diff --git a/packages/typewiz-core/src/transformer.spec.ts b/packages/typewiz-core/src/transformer.spec.ts
new file mode 100644
index 0000000..785b8d0
--- /dev/null
+++ b/packages/typewiz-core/src/transformer.spec.ts
@@ -0,0 +1,23 @@
+import * as ts from 'typescript';
+import { transformSourceCode } from './transformer';
+
+function astPrettyPrint(sourceText: string) {
+    const printer: ts.Printer = ts.createPrinter();
+    return printer.printFile(ts.createSourceFile('test.ts', sourceText, ts.ScriptTarget.Latest));
+}
+
+describe('transformer', () => {
+    it('should instrument function parameters without types', () => {
+        const input = `function (a) { return 5; }`;
+        expect(transformSourceCode(input, 'test.ts')).toMatch(
+            astPrettyPrint(`function (a) { $_$twiz("a", a, 11, "test.ts", {}); return 5; }`),
+        );
+    });
+
+    it('should instrument class method parameters', () => {
+        const input = `class Foo { bar(a) { return 5; } }`;
+        expect(transformSourceCode(input, 'test.ts')).toMatch(
+            astPrettyPrint(`class Foo { bar(a) { $_$twiz("a", a, 17, "test.ts", {}); return 5; } }`),
+        );
+    });
+});
diff --git a/packages/typewiz-core/src/transformer.ts b/packages/typewiz-core/src/transformer.ts
new file mode 100644
index 0000000..f75a10c
--- /dev/null
+++ b/packages/typewiz-core/src/transformer.ts
@@ -0,0 +1,92 @@
+import * as ts from 'typescript';
+
+function updateFunction(node: ts.FunctionDeclaration, instrumentStatements: ReadonlyArray<ts.Statement>) {
+    return ts.updateFunctionDeclaration(
+        node,
+        node.decorators,
+        node.modifiers,
+        node.asteriskToken,
+        node.name,
+        node.typeParameters,
+        node.parameters,
+        node.type,
+        ts.createBlock([...instrumentStatements, ...(node.body ? node.body.statements : [])]),
+    );
+}
+
+function updateMethod(node: ts.MethodDeclaration, instrumentStatements: ReadonlyArray<ts.Statement>) {
+    return ts.updateMethod(
+        node,
+        node.decorators,
+        node.modifiers,
+        node.asteriskToken,
+        node.name,
+        node.questionToken,
+        node.typeParameters,
+        node.parameters,
+        node.type,
+        ts.createBlock([...instrumentStatements, ...(node.body ? node.body.statements : [])]),
+    );
+}
+
+export function visitorFactory(ctx: ts.TransformationContext, source: ts.SourceFile) {
+    const visitor: ts.Visitor = (node: ts.Node): ts.Node => {
+        if (ts.isFunctionDeclaration(node) || ts.isMethodDeclaration(node)) {
+            const instrumentStatements: ts.Statement[] = [];
+            for (const param of node.parameters) {
+                if (!param.type && !param.initializer && node.body) {
+                    const typeInsertionPos = param.name.getEnd() + (param.questionToken ? 1 : 0);
+                    // const opts: IExtraOptions = {};
+                    // if (isArrow) {
+                    //     opts.arrow = true;
+                    // }
+                    // if (!hasParensAroundArguments(node)) {
+                    //     opts.parens = [node.parameters[0].getStart(), node.parameters[0].getEnd()];
+                    // }
+                    instrumentStatements.push(
+                        ts.createStatement(
+                            ts.createCall(
+                                ts.createIdentifier('$_$twiz'),
+                                [],
+                                [
+                                    ts.createLiteral(param.name.getText()),
+                                    ts.createIdentifier(param.name.getText()),
+                                    ts.createNumericLiteral(typeInsertionPos.toString()),
+                                    ts.createLiteral(source.fileName),
+                                    ts.createObjectLiteral(), // TODO: opts
+                                ],
+                            ),
+                        ),
+                    );
+                }
+                if (ts.isFunctionDeclaration(node)) {
+                    return updateFunction(node, instrumentStatements);
+                }
+                if (ts.isMethodDeclaration(node)) {
+                    return updateMethod(node, instrumentStatements);
+                }
+            }
+        }
+
+        return ts.visitEachChild(node, visitor, ctx);
+    };
+
+    return visitor;
+}
+
+export function transformer() {
+    return (ctx: ts.TransformationContext): ts.Transformer<ts.SourceFile> => {
+        return (source: ts.SourceFile) => ts.visitNode(source, visitorFactory(ctx, source));
+    };
+}
+
+export function transformSourceFile(sourceFile: ts.SourceFile) {
+    return ts.transform(sourceFile, [transformer()]).transformed[0];
+}
+
+export function transformSourceCode(sourceText: string, fileName: string) {
+    const sourceFile = ts.createSourceFile(fileName, sourceText, ts.ScriptTarget.Latest, true);
+    const transformed = transformSourceFile(sourceFile);
+    const printer: ts.Printer = ts.createPrinter();
+    return printer.printFile(transformed);
+}