iconara / rechner

This is a toy calculator

Rechner

This is a toy calculator.

Usage

It can calculate simple arithmetic expressions:

require 'rechner'

Rechner.calculate('1 + 2 + 3') # => 6
Rechner.calculate('2 * (7 - 3)') # => 8
Rechner.calculate('8 / 2 - 4') # => 0

It handles references and lets you bind values to them:

require 'rechner'

Rechner.calculate('a + b + 3', a: 1, b: 2) # => 6
Rechner.calculate('a * (b - 3)', a: 2, b: 7) # => 8
Rechner.calculate('a / b - 4', a: 8, b: 2) # => 0

It can prepare an expression for reuse:

require 'rechner'

expression = Rechner.prepare('a + b + 3')
expression.calculate(a: 1, b: 2) # => 6
expression.calculate(a: 2, b: 3) # => 8

It can tell you what references an expression contains:

require 'rechner'

expression = Rechner.prepare('a + b + 3')
expression.references # => [:a, :b]

You can even write your own interpreter for the expressions:

require 'rechner'

class Lispify
  def visit_constant(constant_expression)
    constant_expression.value
  end

  def visit_reference(reference_expression)
    reference_expression.name
  end

  def visit_operator(operator_expression)
    format('(%s %s %s)', operator_expression.operator, operator_expression.left, operator_expression.right)
  end
end

expression = Rechner.parse('a + b + 3')
expression.accept(Lispify.new) # => "(+ a (b + 3))"

Finally, if your Ruby supports named arguments you can compile the expression to native code:

require 'rechner'

expression = Rechner.compile('a + b + 3')
expression.calculate(a: 1, b: 2) # => 6

What happens behind the scenes here is that the expression is rendered as a Ruby expression and compiled to a Proc using Kernel.eval. The Proc has named arguments matching the references in the expression, with default value nil. If you leave out a binding you will get TypeError since the reference will be nil, and if you specify an extra binding you will get ArgumentError since there is no such named argument.

Known issues & limitations

This is a toy calculator. It doesn't have proper error handling. Don't use it.

License & Copyright

© 2017 Theo Hultberg, see LICENSE.txt (BSD 3-Clause).