Lesson 33 · Assignment - Build a Programming Language

Assignment

A let creates a binding; assignment updates one. Today you evaluate x = value, changing what an existing name points to - the piece a loop needs to make progress.

The goal

Evaluate assignment to an existing variable, and error when the name was never bound.

Start here - the target

TO DO

Scenario: Reassigning a variable

Giventhe source let x = 1; x = 2; x

Whenthe evaluator evaluates the program

Thenthe result is 2

Andassigning to a name that was never bound, as in y = 3, yields the error identifier not found: y

Background

let and assignment are different operations: let x = 1 creates a new binding, while x = 2 updates an existing one. Assignment requires the name to already exist - assigning to an unbound y is the same identifier not found error you built last lesson, not a silent new binding. That distinction keeps typos from quietly creating variables.

You will parse x = value as an assignment (an identifier on the left, =, and a value) and evaluate it by updating the environment in place. This is the missing piece for loops: without reassignment a while could never change its condition, so a counter could never advance. That is exactly what the chapter’s final lesson puts to work.

Make it work

// parse x = value as an Assignment node (name + value expression)
// eval: require the name already exists in the environment, then update it
func (e *Environment) Assign(name string, val Object) bool {
  if _, ok := e.store[name]; !ok { return false }   // not bound -> caller errors
  e.store[name] = val; return true
}

CheckpointDONE

Assignment updates an existing binding, and assigning an unbound name errors. Commit.