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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/

package eval

// An Expr is an arithmetic expression.
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


// A Var identifies a variable, e.g., x.
type Var string

// A literal is a numeric constant, e.g., 3.141.
type literal float64

// A unary represents a unary operator expression, e.g., -x.
type unary struct {
    op rune // one of '+', '-'
    x  Expr

// A binary represents a binary operator expression, e.g., x+y.
type binary struct {
    op   rune // one of '+', '-', '*', '/'
    x, y Expr

// A call represents a function call expression, e.g., sin(x).
type call struct {
    fn   string // one of "pow", "sin", "sqrt"
    args []Expr