chervil/spec/interpreter_spec.rb

module Chervil
  RSpec.describe Interpreter do
    def interpret(source, env)
      lexer = Lexer.new(source)
      tree = Parser.new(lexer).parse
      Interpreter.new(tree, env).interpret
    end

    it 'interprets a definition and variable usage' do
      env = Env.new
      interpret('(define x 5)', env)
      expect(env.get('x')).to eq(AST::Number.new(5.0))
      expect(interpret('x', env).first).to eq(5.0)
    end

    it 'interprets a recursive function' do
      env = Env.new
      interpret('(define (fact x) (if (= x 0) 1 (* x (fact (- x 1)))))', env)
      expect(interpret('(fact 5)', env).first).to eq(120.0)
    end

    it 'supports closures' do
      env = Env.new
      interpret('(define one 1)', env)
      interpret('(define (plus-one x) (+ x one))', env)
      expect(interpret('(plus-one 2)', env).first).to eq(3.0)
    end

    it "doesn't leave variables bound after function execution" do
      env = Env.new
      interpret('(define (plus-one x) (+ 1 x))', env)
      interpret('(plus-one 5)', env)
      expect(env.get('x')).to eq(nil)
    end

    it 'returns a type error for invalid operation' do
      expect(interpret('(+ 1 "hello")', Env.new).first).to eq(
        Error.new('Expected an argument of type number but got string')
      )
    end
  end
end