Slightly confusing error message related to unboxing when trying to call a non-function

[jiribenes]

Note

This issue is probably low priority, I just wanted to write it down:

The following code tries to use a Scala-ism: using function call to call some .apply method in the style of .get:

def getFirst(words: List[String]): String = words(0)

returns the following, slightly confusing, error:

Unbox requires a boxed type, but got List[String].

It is, of course, confusing, because the user didn't write neither a box/unbox, nor a first-class function type, which causes a lot of confusion. An ideal message (given the annotations) here for a language without boxing could be something like:

Trying to call a value of type `List[String]` as a function with 1 argument of type `Int`.

(we can bikeshed this later, it also depends on the amount of information we have at that point, of course!)

Other systems:

### Elm:
The `words` value is not a function, but it was given 1 argument.

### Haskell:
- Couldn't match expected type: t0 -> String with actual type: [String]
- The function ‘words’ is applied to one value argument, but its type ‘[String]’ has none

### HMloc:
[ERROR] Type `string list` does not match `int -> _`

◉ (string list) is here
▲ - l.2 let getFirst (words: list[string]) = words 0
│ 
│  - l.2 let getFirst (words: list[string]) = words 0
│ 
◉ ('a) is assumed here
│  - l.2 let getFirst (words: list[string]) = words 0
▼ 
◉ (int -> _) is here
   - l.2 let getFirst (words: list[string]) = words 0

[jiribenes]

I think the message here could be slightly more customised for situation where the unbox parent is a function call & the unbox is synthesised: 👀

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

Lines 514 to 523 in 92a6e0f

	case _ =>
	Context.annotationOption(Annotations.UnboxParentDef, u) match {
	case Some(source.DefDef(id, annot, block)) =>
	// Since this `unbox` was synthesized by the compiler from `def foo = E`,
	// it's possible that the user simply doesn't know that they should have used the `val` keyword to specify a value
	// instead of using `def`; see [issue #130](https://github.com/effekt-lang/effekt/issues/130) for more details
	Context.abort(pretty"Expected the right-hand side of a `def` binding to be a block, but got a value of type $vtpe.\nMaybe try `val` if you're defining a value.")
	case _ =>
	Context.abort(pretty"Unbox requires a boxed type, but got $vtpe.")
	}

[jiribenes]

Here's a quick patch with a hack that allows a slightly specialised error message [error] Trying to call a value `words` of type List[String] as if it was a function. for this case:

diff --git i/effekt/shared/src/main/scala/effekt/Typer.scala w/effekt/shared/src/main/scala/effekt/Typer.scala
index 48849181..45bf4bd0 100644
--- i/effekt/shared/src/main/scala/effekt/Typer.scala
+++ w/effekt/shared/src/main/scala/effekt/Typer.scala
@@ -518,7 +518,9 @@ object Typer extends Phase[NameResolved, Typechecked] {
                 // it's possible that the user simply doesn't know that they should have used the `val` keyword to specify a value
                 // instead of using `def`; see [issue #130](https://github.com/effekt-lang/effekt/issues/130) for more details
                 Context.abort(pretty"Expected the right-hand side of a `def` binding to be a block, but got a value of type $vtpe.\nMaybe try `val` if you're defining a value.")
+              case Some(source.IdTarget(id)) =>
+                Context.abort(pretty"Trying to call a value `$id` of type $vtpe as if it was a function.")
               case _ =>
                 Context.abort(pretty"Unbox requires a boxed type, but got $vtpe.")
             }
         }
diff --git i/effekt/shared/src/main/scala/effekt/context/Annotations.scala w/effekt/shared/src/main/scala/effekt/context/Annotations.scala
index bfa619d3..ffff6c71 100644
--- i/effekt/shared/src/main/scala/effekt/context/Annotations.scala
+++ w/effekt/shared/src/main/scala/effekt/context/Annotations.scala
@@ -298,7 +298,7 @@ object Annotations {
    * Introduced by the pretyper.
    * Used by typer in order to display a more precise error message.
    */
-  val UnboxParentDef = Annotation[source.Unbox, source.Def](
+  val UnboxParentDef = Annotation[source.Unbox, source.Def | source.IdTarget](
     "UnboxParentDef",
     "the parent definition of an Unbox if it was synthesized"
   )
diff --git i/effekt/shared/src/main/scala/effekt/typer/BoxUnboxInference.scala w/effekt/shared/src/main/scala/effekt/typer/BoxUnboxInference.scala
index 7b0a47da..69e8c25b 100644
--- i/effekt/shared/src/main/scala/effekt/typer/BoxUnboxInference.scala
+++ w/effekt/shared/src/main/scala/effekt/typer/BoxUnboxInference.scala
@@ -123,7 +123,9 @@ object BoxUnboxInference extends Phase[NameResolved, NameResolved] {
     case source.ExprTarget(receiver) => source.ExprTarget(rewriteAsBlock(receiver))
     case t: source.IdTarget => t.definition match {
       case sym: (ValueSymbol | symbols.RefBinder) =>
-        source.ExprTarget(source.Unbox(source.Var(t.id).inheritPosition(t)).inheritPosition(t)).inheritPosition(t)
+        val u: source.Term.Unbox = source.Unbox(source.Var(t.id).inheritPosition(t))
+        C.annotate(Annotations.UnboxParentDef, u, t)
+        source.ExprTarget(u.inheritPosition(t)).inheritPosition(t)
       case sym: BlockSymbol =>
         t
     }