Rewrite MacroEvaluator to support flatten, and container-making macros

src/main/java/com/amazon/ion/impl/macro/EExpressionArgsReader.java

@@ -231,15 +231,16 @@ private void readStreamAsExpressionGroup(
      * @param expressions receives the expressions as they are materialized.
      */
     protected void readValueAsExpression(boolean isImplicitRest, List<Expression.EExpressionBodyExpression> expressions) {
-        if (isMacroInvocation()) {
+        if (isImplicitRest && !isContainerAnExpressionGroup()) {
+            readStreamAsExpressionGroup(expressions);
+            return;
+        } else if (isMacroInvocation()) {
             collectEExpressionArgs(expressions); // TODO avoid recursion
             return;
         }
         IonType type = reader.encodingType();
         List<SymbolToken> annotations = getAnnotations();
-        if (isImplicitRest && !isContainerAnExpressionGroup()) {
-            readStreamAsExpressionGroup(expressions);
-        } else if (IonType.isContainer(type)) {
+        if (IonType.isContainer(type) && !reader.isNullValue()) {
             readContainerValueAsExpression(type, annotations, expressions);
         } else {
             readScalarValueAsExpression(type, annotations, expressions);

src/main/java/com/amazon/ion/impl/macro/Environment.kt

@@ -21,6 +21,16 @@
     val parentEnvironment: Environment?,
 ) {
     fun createChild(arguments: List<Expression>, argumentIndices: List<Int>) = Environment(arguments, argumentIndices, this)
+
+    override fun toString() = """
+        |Environment(
+        |    argumentIndices: $argumentIndices,
+        |    argumentExpressions: [${arguments.mapIndexed { index, expression -> "\n|        $index. $expression" }.joinToString() }
+        |    ],
+        |    parent: ${parentEnvironment.toString().lines().joinToString("\n|        ")},
+        |)
+    """.trimMargin()
+
     companion object {
         @JvmStatic
         val EMPTY = Environment(emptyList(), emptyList(), null)

src/main/java/com/amazon/ion/impl/macro/Expression.kt

@@ -52,7 +52,21 @@
      * These expressions are the only ones that may be the output from the macro evaluator.
      * All [DataModelExpression]s are also valid to use as [TemplateBodyExpression]s and [EExpressionBodyExpression]s.
      */
-    sealed interface DataModelExpression : Expression, EExpressionBodyExpression, TemplateBodyExpression
+    sealed interface DataModelExpression : Expression, EExpressionBodyExpression, TemplateBodyExpression, ExpansionOutputExpression
+
+    /** Output of a macro expansion (internal to the macro evaluator) */
+    sealed interface ExpansionOutputExpressionOrContinue
+    /** Output of the macro evaluator */
+    sealed interface ExpansionOutputExpression : ExpansionOutputExpressionOrContinue
+
+    /**
+     * Indicates to the macro evaluator that the current expansion did not produce a value this time, but it may
+     * produce more expressions. The macro evaluator should request another expression from that macro.
+     */
+    data object ContinueExpansion : ExpansionOutputExpressionOrContinue
+
+    /** Signals the end of an expansion in the macro evaluator. */
+    data object EndOfExpansion : ExpansionOutputExpression
 
     /**
      * Interface for expressions that are _values_ in the Ion data model.
@@ -69,6 +83,8 @@
 
     /**
      * A temporary placeholder that is used only while a macro or e-expression is partially compiled.
+     *
+     * TODO: See if we can get rid of this by e.g. using nulls during macro compilation.
      */
     object Placeholder : TemplateBodyExpression, EExpressionBodyExpression
 
@@ -221,21 +237,25 @@
      */
     data class VariableRef(val signatureIndex: Int) : TemplateBodyExpression
 
+    sealed interface InvokableExpression : HasStartAndEnd, Expression {
+        val macro: Macro
+    }
+
     /**
      * A macro invocation that needs to be expanded.
      */
     data class MacroInvocation(
-        val macro: Macro,
+        override val macro: Macro,
         override val selfIndex: Int,
         override val endExclusive: Int
-    ) : TemplateBodyExpression, HasStartAndEnd
+    ) : TemplateBodyExpression, HasStartAndEnd, InvokableExpression
 
     /**
      * An e-expression that needs to be expanded.
      */
     data class EExpression(
-        val macro: Macro,
+        override val macro: Macro,
         override val selfIndex: Int,
         override val endExclusive: Int
-    ) : EExpressionBodyExpression, HasStartAndEnd
+    ) : EExpressionBodyExpression, HasStartAndEnd, InvokableExpression
 }