feat(machine): pretty-print Fn and Cmd

Author: sharpvik

machine/cmd.go

@@ -1,5 +1,7 @@
 package machine
 
+import "fmt"
+
 // Cmd (a.k.a. "Command") is a
 //
 //  1. sequence of executable instructions
@@ -61,3 +63,7 @@ func (cmd Cmd) call() Object {
 
 	return cmd.stack.Pop()
 }
+
+func (cmd Cmd) String() string {
+	return fmt.Sprintf("Cmd<%d>(%d/%d)", cmd.ip, len(cmd.args), cmd.argc)
+}

machine/fn.go

@@ -1,5 +1,7 @@
 package machine
 
+import "fmt"
+
 // Fn (a.k.a "Builtin Function") is a special type that implement the Function
 // interface. It is used to carry stdlib functions.
 type Fn struct {
@@ -28,3 +30,7 @@ func (fn Fn) Feed(arg Object) Object {
 func (fn Fn) call() Object {
 	return fn.eval(fn.args)
 }
+
+func (fn Fn) String() string {
+	return fmt.Sprintf("Fn(%d/%d)", len(fn.args), fn.argc)
+}