Fix #661 by improving type inference (#662)

We now order block arguments to first type check those that do not
subtract effects. This hopefully gives enough info about type arguments
to uniquely determine effects of other block arguments.

effekt/shared/src/main/scala/effekt/Typer.scala

@@ -1198,10 +1198,10 @@ object Typer extends Phase[NameResolved, Typechecked] {
       Context.abort(s"Wrong number of type arguments, given ${targs.size}, but ${name} expects ${funTpe.tparams.size}.")
 
     if (vargs.size != funTpe.vparams.size)
-      Context.error(s"Wrong number of value arguments, given ${vargs.size}, but ${name} expects ${funTpe.vparams.size}.")
+      Context.abort(s"Wrong number of value arguments, given ${vargs.size}, but ${name} expects ${funTpe.vparams.size}.")
 
     if (bargs.size != funTpe.bparams.size)
-      Context.error(s"Wrong number of block arguments, given ${bargs.size}, but ${name} expects ${funTpe.bparams.size}.")
+      Context.abort(s"Wrong number of block arguments, given ${bargs.size}, but ${name} expects ${funTpe.bparams.size}.")
 
     val callsite = currentCapture
 
@@ -1222,7 +1222,16 @@ object Typer extends Phase[NameResolved, Typechecked] {
       effs = effs ++ eff
     }
 
-    (bps zip bargs zip captArgs) foreach { case ((tpe, expr), capt) =>
+    // To improve inference, we first type check block arguments that DO NOT subtract effects,
+    // since those need to be fully known.
+
+    val (withoutEffects, withEffects) = (bps zip (bargs zip captArgs)).partitionMap {
+      // TODO refine and check that eff.args refers to (inferred) type arguments of this application (`typeArgs`)
+      case (tpe : FunctionType, rest) if tpe.effects.exists { eff => eff.args.nonEmpty } => Right((tpe, rest))
+      case (tpe, rest) => Left((tpe, rest))
+    }
+
+    (withoutEffects ++ withEffects) foreach { case (tpe, (expr, capt)) =>
       flowsInto(capt, callsite)
       // capture of block <: ?C
       flowingInto(capt) {

examples/pos/issue661.check

@@ -0,0 +1,3 @@
+2
+3
+4

examples/pos/issue661.effekt

@@ -0,0 +1,21 @@
+interface Iter[T] {
+  def yield(value: T): Unit
+}
+
+def iter[T](self: List[T]): Unit / {Iter[T]} =
+  self match {
+    case Cons(head, tail) => do yield(head); iter(tail)
+    case Nil() => ()
+  }
+
+def my_map[T, U] {action: () => Unit / {Iter[T]}} {f: T => U}: Unit / {Iter[U]} =
+  try action()
+  with Iter[T] {
+    def yield(value) = { do yield(f(value)); resume(()) }
+  }
+
+def main() =
+  try my_map { iter([1, 2, 3]) } { x => x+1 }
+  with Iter[Int] {
+    def yield(x) = { println(x); resume(()) }
+  }