Scaffolding for OptionalCall

Adds a new `optional-call` terminal and sets up the appropriate handling in the 
visitors, with lowering/reactivefunction/codegen as todos for now and 
implemented in follow-ups.

compiler/forget/src/HIR/BuildHIR.ts

@@ -11,6 +11,7 @@ import { Expression } from "@babel/types";
 import invariant from "invariant";
 import { CompilerError, ErrorSeverity } from "../CompilerError";
 import { Err, Ok, Result } from "../Utils/Result";
+import todo from "../Utils/todo";
 import { assertExhaustive } from "../Utils/utils";
 import { Environment, EnvironmentOptions } from "./Environment";
 import {
@@ -939,6 +940,12 @@ function lowerExpression(
         loc: exprLoc,
       };
     }
+    case "OptionalCallExpression": {
+      const expr = exprPath as NodePath<t.OptionalCallExpression>;
+      const _optional = expr.get("optional");
+
+      return todo("OptionalCallExpression");
+    }
     case "CallExpression": {
       const expr = exprPath as NodePath<t.CallExpression>;
       const calleePath = expr.get("callee");

compiler/forget/src/HIR/HIR.ts

@@ -251,7 +251,8 @@ export type Terminal =
   | DoWhileTerminal
   | WhileTerminal
   | LogicalTerminal
-  | TernaryTerminal;
+  | TernaryTerminal
+  | OptionalCallTerminal;
 
 /**
  * Terminal nodes allowed for a value block
@@ -352,6 +353,14 @@ export type TernaryTerminal = {
   loc: SourceLocation;
 };
 
+export type OptionalCallTerminal = {
+  kind: "optional-call";
+  test: BlockId;
+  fallthrough: BlockId;
+  id: InstructionId;
+  loc: SourceLocation;
+};
+
 /**
  * Instructions generally represent expressions but with all nesting flattened away,
  * such that all operands to each instruction are either primitive values OR are

compiler/forget/src/HIR/HIRBuilder.ts

@@ -624,6 +624,7 @@ export function reversePostorderBlocks(func: HIR): void {
         }
         break;
       }
+      case "optional-call":
       case "ternary":
       case "logical": {
         visit(terminal.test);

compiler/forget/src/HIR/PrintHIR.ts

@@ -79,6 +79,7 @@ export function printMixedHIR(
     return printInstruction(value);
   }
   switch (value.kind) {
+    case "optional-call":
     case "branch":
     case "if":
     case "logical":
@@ -155,6 +156,10 @@ export function printTerminal(terminal: Terminal): Array<string> | string {
       value = `[${terminal.id}] Ternary test:bb${terminal.test} fallthrough=bb${terminal.fallthrough}`;
       break;
     }
+    case "optional-call": {
+      value = `[${terminal.id}] OptionalCall test:bb${terminal.test} fallthrough=bb${terminal.fallthrough}`;
+      break;
+    }
     case "throw": {
       value = `[${terminal.id}] Throw ${printPlace(terminal.value)}`;
       break;

compiler/forget/src/HIR/visitors.ts

@@ -562,6 +562,17 @@ export function mapTerminalSuccessors(
         loc: terminal.loc,
       };
     }
+    case "optional-call": {
+      const test = fn(terminal.test);
+      const fallthrough = fn(terminal.fallthrough);
+      return {
+        kind: "optional-call",
+        test,
+        fallthrough,
+        id: makeInstructionId(0),
+        loc: terminal.loc,
+      };
+    }
     case "return": {
       return {
         kind: "return",
@@ -655,6 +666,7 @@ export function* eachTerminalSuccessor(terminal: Terminal): Iterable<BlockId> {
       }
       break;
     }
+    case "optional-call":
     case "ternary":
     case "logical": {
       yield terminal.test;
@@ -719,6 +731,7 @@ export function mapTerminalOperands(
       }
       break;
     }
+    case "optional-call":
     case "ternary":
     case "logical":
     case "do-while":
@@ -765,6 +778,7 @@ export function* eachTerminalOperand(terminal: Terminal): Iterable<Place> {
       }
       break;
     }
+    case "optional-call":
     case "ternary":
     case "logical":
     case "do-while":

compiler/forget/src/ReactiveScopes/BuildReactiveFunction.ts

@@ -29,6 +29,7 @@ import {
   ReactiveValue,
   Terminal,
 } from "../HIR/HIR";
+import todo from "../Utils/todo";
 import { assertExhaustive } from "../Utils/utils";
 
 /**
@@ -481,6 +482,9 @@ class Driver {
         this.visitBlock(this.cx.ir.blocks.get(fallthroughId)!, blockValue);
         break;
       }
+      case "optional-call": {
+        todo("BuildReactiveFunction: support optional-call terminal");
+      }
       case "goto": {
         switch (terminal.variant) {
           case GotoVariant.Break: {

compiler/forget/src/Utils/VisualizeHIRMermaid.ts

@@ -107,6 +107,10 @@ function printTerminalLabel(terminal: Terminal): string {
       buffer.push("Ternary");
       break;
     }
+    case "optional-call": {
+      buffer.push("OptionalCall");
+      break;
+    }
     case "do-while": {
       buffer.push("DoWhile");
       break;
@@ -157,6 +161,11 @@ function printTerminalArrows(blockId: BlockId, terminal: Terminal): string {
       buffer.push(printJumpArrow(blockId, terminal.fallthrough, "fallthrough"));
       break;
     }
+    case "optional-call": {
+      buffer.push(printJumpArrow(blockId, terminal.test, "test"));
+      buffer.push(printJumpArrow(blockId, terminal.fallthrough, "fallthrough"));
+      break;
+    }
     case "goto": {
       buffer.push(printJumpArrow(blockId, terminal.block, null));
       break;