Subtext

// Expression evaluator in Subtext
// Demonstrates choices, pattern matching, conditionals, and exports

expr: choice {
  literal?: 0
  plus?: record {
    left: expr
    right: expr
  }
}

eval-expr = do {
  x: expr
  try {
    x.literal?
  } else {
    x.plus?.left eval-expr() +(x.plus?.right eval-expr())
  }
}

polarity = do {
  n: 0
  try {
    check n <? 0
    'negative'
  } else {
    check n =? 0
    'zero'
  } else {
    'positive'
  }
}

integral-divide = do {
  numerator: 1
  divisor: 1
  ratio = numerator /(divisor) floor()
  export remainder = numerator -(ratio * divisor)
}

absolute-value = do {
  n: 0
  optionally {
    check n <? 0
    negate()
  }
}

test {
  1 polarity() =? 'positive'
}

test {
  -1 polarity() =? 'negative'
}

test {
  0 polarity() =? 'zero'
}

test {
  x = 7 integral-divide(3)
  check x =? 2
  check x~remainder =? 1
}