eval

README

Exercise 7.13 (P205)

Add a String method to Expr to pretty-print the syntax tree. Check that the results, when parsed again, yield an equivalent tree.

Documentation

Overview

Package eval provides an expression evaluator.

Index

• func Format(e Expr) string
• type Env
• type Expr
  • func Parse(input string) (_ Expr, err error)
• type Var
  • func (v Var) Check(vars map[Var]bool) error
  • func (v Var) Eval(env Env) float64
  • func (v Var) String() string

Functions

func Format

func Format(e Expr) string

Format formats an expression as a string. It does not attempt to remove unnecessary parens.

Types

type Env

type Env map[Var]float64

type Expr

type Expr interface {
	// Eval returns the value of this Expr in the environment env.
	Eval(env Env) float64
	// Check reports errors in this Expr and adds its Vars to the set.
	Check(vars map[Var]bool) error
	// String returns a human readable string of the Expr
	String() string
}

An Expr is an arithmetic expression.

func Parse

func Parse(input string) (_ Expr, err error)

Parse parses the input string as an arithmetic expression.

expr = num                         a literal number, e.g., 3.14159
     | id                          a variable name, e.g., x
     | id '(' expr ',' ... ')'     a function call
     | '-' expr                    a unary operator (+-)
     | expr '+' expr               a binary operator (+-*/)

type Var

type Var string

A Var identifies a variable, e.g., x.

func (Var) Check

func (v Var) Check(vars map[Var]bool) error

func (Var) Eval

func (v Var) Eval(env Env) float64

func (Var) String

func (v Var) String() string