server.rb

require 'sinatra'

# Ex. http://localhost:4567/calc/add?x=1&y=2
get '/calc/:operation' do
  operations = { 'add' => ->(x, y) {x + y},
                 'sub' => ->(x, y) {x - y},
                 'mul' => ->(x, y) {x * y},
                 'div' => ->(x, y) {x / y},
                 'mod' => ->(x, y) {x % y} }
  # Yes I could DRY that further but I don't trust user input
  
  operation = params['operation']
  x, y = params['x'].to_f, params['y'].to_f
  result = operations[operation].call(x, y)
  result.to_s
end

MIT License