diff --git a/go.sum b/go.sum
index 84dc6eb2..2eea0ac2 100644
--- a/go.sum
+++ b/go.sum
@@ -96,6 +96,7 @@ golang.org/x/sys v0.0.0-20190813064441-fde4db37ae7a/go.mod h1:h1NjWce9XRLGQEsW7w
 golang.org/x/text v0.3.0 h1:g61tztE5qeGQ89tm6NTjjM9VPIm088od1l6aSorWRWg=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/tools v0.0.0-20190328211700-ab21143f2384/go.mod h1:LCzVGOaR6xXOjkQ3onu1FJEFr0SW1gC7cKk1uF8kGRs=
+golang.org/x/tools v0.0.0-20190425163242-31fd60d6bfdc h1:N3zlSgxkefUH/ecsl37RWTkESTB026kmXzNly8TuZCI=
 golang.org/x/tools v0.0.0-20190425163242-31fd60d6bfdc/go.mod h1:RgjU9mgBXZiqYHBnxXauZ1Gv1EHHAz9KjViQ78xBX0Q=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
diff --git a/pkg/compiler/compiler.go b/pkg/compiler/compiler.go
index 083b6e3c..a4915366 100644
--- a/pkg/compiler/compiler.go
+++ b/pkg/compiler/compiler.go
@@ -16,7 +16,7 @@ type Compiler struct {
 func New(setters ...Option) *Compiler {
 	c := &Compiler{}
 	c.NamespaceContainer = newRootNamespace()
-	c.funcs = make(map[string]core.Function)
+	c.funcs = make(core.Functions)
 
 	opts := &Options{}
 
diff --git a/pkg/compiler/namespace.go b/pkg/compiler/namespace.go
index bae47a38..85121532 100644
--- a/pkg/compiler/namespace.go
+++ b/pkg/compiler/namespace.go
@@ -1,10 +1,11 @@
 package compiler
 
 import (
-	"github.com/MontFerret/ferret/pkg/runtime/core"
-	"github.com/pkg/errors"
 	"regexp"
 	"strings"
+
+	"github.com/MontFerret/ferret/pkg/runtime/core"
+	"github.com/pkg/errors"
 )
 
 var fnNameValidation = regexp.MustCompile("^[a-zA-Z]+[a-zA-Z0-9_]*(::[a-zA-Z]+[a-zA-Z0-9_]*)*$")
@@ -34,7 +35,8 @@ func (nc *NamespaceContainer) Namespace(name string) core.Namespace {
 
 func (nc *NamespaceContainer) RegisterFunction(name string, fun core.Function) error {
 	nsName := nc.makeFullName(name)
-	_, exists := nc.funcs[nsName]
+
+	_, exists := nc.funcs.Get(nsName)
 
 	if exists {
 		return errors.Errorf("function already exists: %s", name)
@@ -49,13 +51,13 @@ func (nc *NamespaceContainer) RegisterFunction(name string, fun core.Function) e
 		return errors.Errorf("invalid function or namespace name: %s", nsName)
 	}
 
-	nc.funcs[strings.ToUpper(nsName)] = fun
+	nc.funcs.Set(nsName, fun)
 
 	return nil
 }
 
 func (nc *NamespaceContainer) RemoveFunction(name string) {
-	delete(nc.funcs, strings.ToUpper(nc.makeFullName(name)))
+	nc.funcs.Unset(nc.makeFullName(name))
 }
 
 func (nc *NamespaceContainer) RegisterFunctions(funcs core.Functions) error {
diff --git a/pkg/compiler/visitor.go b/pkg/compiler/visitor.go
index dedd6cad..4764aeef 100644
--- a/pkg/compiler/visitor.go
+++ b/pkg/compiler/visitor.go
@@ -24,11 +24,11 @@ type (
 	visitor struct {
 		*fql.BaseFqlParserVisitor
 		src   string
-		funcs map[string]core.Function
+		funcs core.Functions
 	}
 )
 
-func newVisitor(src string, funcs map[string]core.Function) *visitor {
+func newVisitor(src string, funcs core.Functions) *visitor {
 	return &visitor{
 		&fql.BaseFqlParserVisitor{},
 		src,
@@ -1104,7 +1104,7 @@ func (v *visitor) doVisitFunctionCallExpression(context *fql.FunctionCallExpress
 
 	name += context.Identifier().GetText()
 
-	fun, exists := v.funcs[name]
+	fun, exists := v.funcs.Get(name)
 
 	if !exists {
 		return nil, core.Error(core.ErrNotFound, fmt.Sprintf("function: '%s'", name))
diff --git a/pkg/runtime/core/function.go b/pkg/runtime/core/function.go
index 6c67a6ff..aa609332 100644
--- a/pkg/runtime/core/function.go
+++ b/pkg/runtime/core/function.go
@@ -3,15 +3,12 @@ package core
 import (
 	"context"
 	"fmt"
+	"strings"
 )
 
 const MaxArgs = 65536
 
 type (
-	Function = func(ctx context.Context, args ...Value) (Value, error)
-
-	Functions map[string]Function
-
 	Namespace interface {
 		Namespace(name string) Namespace
 		RegisterFunction(name string, fun Function) error
@@ -36,3 +33,29 @@ func ValidateArgs(args []Value, minimum, maximum int) error {
 
 	return nil
 }
+
+type (
+	// Functions is a map of functions and their names.
+	Functions map[string]Function
+
+	// Function is a common interface for all functions of FQL.
+	Function = func(ctx context.Context, args ...Value) (Value, error)
+)
+
+// Get returns the function with the given name. If the function
+// does not exist it returns nil, false.
+func (fns Functions) Get(name string) (Function, bool) {
+	fn, exists := fns[strings.ToUpper(name)]
+	return fn, exists
+}
+
+// Set sets the function with the given name. If the function
+// with the such name already exists it will be overwritten.
+func (fns Functions) Set(name string, fn Function) {
+	fns[strings.ToUpper(name)] = fn
+}
+
+// Unset delete the function with the given name.
+func (fns Functions) Unset(name string) {
+	delete(fns, strings.ToUpper(name))
+}
diff --git a/pkg/stdlib/datetime/lib.go b/pkg/stdlib/datetime/lib.go
index 3daa77b2..55d7436c 100644
--- a/pkg/stdlib/datetime/lib.go
+++ b/pkg/stdlib/datetime/lib.go
@@ -2,8 +2,8 @@ package datetime
 
 import "github.com/MontFerret/ferret/pkg/runtime/core"
 
-func NewLib() map[string]core.Function {
-	return map[string]core.Function{
+func NewLib() core.Functions {
+	return core.Functions{
 		"NOW":                Now,
 		"DATE":               Date,
 		"DATE_DAYOFWEEK":     DateDayOfWeek,