expression.hpp

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#ifndef EXPRESSION_HPP
#define EXPRESSION_HPP

#include <string>
#include <vector>

#include "token.hpp"
#include "atom.hpp"

// forward declare Environment
class Environment;

class Expression {
public:

  typedef std::vector<Expression>::const_iterator ConstIteratorType;

  Expression();

  Expression(const Atom & a);

  Expression(const Expression & a);

  Expression & operator=(const Expression & a);

  Atom & head();

  const Atom & head() const;

  void append(const Atom & a);

  Expression * tail();

  ConstIteratorType tailConstBegin() const noexcept;

  ConstIteratorType tailConstEnd() const noexcept;

  bool isHeadNumber() const noexcept;

  bool isHeadSymbol() const noexcept;

  Expression eval(Environment & env);

  bool operator==(const Expression & exp) const noexcept;
  
private:

  // the head of the expression
  Atom m_head;

  // the tail list is expressed as a vector for access efficiency
  // and cache coherence, at the cost of wasted memory.
  std::vector<Expression> m_tail;

  // convenience typedef
  typedef std::vector<Expression>::iterator IteratorType;
  
  // internal helper methods
  Expression handle_lookup(const Atom & head, const Environment & env);
  Expression handle_define(Environment & env);
  Expression handle_begin(Environment & env);
};

std::ostream & operator<<(std::ostream & out, const Expression & exp);

bool operator!=(const Expression & left, const Expression & right) noexcept;
  
#endif