Spanned holes: threading positions through holes (#1085)

Every hole is now internally associated with a span which is printed
when the hole is encountered:
```sh
$ for BACKEND in chez-callcc llvm js; do effekt hole.effekt --backend $BACKEND; done
```

```log
# Chez
Exception: hole.effekt:8:15: not implemented yet
[error] Process exited with non-zero exit code 255.

# LLVM
PANIC: hole.effekt:8:15: not implemented yet
[error] Process exited with non-zero exit code 1.

# JS
.../out/hole.js:208
    else throw e
         ^
hole.effekt:8:15: not implemented yet
(Use `node --trace-uncaught ...` to show where the exception was thrown)

Node.js v22.16.0
[error] Process exited with non-zero exit code 1.
```

Author: jiribenes

effekt/jvm/src/test/scala/effekt/core/PatternMatchingTests.scala

@@ -47,7 +47,7 @@ class PatternMatchingTests extends CoreTests {
   }
 
   test("Sanity check: compiling empty list of clauses") {
-    assertEquals(compile(Nil), core.Hole())
+    assertEquals(compile(Nil), core.Hole(effekt.source.Span.missing))
   }
 
   test("Simple guard") {

effekt/shared/src/main/scala/effekt/core/Parser.scala

@@ -298,7 +298,7 @@ class CoreParsers(names: Names) extends EffektLexers {
       }
     | (`if` ~> `(` ~/> pure <~ `)`) ~ stmt ~ (`else` ~> stmt) ^^ Stmt.If.apply
     | `region` ~> blockLit ^^ Stmt.Region.apply
-    | `<>` ^^^ Hole()
+    | `<>` ^^^ Hole(effekt.source.Span.missing)
     | (pure <~ `match`) ~/ (`{` ~> many(clause) <~ `}`) ~ (`else` ~> stmt).? ^^ Stmt.Match.apply
     )
 

effekt/shared/src/main/scala/effekt/core/PatternMatchingCompiler.scala

@@ -86,7 +86,7 @@ object PatternMatchingCompiler {
    */
   def compile(clauses: List[Clause]): core.Stmt = {
     // This shouldn't be reachable anymore since we specialize matching on void before calling compile
-    if (clauses.isEmpty) return core.Hole()
+    if (clauses.isEmpty) return core.Hole(effekt.source.Span.missing)
 
     // (0) normalize clauses
     val normalized @ (headClause :: remainingClauses) = clauses.map(normalize) : @unchecked

effekt/shared/src/main/scala/effekt/core/PolymorphismBoxing.scala

@@ -269,7 +269,7 @@ object PolymorphismBoxing extends Phase[CoreTransformed, CoreTransformed] {
           val boxedResult = transformArg(originalResult)
           coerce(Stmt.Region(BlockLit(tparams, cparams, vparams, bparams, coerce(body, boxedResult))), originalResult)
       }
-    case Stmt.Hole() => Stmt.Hole()
+    case Stmt.Hole(span) => Stmt.Hole(span)
   }
 
   def transform(expr: Expr)(using Context, DeclarationContext): Bind[Expr] = expr match {

effekt/shared/src/main/scala/effekt/core/PrettyPrinter.scala

@@ -235,8 +235,8 @@ object PrettyPrinter extends ParenPrettyPrinter {
       val d = default.map { body => space <> "else" <+> braces(nest(line <> toDocStmts(body))) }.getOrElse { emptyDoc }
       toDoc(sc) <+> "match" <+> cs <> d
 
-    case Hole() =>
-      "<>"
+    case Hole(span) =>
+      "<>" <+> s"// @ ${span.range.from.format}"
   }
 
   def toDoc(tpe: core.BlockType): Doc = tpe match {

effekt/shared/src/main/scala/effekt/core/Recursive.scala

@@ -83,7 +83,7 @@ class Recursive(
     case Stmt.Shift(prompt, body) => process(prompt); process(body)
     case Stmt.Resume(k, body) => process(k); process(body)
     case Stmt.Region(body) => process(body)
-    case Stmt.Hole() => ()
+    case Stmt.Hole(span) => ()
   }
 
   def process(e: Expr): Unit = e match {

effekt/shared/src/main/scala/effekt/core/Transformer.scala

@@ -470,7 +470,7 @@ object Transformer extends Phase[Typechecked, CoreTransformed] {
       Context.bind(Region(BlockLit(Nil, List(region.capture), Nil, List(cap), transform(body))))
 
     case source.Hole(id, stmts, span) =>
-      Context.bind(core.Hole())
+      Context.bind(core.Hole(span))
 
     case a @ source.Assign(id, expr, _) =>
       val sym = a.definition

effekt/shared/src/main/scala/effekt/core/Tree.scala

@@ -303,7 +303,7 @@ enum Stmt extends Tree {
   case Resume(k: BlockVar, body: Stmt)
 
   // Others
-  case Hole()
+  case Hole(span: effekt.source.Span)
 
   val tpe: ValueType = Type.inferType(this)
   val capt: Captures = Type.inferCapt(this)
@@ -637,7 +637,7 @@ object Variables {
     case Stmt.Reset(body) => free(body)
     case Stmt.Shift(prompt, body) => free(prompt) ++ free(body)
     case Stmt.Resume(k, body) => free(k) ++ free(body)
-    case Stmt.Hole() => Variables.empty
+    case Stmt.Hole(span) => Variables.empty
   }
 
   def bound(t: ValueParam): Variables = Variables.value(t.id, t.tpe)

effekt/shared/src/main/scala/effekt/core/Type.scala

@@ -225,7 +225,7 @@ object Type {
     }
     case Stmt.Region(body) => body.returnType
 
-    case Stmt.Hole() => TBottom
+    case Stmt.Hole(span) => TBottom
   }
 
   def inferCapt(stmt: Stmt): Captures = stmt match {
@@ -245,7 +245,7 @@ object Type {
     case Stmt.Shift(prompt, body) => prompt.capt ++ body.capt
     case Stmt.Resume(k, body) => k.capt ++ body.capt
     case Stmt.Region(body) => body.capt
-    case Stmt.Hole() => Set.empty
+    case Stmt.Hole(span) => Set.empty
   }
 
   def inferType(expr: Expr): ValueType = expr match {

effekt/shared/src/main/scala/effekt/core/optimizer/BindSubexpressions.scala

@@ -86,7 +86,7 @@ object BindSubexpressions {
     case Stmt.Reset(body) => Stmt.Reset(transform(body))
     case Stmt.Shift(prompt, body) => Stmt.Shift(transform(prompt), transform(body))
     case Stmt.Resume(k, body) => Stmt.Resume(transform(k), transform(body))
-    case Stmt.Hole() => Stmt.Hole()
+    case Stmt.Hole(span) => Stmt.Hole(span)
   }
 
   def transform(b: Block)(using Env): Bind[Block] = b match {