lang: Add function values and lambdas

This adds a giant missing piece of the language: proper function values!
It is lovely to now understand why early programming language designers
didn't implement these, but a joy to now reap the benefits of them. In
adding these, many other changes had to be made to get them to "fit"
correctly. This improved the code and fixed a number of bugs.
Unfortunately this touched many areas of the code, and since I was
learning how to do all of this for the first time, I've squashed most of
my work into a single commit. Some more information:

* This adds over 70 new tests to verify the new functionality.

* Functions, global variables, and classes can all be implemented
natively in mcl and built into core packages.

* A new compiler step called "Ordering" was added. It is called by the
SetScope step, and determines statement ordering and shadowing
precedence formally. It helped remove at least one bug and provided the
additional analysis required to properly capture variables when
implementing function generators and closures.

* The type unification code was improved to handle the new cases.

* Light copying of Node's allowed our function graphs to be more optimal
and share common vertices and edges. For example, if two different
closures capture a variable $x, they'll both use the same copy when
running the function, since the compiler can prove if they're identical.

* Some areas still need improvements, but this is ready for mainstream
testing and use!

lang/funcs/simple/simple.go

@@ -26,6 +26,13 @@ import (
 	"github.com/purpleidea/mgmt/util/errwrap"
 )
 
+const (
+	// DirectInterface specifies whether we should use the direct function
+	// API or not. If we don't use it, then these simple functions are
+	// wrapped with the struct below.
+	DirectInterface = false // XXX: fix any bugs and set to true!
+)
+
 // RegisteredFuncs maps a function name to the corresponding static, pure func.
 var RegisteredFuncs = make(map[string]*types.FuncValue) // must initialize
 
@@ -38,7 +45,7 @@ func Register(name string, fn *types.FuncValue) {
 	RegisteredFuncs[name] = fn // store a copy for ourselves
 
 	// register a copy in the main function database
-	funcs.Register(name, func() interfaces.Func { return &simpleFunc{Fn: fn} })
+	funcs.Register(name, func() interfaces.Func { return &WrappedFunc{Fn: fn} })
 }
 
 // ModuleRegister is exactly like Register, except that it registers within a
@@ -47,9 +54,9 @@ func ModuleRegister(module, name string, fn *types.FuncValue) {
 	Register(module+funcs.ModuleSep+name, fn)
 }
 
-// simpleFunc is a scaffolding function struct which fulfills the boiler-plate
+// WrappedFunc is a scaffolding function struct which fulfills the boiler-plate
 // for the function API, but that can run a very simple, static, pure function.
-type simpleFunc struct {
+type WrappedFunc struct {
 	Fn *types.FuncValue
 
 	init *interfaces.Init
@@ -60,9 +67,22 @@ type simpleFunc struct {
 	closeChan chan struct{}
 }
 
+// ArgGen returns the Nth arg name for this function.
+func (obj *WrappedFunc) ArgGen(index int) (string, error) {
+	typ := obj.Fn.Type()
+	if typ.Kind != types.KindFunc {
+		return "", fmt.Errorf("expected %s, got %s", types.KindFunc, typ.Kind)
+	}
+	seq := typ.Ord
+	if l := len(seq); index >= l {
+		return "", fmt.Errorf("index %d exceeds arg length of %d", index, l)
+	}
+	return seq[index], nil
+}
+
 // Validate makes sure we've built our struct properly. It is usually unused for
 // normal functions that users can use directly.
-func (obj *simpleFunc) Validate() error {
+func (obj *WrappedFunc) Validate() error {
 	if obj.Fn == nil { // build must be run first
 		return fmt.Errorf("type is still unspecified")
 	}
@@ -70,7 +90,7 @@ func (obj *simpleFunc) Validate() error {
 }
 
 // Info returns some static info about itself.
-func (obj *simpleFunc) Info() *interfaces.Info {
+func (obj *WrappedFunc) Info() *interfaces.Info {
 	return &interfaces.Info{
 		Pure: true,
 		Memo: false, // TODO: should this be something we specify here?
@@ -80,14 +100,14 @@ func (obj *simpleFunc) Info() *interfaces.Info {
 }
 
 // Init runs some startup code for this function.
-func (obj *simpleFunc) Init(init *interfaces.Init) error {
+func (obj *WrappedFunc) Init(init *interfaces.Init) error {
 	obj.init = init
 	obj.closeChan = make(chan struct{})
 	return nil
 }
 
 // Stream returns the changing values that this func has over time.
-func (obj *simpleFunc) Stream() error {
+func (obj *WrappedFunc) Stream() error {
 	defer close(obj.init.Output) // the sender closes
 	for {
 		select {
@@ -141,7 +161,7 @@ func (obj *simpleFunc) Stream() error {
 }
 
 // Close runs some shutdown code for this function and turns off the stream.
-func (obj *simpleFunc) Close() error {
+func (obj *WrappedFunc) Close() error {
 	close(obj.closeChan)
 	return nil
 }