Lesson 29 · Wrong argument counts - Build an Expression Evaluator

Wrong argument counts

A known function called with the wrong number of arguments would read past its arguments or ignore them. Today you check each built-in's arity and report a clear error when it does not match.

The goal

Report a positioned error when a built-in function is called with the wrong number of arguments.

Start here - the target

TO DO

Scenario: Functions validate how many arguments they receive

Giventhe expression string "sqrt(1, 2)"

Whenit is evaluated with EvalString

Thenevaluation fails with: sqrt expects 1 argument, got 2 at position 0

Andevaluating "max()" fails with: max expects at least 1 argument, got 0 at position 0

Background

Every built-in has an arity, a required number of arguments: sqrt and abs take exactly one, pow exactly two, and max and min at least one. Calling one the wrong way, like sqrt(1, 2), previously ignored the extra argument, and max() would have crashed indexing an empty slice. Guarding each function’s argument count before using the arguments turns both into a clear message that says what the function expected and what it got, at the call’s position.

This is the last error class. The message wording, expects 1 argument for a fixed arity and expects at least 1 argument for a variadic one, tells the user precisely how to fix the call. With parse errors, arithmetic errors, undefined-name errors, and now arity errors all handled, every way an expression can be malformed produces a clear, positioned message instead of a crash or a wrong answer. The library is complete and robust; the capstone puts it all on display.

Make it work

// check arity before indexing into args
case "sqrt":
  if len(args) != 1 {
    return 0, fmt.Errorf("sqrt expects 1 argument, got %d at position %d", len(args), n.Pos)
  }
  return math.Sqrt(args[0]), nil
case "max":
  if len(args) < 1 {
    return 0, fmt.Errorf("max expects at least 1 argument, got %d at position %d", len(args), n.Pos)
  }
  // ... fold as before ...

CheckpointDONE

Built-in functions reject calls with the wrong number of arguments. Commit and stop here.