Lesson 13 · Evaluating a number - Build an Expression Evaluator

Evaluating a number

A tree is only useful once you can reduce it to a value. Today you write the evaluator's first case and the one-call convenience that ties tokenizing, parsing, and evaluating together, so the library can compute a whole expression string.

The goal

Evaluate a Number node to its float64 value, and add EvalString to run a whole string.

Start here - the target

TO DO

Scenario: A number expression evaluates to its value

Giventhe expression string "42"

Whenit is evaluated with EvalString

Thenthe result is 42 and no error is returned

Andevaluating "3.14" gives 3.14

Background

Evaluation is a tree walk: a recursive function that turns each node into a number. Today it handles the one leaf case, a Num, by returning its stored value. Eval takes an Env, a map from variable names to values, and returns a value and an error. Both ride along mostly unused right now, the way Parse’s error did: the environment stays empty until the variables chapter, and the error stays nil until the errors chapter, but declaring them now keeps the signature stable for every caller.

EvalString is the library’s headline entry point: give it a string, and it tokenizes, parses, and evaluates in one call, returning the number or the first error along the way. From here on you can assert on whole expressions end to end, which is exactly how the rest of this chapter pins each operator’s behavior to an exact result.

Make it work

type Env map[string]float64          // variable names to values, empty for now
func Eval(e Expr, env Env) (float64, error) {
  switch n := e.(type) {
  case *Num:
    return n.Val, nil
  }
  return 0, fmt.Errorf("cannot evaluate")
}
func EvalString(in string, env Env) (float64, error) {
  e, err := Parse(in)
  if err != nil { return 0, err }
  return Eval(e, env)
}

CheckpointDONE

The evaluator reduces a number to its value, and EvalString runs a whole expression string. Commit and stop here.