diff --git a/context.go b/context.go
index d01c7d1..37fd1b9 100644
--- a/context.go
+++ b/context.go
@@ -4,6 +4,8 @@
 // context.go
 package expr
 
+type FuncTemplate func(ctx exprContext, name string, args []any) (result any, err error)
+
 type exprFunc interface {
 	Name() string
 	MinArgs() int
@@ -15,4 +17,5 @@ type exprContext interface {
 	SetValue(varName string, value any)
 	GetFuncInfo(name string) exprFunc
 	Call(name string, args []any) (result any, err error)
+	RegisterFunc(name string, f FuncTemplate, minArgs, maxArgs int)
 }
diff --git a/helpers.go b/helpers.go
index f62e5f7..d524e2c 100644
--- a/helpers.go
+++ b/helpers.go
@@ -33,7 +33,7 @@ func EvalStringV(source string, args []EvalArg) (result any, err error) {
 	for _, arg := range args {
 		if isFunc(arg.value) {
 			if f, ok := arg.value.(FuncTemplate); ok {
-				ctx.addFunc(arg.name, f)
+				ctx.RegisterFunc(arg.name, f, 0, -1)
 			} else {
 				err = fmt.Errorf("invalid function specification: %q", arg.name)
 			}
diff --git a/simple-func-store.go b/simple-func-store.go
index 90b7593..cfd1d6e 100644
--- a/simple-func-store.go
+++ b/simple-func-store.go
@@ -3,8 +3,6 @@ package expr
 
 import "fmt"
 
-type FuncTemplate func(ctx exprContext, name string, args []any) (result any, err error)
-
 type SimpleFuncStore struct {
 	varStore  map[string]any
 	funcStore map[string]FuncTemplate
@@ -52,7 +50,7 @@ func (ctx *SimpleFuncStore) GetFuncInfo(name string) (f exprFunc) {
 	return
 }
 
-func (ctx *SimpleFuncStore) addFunc(name string, f FuncTemplate) {
+func (ctx *SimpleFuncStore) RegisterFunc(name string, f FuncTemplate, minArgs, maxArgs int) {
 	ctx.funcStore[name] = f
 }
 
diff --git a/simple-var-store.go b/simple-var-store.go
index 8f4e5dc..ae89a36 100644
--- a/simple-var-store.go
+++ b/simple-var-store.go
@@ -27,3 +27,6 @@ func (ctx *SimpleVarStore) GetFuncInfo(name string) (f exprFunc) {
 func (ctx *SimpleVarStore) Call(name string, args []any) (result any, err error) {
 	return
 }
+
+func (ctx *SimpleVarStore) RegisterFunc(name string, f FuncTemplate, minArgs, maxArgs int) {
+}