good good study, day day up!

vendor/gopl.io/ch7/eval/ast.go

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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
+}
+
+//!+ast
+
+// 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
+}
+
+//!-ast