initial recast setup

src/lang/recast.test.ts

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+import { recast } from "./recast";
+import { Program } from "./abstractSyntaxTree";
+import { abstractSyntaxTree } from "./abstractSyntaxTree";
+import { lexer } from "./tokeniser";
+import { Token } from "./tokeniser";
+
+describe("recast", () => {
+  it("recasts a simple program", () => {
+    const code = "1 + 2";
+    const {ast, tokens } = code2ast(code);
+    const recasted = recast(ast);
+    expect(recasted).toBe(code);
+  });
+});
+
+// helpers
+
+function code2ast(code: string): { ast: Program; tokens: Token[]} {
+  const tokens = lexer(code);
+  const ast = abstractSyntaxTree(tokens);
+  return {
+    ast,
+    tokens
+  }
+}

src/lang/recast.ts

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+import { Program, BinaryExpression, BinaryPart } from "./abstractSyntaxTree";
+
+export function recast(ast: Program, previousWrittenCode = ''): string {
+  return ast.body
+    .map((statement) => {
+        if(statement.type === "ExpressionStatement") {
+            if(statement.expression.type === "BinaryExpression") {
+                return recastBinaryExpression(statement.expression);
+            }
+        }
+      return statement.type;
+    })
+    .join("\n");
+}
+
+function recastBinaryExpression(expression: BinaryExpression): string {
+    return `${recastBinaryPart(expression.left)} ${expression.operator} ${recastBinaryPart(expression.right)}`
+}
+
+function recastBinaryPart(part: BinaryPart): string {
+    if(part.type === "Literal") {
+        return String(part?.value);
+    } else if(part.type === "Identifier") {
+        return part.name;
+    }
+    throw new Error(`Cannot recast ${part}`);
+}