implement cd() only for issue #144

docs/types/builtin-function.md

@@ -116,10 +116,35 @@ type("") # "STRING"
 type({}) # "HASH"
 ```
 
+### cd() or cd(path)
+
+Sets the current working directory to `$HOME` or the given path.
+
+Note that the path may have a `"~/"` prefix which will be replaced
+with `$HOME/`. 
+
+The path to the current working directory can be seen with `pwd()`
+
+``` bash
+cd()
+pwd()   # /home/user
+
+cd("~/git/abs")
+pwd()   # /home/user/git/abs
+
+cd("/usr/local/bin")
+pwd()   # /usr/local/bin
+```
+
 ### pwd()
 
-Returns the working directory where the script was started for -- equivalent
-to `env("PWD")`:
+Returns the path to the current working directory -- equivalent
+to `env("PWD")`. 
+
+If executed from a script this will initially be the directory
+containing the script.
+
+To change the working directory, see `cd()`.
 
 ``` bash
 pwd() # /go/src/github.com/abs-lang/abs

evaluator/evaluator_test.go

@@ -657,6 +657,7 @@ func TestBuiltinFunctions(t *testing.T) {
 		{`arg("o")`, "argument 0 to arg(...) is not supported (got: o, allowed: NUMBER)"},
 		{`arg(3)`, ""},
 		{`pwd().split("").reverse().slice(0, 33).reverse().join("").replace("\\", "/", -1).suffix("/evaluator")`, true}, // Little trick to get travis to run this test, as the base path is not /go/src/
+		{`cd("/usr/bin"); pwd()`, "/usr/bin"},
 		{`rand(1)`, 0},
 		{`int(10)`, 10},
 		{`int(10.5)`, 10},

evaluator/functions.go

@@ -57,6 +57,11 @@ func getFns() map[string]*object.Builtin {
 			Types: []string{},
 			Fn:    pwdFn,
 		},
+		// cd(path)
+		"cd": &object.Builtin{
+			Types: []string{},
+			Fn:    cdFn,
+		},
 		// echo(arg:"hello")
 		"echo": &object.Builtin{
 			Types: []string{},
@@ -411,6 +416,28 @@ func pwdFn(args ...object.Object) object.Object {
 	return &object.String{Token: tok, Value: dir}
 }
 
+// cd(path)
+func cdFn(args ...object.Object) object.Object {
+	// Default cd $HOME
+	path := os.Getenv("HOME")
+	if len(args) == 1 {
+		// arg: path
+		pathStr := args[0].(*object.String)
+		rawPath := pathStr.Value
+		// expand ~/ path prefix
+		if strings.HasPrefix(rawPath, "~/") {
+			path = strings.Replace(rawPath, "~/", path+"/", 1)
+		} else if len(rawPath) > 0 {
+			path = rawPath
+		}
+	}
+	error := os.Chdir(path)
+	if error != nil {
+		return newError(tok, error.Error())
+	}
+	return NULL
+}
+
 // echo(arg:"hello")
 func echoFn(args ...object.Object) object.Object {
 	if len(args) == 0 {