("f"), App(Var("x"), Var("x"))))))
   println(pretty"Fix point combinator: ${fixpoint}")
+
+  println(show(len"hello, world"))
 }

examples/pos/string_interpolation_literal.check

@@ -0,0 +1,5 @@
+42
+[WARNING] Frobnicators have been jabberwocked!
+-1
+[ERROR] Stuff went wrong!
+[ERROR] Aborting!

examples/pos/string_interpolation_literal.effekt

@@ -0,0 +1,21 @@
+effect log(msg: String): Unit
+
+def error { body: => Unit / literal }: Unit / log =
+  try body() with literal { x => do log("[ERROR] " ++ x); resume(()) }
+
+def warn { body: => Unit / literal }: Unit / log =
+  try body() with literal { x => do log("[WARNING] " ++ x); resume(()) }
+
+def doc { body: => Unit / {literal} }: Unit =
+  try body() with literal { _ => resume(()) }
+
+def main() = try {
+  doc"This is the doc string for my main function!"
+
+  println(42)
+  warn"Frobnicators have been jabberwocked!"
+
+  println(-1)
+  error"Stuff went wrong!"
+  error"Aborting!"
+} with log { msg => println(msg); resume(()) }