eval

README

Exercise 7.14 (P205)

Define a new concrete type that satisfies the Expr interface and provides a new operation such as computing the minimum value of its operands. Since the Parse function does not create instances of this new type, to use it you will need to construct a syntax tree directly (or extend the parser).

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