From 160e1456d36951b1dabdda0a5d5e7b7255f142d9 Mon Sep 17 00:00:00 2001
From: Qi Xiao <xiaqqaix@gmail.com>
Date: Sun, 21 Feb 2016 12:58:20 +0100
Subject: [PATCH] Add $pwd.

---
 eval/eval.go |  1 +
 eval/pwd.go  | 24 ++++++++++++++++++++++++
 2 files changed, 25 insertions(+)
 create mode 100644 eval/pwd.go

diff --git a/eval/eval.go b/eval/eval.go
index 4e8c0153..03603b83 100644
--- a/eval/eval.go
+++ b/eval/eval.go
@@ -60,6 +60,7 @@ func NewEvaler(st *store.Store) *Evaler {
 		"true":  NewRoVariable(Bool(true)),
 		"false": NewRoVariable(Bool(false)),
 		"paths": &EnvPathList{envName: "PATH"},
+		"pwd":   PwdVariable{},
 	}
 	for _, b := range builtinFns {
 		ev.global[FnPrefix+b.Name] = NewRoVariable(b)
diff --git a/eval/pwd.go b/eval/pwd.go
new file mode 100644
index 00000000..5ff03e6a
--- /dev/null
+++ b/eval/pwd.go
@@ -0,0 +1,24 @@
+package eval
+
+import "os"
+
+// PwdVariable is a variable whose value always reflects the current working
+// directory. Setting it changes the current working directory.
+type PwdVariable struct{}
+
+var _ Variable = PwdVariable{}
+
+func (PwdVariable) Get() Value {
+	pwd, err := os.Getwd()
+	maybeThrow(err)
+	return String(pwd)
+}
+
+func (PwdVariable) Set(v Value) {
+	path, ok := v.(String)
+	if !ok {
+		throw(ErrPathMustBeString)
+	}
+	err := os.Chdir(string(path))
+	maybeThrow(err)
+}