Update to gobook@c6d7a22edd03e7738c38d5baa2174ff325d8d863

ch7/eval/eval.go

@@ -0,0 +1,70 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 198.
+
+// Package eval provides an expression evaluator.
+package eval
+
+import (
+	"fmt"
+	"math"
+)
+
+//!+env
+
+type Env map[Var]float64
+
+//!-env
+
+//!+Eval1
+
+func (v Var) Eval(env Env) float64 {
+	return env[v]
+}
+
+func (l literal) Eval(_ Env) float64 {
+	return float64(l)
+}
+
+//!-Eval1
+
+//!+Eval2
+
+func (u unary) Eval(env Env) float64 {
+	switch u.op {
+	case '+':
+		return +u.x.Eval(env)
+	case '-':
+		return -u.x.Eval(env)
+	}
+	panic(fmt.Sprintf("unsupported unary operator: %q", u.op))
+}
+
+func (b binary) Eval(env Env) float64 {
+	switch b.op {
+	case '+':
+		return b.x.Eval(env) + b.y.Eval(env)
+	case '-':
+		return b.x.Eval(env) - b.y.Eval(env)
+	case '*':
+		return b.x.Eval(env) * b.y.Eval(env)
+	case '/':
+		return b.x.Eval(env) / b.y.Eval(env)
+	}
+	panic(fmt.Sprintf("unsupported binary operator: %q", b.op))
+}
+
+func (c call) Eval(env Env) float64 {
+	switch c.fn {
+	case "pow":
+		return math.Pow(c.args[0].Eval(env), c.args[1].Eval(env))
+	case "sin":
+		return math.Sin(c.args[0].Eval(env))
+	case "sqrt":
+		return math.Sqrt(c.args[0].Eval(env))
+	}
+	panic(fmt.Sprintf("unsupported function call: %s", c.fn))
+}
+
+//!-Eval2