diff --git a/src/lang/debug/repl.ts b/src/lang/debug/repl.ts
index 513b754..82fbdf5 100644
--- a/src/lang/debug/repl.ts
+++ b/src/lang/debug/repl.ts
@@ -19,7 +19,7 @@ const C = {
   Bold: "\x1b[1m",
 };
 
-const program = Program.makeEmpty();
+const program = Program.make();
 
 function runSource(inputRaw: string, isRepl: boolean): boolean {
   const input = inputRaw.trim();
diff --git a/src/lang/primitive.ts b/src/lang/primitive.ts
new file mode 100644
index 0000000..aa90725
--- /dev/null
+++ b/src/lang/primitive.ts
@@ -0,0 +1,57 @@
+import { Value } from "./eval/value";
+import { Implementation, Program } from "./program";
+import { ThrownRuntimeError } from "./eval/error";
+import { FunctionName } from "./expr";
+import { valueToString } from "./debug/value_show";
+
+// TODO: Primitive functions like +, -, *, div, <, <=, ==, mod
+
+export function installPrimitives(program: Program) {
+  function R(name: FunctionName, implementation: Implementation) {
+    Program.registerPrimitive(program, name, implementation);
+  }
+
+  // addition
+  R("+", (args: Value[]): Value => {
+    let sum = 0;
+    for (const arg of args) {
+      if (arg.tag !== "number") {
+        throw ThrownRuntimeError.error({ tag: "TypeMismatch", expected: "number", received: arg });
+      }
+      sum += arg.value;
+    }
+    return Value.number(sum);
+  });
+
+  // multiplication
+  R("*", (args: Value[]): Value => {
+    let product = 1;
+    for (const arg of args) {
+      if (arg.tag !== "number") {
+        throw ThrownRuntimeError.error({ tag: "TypeMismatch", expected: "number", received: arg });
+      }
+      product = product*arg.value;
+    }
+    return Value.number(product);
+  });
+
+  // string concat
+  R("++", (args: Value[]): Value => {
+    let sum = "";
+    for (const arg of args) {
+      if (arg.tag !== "string") {
+        throw ThrownRuntimeError.error({ tag: "TypeMismatch", expected: "number", received: arg });
+      }
+      sum += arg.value;
+    }
+    return Value.string(sum);
+  });
+
+  R("log", (args: Value[]): Value => {
+    for (const arg of args) {
+      console.log(valueToString(arg));
+    }
+    return Value.tuple([]);
+  });
+
+}
diff --git a/src/lang/program.ts b/src/lang/program.ts
index ac5a10d..24fe0ee 100644
--- a/src/lang/program.ts
+++ b/src/lang/program.ts
@@ -1,6 +1,7 @@
 import { ThrownRuntimeError } from "./eval/error";
 import { Value } from "./eval/value";
 import { Expr, FunctionName, ProductPattern } from "./expr";
+import { installPrimitives } from "./primitive";
 
 export type Timestamp = number;
 
@@ -51,39 +52,56 @@ export type UserFunctionDefinition = {
 
 export type PrimitiveFunctionDefinition = {
   name: FunctionName,
-  implementation: (args: Value[]) => Value,
+  implementation: Implementation,
 }
 
+export type Implementation = (args: Value[]) => Value
+
 export namespace Program {
 
   type Error =
     | { tag: "DuplicateFunctionName", name: FunctionName }
-    | { tag: "FunctionNotFound", name: FunctionName };
+    | { tag: "FunctionNotFound", name: FunctionName }
+    | { tag: "CannotEditPrimitiveFunction", name: FunctionName }
+    | { tag: "CannotDeletePrimitiveFunction", name: FunctionName }
+    | { tag: "PrimitiveFunctionAlreadyExists", name: FunctionName }
 
   type Result<T> =
     | { tag: "ok", value: T }
-    | { tag: "error", error: Error };
-  // | { tag: "ParseError", message: string } // TODO
+    | { tag: "error", error: Error }
 
   export namespace Result {
     export function ok<T>(value: T): Result<T> { return { tag: "ok", value } }
     export function error<T>(error: Error): Result<T> { return { tag: "error", error } }
   }
 
-  export function makeEmpty(): Program {
-    return {
+  function primitive(name: FunctionName, implementation: Implementation): FunctionDefinition {
+    return { tag: "primitive", def: { name, implementation } };
+  }
+
+  export function registerPrimitive(program: Program, name: FunctionName, implementation: Implementation): void {
+    const fn = program.function_definitions.get(name);
+    if (fn !== undefined) {
+      throw { tag: "PrimitiveFunctionAlreadyExists", name };
+    }
+    program.function_definitions.set(name, primitive(name, implementation));
+    return undefined;
+  }
+
+  export function make(): Program {
+    const program: Program = {
       function_definitions: new Map(),
       function_definition_order: [],
     };
+
+    installPrimitives(program);
+    return program;
   }
-  // TODO: Primitive functions like +, -, *, div, <, <=, ==, mod
 
-  // TODO: function to create initial program (with the above primitive functions otherwise empty)
-
-  // may throw `ThrownRuntimeError`
+  // may throw `ThrownRuntimeError`. This is used by evaluator.
   export function lookup_function(program: Program, name: FunctionName): FunctionDefinition {
     const fn = program.function_definitions.get(name);
-    if (!fn) {
+    if (fn === undefined) {
       throw ThrownRuntimeError.error({
         tag: "FunctionLookupFailure",
         name,
@@ -93,44 +111,86 @@ export namespace Program {
   }
 
   export type CreateFunction = {
+    name: FunctionName,
+    parameters: ProductPattern[],
+    body: Expr,
+    raw_parameters: string,
+    raw_body: string,
+  }
+
+  export function registerFunction(
+    program: Program, 
+    { name, body, parameters, raw_parameters, raw_body }: CreateFunction
+  ): Result<void> {
+    if (program.function_definitions.has(name)) {
+      return Result.error({ tag: "DuplicateFunctionName", name });
+    }
+
+    const now: Timestamp = Date.now();
+
+    const newFunction: UserFunctionDefinition = {
+      name,
+      raw_parameters,
+      raw_body,
+      parameters,
+      body,
+      created_at: now,
+      last_modified_at: now,
+    };
+
+    program.function_definitions.set(name, { tag: "user", def: newFunction });
+    program.function_definition_order.push(name);
+
+    return Result.ok(undefined);
+  }
+
+  export type UpdateFunction = {
+    parameters: ProductPattern[],
+    body: Expr,
     raw_name: string,
     raw_parameters: string,
     raw_body: string,
   }
 
-  export type UpdateFunction = {
-    raw_name?: string,
-    raw_parameters?: string,
-    raw_body?: string,
+  export function updateFunction(
+    program: Program,
+    name: FunctionName,
+    { parameters, body, raw_parameters, raw_body }: UpdateFunction
+  ): Result<void> {
+    const existingEntry = program.function_definitions.get(name);
+    if (existingEntry === undefined) {
+      return Result.error({ tag: "FunctionNotFound", name });
+    }
+    if (existingEntry.tag === "primitive") {
+      return Result.error({ tag: "CannotEditPrimitiveFunction", name });
+    }
+    const def = existingEntry.def;
+    def.parameters = parameters;
+    def.body = body;
+    def.raw_parameters = raw_parameters;
+    def.raw_body = raw_body;
+
+    def.last_modified_at = Date.now();
+
+    return Result.ok(undefined);
   }
 
-  export function add_user_function(program: Program, description: CreateFunction): Result<void> {
-    // TODO:
-    // - parsing/validation
-    //   - raw_name (check if function already exists)
-    //   - raw_parameters
-    //   - raw_body
-    // - compute timestamp for now
-    return (0 as any);
-  }
+  export function deleteFunction(program: Program, name: FunctionName): Result<void> {
+    const existingEntry = program.function_definitions.get(name);
+    if (!existingEntry) {
+      return Result.error({ tag: "FunctionNotFound", name });
+    }
+    if (existingEntry.tag === "primitive") {
+       return Result.error({ tag: "CannotDeletePrimitiveFunction", name });
+    }
+    program.function_definitions.delete(name);
 
-  // TODO: What about result type? Should it on deletion return the original data of the function, and if there's a failure, how detailed should it be?
-  export function delete_user_function(program: Program, name: FunctionName): Result<void> {
-    // TODO:
-    // - see if the user function exists
-    // - if it does, delete it
-    // - if it doesn't ???
-    return (0 as any);
-  }
+    const orderIndex = program.function_definition_order.indexOf(name);
+    if (orderIndex !== -1) {
+      program.function_definition_order.splice(orderIndex, 1);
+    }
 
-  export function update_user_function(program: Program, name: FunctionName): Result<void> {
-    // TODO:
-    return (0 as any);
-  }
-
-  export function get_user_function(program: Program, name: FunctionName): Result<UserFunctionDefinition> {
-    // TODO:
-    return (0 as any);
+    return Result.ok(undefined);
   }
 
 }
diff --git a/tmp_repl/tmp_repl.md b/tmp_repl/tmp_repl.md
index 5bc61d2..6358c6e 100644
--- a/tmp_repl/tmp_repl.md
+++ b/tmp_repl/tmp_repl.md
@@ -2,14 +2,6 @@
 
 npm run start
 
-
-npm install -D @electron-forge/cli @electron-forge/maker-deb @electron-forge/maker-rpm @electron-forge/maker-squirrel @electron-forge/maker-zip @electron-forge/plugin-auto-unpack-natives @electron-forge/plugin-fuses @electron-forge/plugin-vite @electron/fuses @types/electron-squirrel-startup @typescript-eslint/eslint-plugin @typescript-eslint/parser electron eslint eslint-plugin-import typescript vite
-
-npm install electron-squirrel-startup
-
-npm install -D sass-embedded
-
-
 # Tests
 
 npx ts-node src/parser/cursor.test.ts