lang: Initial implementation of the mgmt language

This is an initial implementation of the mgmt language. It is a
declarative (immutable) functional, reactive, domain specific
programming language. It is intended to be a language that is:

* safe
* powerful
* easy to reason about

With these properties, we hope this language, and the mgmt engine will
allow you to model the real-time systems that you'd like to automate.

This also includes a number of other associated changes. Sorry for the
large size of this patch.

lang/funcs/structs/var.go

@@ -0,0 +1,141 @@
+// Mgmt
+// Copyright (C) 2013-2018+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package structs
+
+import (
+	"fmt"
+
+	"github.com/purpleidea/mgmt/lang/interfaces"
+	"github.com/purpleidea/mgmt/lang/types"
+
+	errwrap "github.com/pkg/errors"
+)
+
+// VarFunc is a function that passes through a function that came from a bind
+// lookup. It exists so that the reactive function engine type checks correctly.
+type VarFunc struct {
+	Type *types.Type // this is the type of the var's value that we hold
+	Func interfaces.Func
+	Edge string // name of the edge used
+
+	init   *interfaces.Init
+	last   types.Value // last value received to use for diff
+	result types.Value // last calculated output
+
+	closeChan chan struct{}
+}
+
+// Validate makes sure we've built our struct properly.
+func (obj *VarFunc) Validate() error {
+	if obj.Type == nil {
+		return fmt.Errorf("must specify a type")
+	}
+	if obj.Func == nil {
+		return fmt.Errorf("must specify a func")
+	}
+	if obj.Edge == "" {
+		return fmt.Errorf("must specify an edge name")
+	}
+
+	// we're supposed to call Validate() before we ever call Info()
+	if err := obj.Func.Validate(); err != nil {
+		return errwrap.Wrapf(err, "func did not validate")
+	}
+
+	typ := obj.Func.Info().Sig
+	if err := obj.Type.Cmp(typ.Out); err != nil {
+		return errwrap.Wrapf(err, "expr type must match func out type")
+	}
+	return nil
+}
+
+// Info returns some static info about itself.
+func (obj *VarFunc) Info() *interfaces.Info {
+	typ := &types.Type{
+		Kind: types.KindFunc, // function type
+		Map:  map[string]*types.Type{obj.Edge: obj.Type},
+		Ord:  []string{obj.Edge},
+		Out:  obj.Type, // this is the output type for the expression
+	}
+
+	return &interfaces.Info{
+		Pure: true,
+		Memo: false, // TODO: ???
+		Sig:  typ,
+		Err:  obj.Validate(),
+	}
+}
+
+// Init runs some startup code for this composite function.
+func (obj *VarFunc) Init(init *interfaces.Init) error {
+	obj.init = init
+	obj.closeChan = make(chan struct{})
+	return nil
+}
+
+// Stream takes an input struct in the format as described in the Func and Graph
+// methods of the Expr, and returns the actual expected value as a stream based
+// on the changing inputs to that value.
+func (obj *VarFunc) Stream() error {
+	defer close(obj.init.Output) // the sender closes
+	for {
+		select {
+		case input, ok := <-obj.init.Input:
+			if !ok {
+				return nil // can't output any more
+			}
+			//if err := input.Type().Cmp(obj.Info().Sig.Input); err != nil {
+			//	return errwrap.Wrapf(err, "wrong function input")
+			//}
+			if obj.last != nil && input.Cmp(obj.last) == nil {
+				continue // value didn't change, skip it
+			}
+			obj.last = input // store for next
+
+			var result types.Value
+			st := input.(*types.StructValue) // must be!
+			value, exists := st.Lookup(obj.Edge)
+			if !exists {
+				return fmt.Errorf("missing expected input argument `%s`", obj.Edge)
+			}
+			result = value
+
+			// skip sending an update...
+			if obj.result != nil && result.Cmp(obj.result) == nil {
+				continue // result didn't change
+			}
+			obj.result = result // store new result
+
+		case <-obj.closeChan:
+			return nil
+		}
+
+		select {
+		case obj.init.Output <- obj.result: // send
+			// pass
+		case <-obj.closeChan:
+			return nil
+		}
+	}
+}
+
+// Close runs some shutdown code for this function and turns off the stream.
+func (obj *VarFunc) Close() error {
+	close(obj.closeChan)
+	return nil
+}