lang: Add modern type unification implementation

This adds a modern type unification algorithm, which drastically
improves performance, particularly for bigger programs.

This required a change to the AST to add TypeCheck methods (for Stmt)
and Infer/Check methods (for Expr). This also changed how the functions
express their invariants, and as a result this was changed as well.

This greatly improves the way we express these invariants, and as a
result it makes adding new polymorphic functions significantly easier.

This also makes error output for the user a lot better in pretty much
all scenarios.

The one downside of this patch is that a good chunk of it is merged in
this giant single commit since it was hard to do it step-wise. That's
not the end of the world.

This couldn't be done without the guidance of Sam who helped me in
explaining, debugging, and writing all the sneaky algorithmic parts and
much more. Thanks again Sam!

Co-authored-by: Samuel Gélineau <gelisam@gmail.com>

lang/funcs/wrapped/wrapped.go

@@ -0,0 +1,182 @@
+// Mgmt
+// Copyright (C) 2013-2024+ 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 <https://www.gnu.org/licenses/>.
+//
+// Additional permission under GNU GPL version 3 section 7
+//
+// If you modify this program, or any covered work, by linking or combining it
+// with embedded mcl code and modules (and that the embedded mcl code and
+// modules which link with this program, contain a copy of their source code in
+// the authoritative form) containing parts covered by the terms of any other
+// license, the licensors of this program grant you additional permission to
+// convey the resulting work. Furthermore, the licensors of this program grant
+// the original author, James Shubin, additional permission to update this
+// additional permission if he deems it necessary to achieve the goals of this
+// additional permission.
+
+package wrapped
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/purpleidea/mgmt/lang/interfaces"
+	"github.com/purpleidea/mgmt/lang/types"
+	"github.com/purpleidea/mgmt/util/errwrap"
+)
+
+var _ interfaces.Func = &Func{} // ensure it meets this expectation
+
+// Func is a wrapped scaffolding function struct which fulfills the boiler-plate
+// for the function API, but that can run a very simple, static, pure, function.
+// It can be wrapped by other structs that support polymorphism in various ways.
+type Func struct {
+	// Name is a unique string name for the function.
+	Name string
+
+	// Type is the type of the function. It can include unification
+	// variables when this struct is wrapped in one that can build this out.
+	Type *types.Type
+
+	// Fn is the concrete version of our chosen function.
+	Fn *types.FuncValue
+
+	init *interfaces.Init
+	last types.Value // last value received to use for diff
+
+	result types.Value // last calculated output
+}
+
+// String returns a simple name for this function. This is needed so this struct
+// can satisfy the pgraph.Vertex interface.
+func (obj *Func) String() string {
+	return fmt.Sprintf("%s @ %p", obj.Name, obj) // be more unique!
+}
+
+// ArgGen returns the Nth arg name for this function.
+func (obj *Func) ArgGen(index int) (string, error) {
+	// If the user specified just a ?1 here, then this might panic if we
+	// wanted to determine the arg length at compile time.
+	seq := obj.Type.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 *Func) Validate() error {
+	if obj.Fn == nil { // build must be run first
+		return fmt.Errorf("func has not been built")
+	}
+	if obj.Fn.T == nil {
+		return fmt.Errorf("func type must not be nil")
+	}
+	if obj.Fn.T.Kind != types.KindFunc {
+		return fmt.Errorf("func must be a kind of func")
+	}
+	return nil
+}
+
+// Info returns some static info about itself.
+func (obj *Func) Info() *interfaces.Info {
+	var typ *types.Type
+	// For speculation we still need to return a type with unification vars.
+	if obj.Type != nil { // && !obj.Type.HasUni() // always return something
+		typ = obj.Type
+	}
+	if obj.Fn != nil { // don't panic if called speculatively
+		typ = obj.Fn.Type()
+	}
+
+	return &interfaces.Info{
+		Pure: true,
+		Memo: false, // TODO: should this be something we specify here?
+		Sig:  typ,
+		Err:  obj.Validate(),
+	}
+}
+
+// Init runs some startup code for this function.
+func (obj *Func) Init(init *interfaces.Init) error {
+	obj.init = init
+	return nil
+}
+
+// Stream returns the changing values that this func has over time.
+func (obj *Func) Stream(ctx context.Context) error {
+	defer close(obj.init.Output) // the sender closes
+	for {
+		select {
+		case input, ok := <-obj.init.Input:
+			if !ok {
+				if len(obj.Fn.Type().Ord) > 0 {
+					return nil // can't output any more
+				}
+				// no inputs were expected, pass through once
+			}
+			if ok {
+				//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
+			}
+
+			values := []types.Value{}
+			for _, name := range obj.Fn.Type().Ord {
+				x := input.Struct()[name]
+				values = append(values, x)
+			}
+
+			if obj.init.Debug {
+				obj.init.Logf("Calling function with: %+v", values)
+			}
+			result, err := obj.Fn.Call(ctx, values) // (Value, error)
+			if err != nil {
+				if obj.init.Debug {
+					obj.init.Logf("Function returned error: %+v", err)
+				}
+				return errwrap.Wrapf(err, "wrapped function errored")
+			}
+			if obj.init.Debug {
+				obj.init.Logf("Function returned with: %+v", result)
+			}
+
+			// TODO: do we want obj.result to be a pointer instead?
+			if obj.result == result {
+				continue // result didn't change
+			}
+			obj.result = result // store new result
+
+		case <-ctx.Done():
+			return nil
+		}
+
+		select {
+		case obj.init.Output <- obj.result: // send
+			if len(obj.Fn.Type().Ord) == 0 {
+				return nil // no more values, we're a pure func
+			}
+		case <-ctx.Done():
+			return nil
+		}
+	}
+}