Add $pwd.

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)

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)
+}