Refactor checker to use Nature (#665)

This PR introduces a new concept inside Expr's type checker - Nature.

It's an abstraction other reflect.Type which carries additional information about possible type, methods and builtin function.

I was able to find and fix a few bugs in type checker and optimizer. Implementation of nature concept has proven itself. As well nature will allow to bring more feature for type checker in future.

Why Nature? I was looking for a name to replace Type. Type name already carries much information, as well can be confused with reflect.Type. Nature name should be understood as "nature of the node", same as "type of the node".

Author: antonmedv

ast/node.go

@@ -3,13 +3,21 @@ package ast
 import (
 	"reflect"
 
+	"github.com/expr-lang/expr/checker/nature"
 	"github.com/expr-lang/expr/file"
 )
 
+var (
+	anyType = reflect.TypeOf(new(any)).Elem()
+)
+
 // Node represents items of abstract syntax tree.
 type Node interface {
 	Location() file.Location
 	SetLocation(file.Location)
+	Nature() nature.Nature
+	SetNature(nature.Nature)
+	Kind() reflect.Kind
 	Type() reflect.Type
 	SetType(reflect.Type)
 	String() string
@@ -25,8 +33,8 @@ func Patch(node *Node, newNode Node) {
 
 // base is a base struct for all nodes.
 type base struct {
-	loc      file.Location
-	nodeType reflect.Type
+	loc    file.Location
+	nature nature.Nature
 }
 
 // Location returns the location of the node in the source code.
@@ -39,14 +47,36 @@ func (n *base) SetLocation(loc file.Location) {
 	n.loc = loc
 }
 
+// Nature returns the nature of the node.
+func (n *base) Nature() nature.Nature {
+	return n.nature
+}
+
+// SetNature sets the nature of the node.
+func (n *base) SetNature(nature nature.Nature) {
+	n.nature = nature
+}
+
+// Kind returns the kind of the node.
+// If the type is nil (meaning unknown) then it returns reflect.Interface.
+func (n *base) Kind() reflect.Kind {
+	if n.nature.Type == nil {
+		return reflect.Interface
+	}
+	return n.nature.Type.Kind()
+}
+
 // Type returns the type of the node.
 func (n *base) Type() reflect.Type {
-	return n.nodeType
+	if n.nature.Type == nil {
+		return anyType
+	}
+	return n.nature.Type
 }
 
 // SetType sets the type of the node.
 func (n *base) SetType(t reflect.Type) {
-	n.nodeType = t
+	n.nature.Type = t
 }
 
 // NilNode represents nil.

builtin/builtin_test.go

@@ -235,7 +235,7 @@ func TestBuiltin_errors(t *testing.T) {
 		{`bitushr(-5, -2)`, "invalid operation: negative shift count -2 (type int) (1:1)"},
 		{`now(nil)`, "invalid number of arguments (expected 0, got 1)"},
 		{`date(nil)`, "interface {} is nil, not string (1:1)"},
-		{`timezone(nil)`, "interface {} is nil, not string (1:1)"},
+		{`timezone(nil)`, "cannot use nil as argument (type string) to call timezone (1:10)"},
 	}
 	for _, test := range errorTests {
 		t.Run(test.input, func(t *testing.T) {