Lesson 24 · Negating numbers - Build a Programming Language

Negating numbers

The other prefix operator is minus, which negates a number. Today you evaluate it, producing a new integer object with the sign flipped.

The goal

Evaluate the prefix - operator on an integer, returning its negation.

Start here - the target

TO DO

Scenario: Evaluating numeric negation

Givena program using the prefix - operator on an integer

Whenthe evaluator evaluates -5 and then -10

Thenthe results are integer objects with values -5 and -10

Andevaluating --5 (minus applied twice) yields 5

Background

The prefix - takes an integer and returns its negation as a new integer object. Applied twice, --5 negates and negates again, landing back on 5. This is the arithmetic counterpart to !, and it rounds out the two prefix operators the language supports.

For today, assume the operand really is an integer - negating a boolean or string is a type error, but reporting those cleanly is a job for the error-handling lesson later in this chapter. Keep the focus narrow: unwrap the integer, flip its sign, wrap it back up.

Make it work

func evalMinus(right Object) Object {
  // for now assume the operand really is an integer;
  // rejecting non-integers is the error-handling lesson's job
  i := right.(*Integer)
  return &Integer{Value: -i.Value}
}

CheckpointDONE

The prefix - operator negates integers. Commit.