diff --git a/checker/checker.go b/checker/checker.go
index d36d0aad7..564086647 100644
--- a/checker/checker.go
+++ b/checker/checker.go
@@ -51,7 +51,7 @@ okay:
 }
 
 type visitor struct {
-	types       conf.TypesTable
+	types       conf.TypeFinder
 	operators   conf.OperatorsTable
 	expect      reflect.Kind
 	collections []reflect.Type
@@ -124,7 +124,7 @@ func (v *visitor) IdentifierNode(node *ast.IdentifierNode) reflect.Type {
 	if v.types == nil {
 		return interfaceType
 	}
-	if t, ok := v.types[node.Value]; ok {
+	if t, ok := v.types.LookupType(node.Value); ok {
 		return t.Type
 	}
 	panic(v.error(node, "unknown name %v", node.Value))
@@ -312,7 +312,7 @@ func (v *visitor) SliceNode(node *ast.SliceNode) reflect.Type {
 }
 
 func (v *visitor) FunctionNode(node *ast.FunctionNode) reflect.Type {
-	if f, ok := v.types[node.Name]; ok {
+	if f, ok := v.types.LookupType(node.Name); ok {
 		if fn, ok := isFuncType(f.Type); ok {
 			if isInterface(fn) {
 				return interfaceType
diff --git a/checker/patcher.go b/checker/patcher.go
index 88f512b5a..314a1d77d 100644
--- a/checker/patcher.go
+++ b/checker/patcher.go
@@ -8,7 +8,7 @@ import (
 
 type operatorPatcher struct {
 	ops   map[string][]string
-	types conf.TypesTable
+	types conf.TypeFinder
 }
 
 func (p *operatorPatcher) Enter(node *ast.Node) {}
diff --git a/docs/Usage.md b/docs/Usage.md
index 778bbb4bf..7335f9069 100644
--- a/docs/Usage.md
+++ b/docs/Usage.md
@@ -21,7 +21,7 @@ fmt.Println(out) // outputs 3
 
 ## Passing in Variables
 
-You can also pass variables into the expression, which can be map or struct:
+You can also pass variables into the expression, which can be map, struct, or lookup function:
 
 ```go
 env := map[string]interface{}{
@@ -35,6 +35,14 @@ env := Env{
 	Bar: ...
 }
 
+// or
+env := func(identifier string) interface{} {
+	switch (ident) {
+	case "Foo": ...
+	case "Bar": ...
+	}
+}
+
 // Pass env option to compile for static type checking.
 program, err := expr.Compile(`Foo == Bar`, expr.Env(env))
 
diff --git a/expr.go b/expr.go
index 82bf16d69..a35f294ff 100644
--- a/expr.go
+++ b/expr.go
@@ -35,13 +35,19 @@ func Eval(input string, env interface{}) (interface{}, error) {
 // If struct is passed, all fields will be treated as variables,
 // as well as all fields of embedded structs and struct itself.
 // If map is passed, all items will be treated as variables.
+// If func(string) interface{} is passed, variables will be
+// looked up dynamically.
 // Methods defined on this type will be available as functions.
 func Env(i interface{}) conf.Option {
 	return func(c *conf.Config) {
 		if _, ok := i.(map[string]interface{}); ok {
 			c.MapEnv = true
+			c.Types = conf.CreateTypesTable(i)
+		} else if fn, ok := i.(func(string) interface{}); ok {
+			c.Types = conf.TypesFunction(fn)
+		} else {
+			c.Types = conf.CreateTypesTable(i)
 		}
-		c.Types = conf.CreateTypesTable(i)
 	}
 }
 
diff --git a/expr_test.go b/expr_test.go
index 61da6517a..a9efbaebf 100644
--- a/expr_test.go
+++ b/expr_test.go
@@ -903,3 +903,53 @@ type segment struct {
 	Destination string
 	Date        time.Time
 }
+
+func envFunction(key string) interface{} {
+	switch key {
+	case "name":
+		return "Dave"
+	case "i":
+		return int(10)
+	case "upper":
+		return strings.ToUpper
+	case "now":
+		return time.Now()
+	}
+	return nil
+}
+
+func TestEval_func(t *testing.T) {
+	tests := []struct {
+		Input  string
+		Result interface{}
+	}{
+		{`i + i`, int(20)},
+		{`name`, `Dave`},
+		{`upper(name)`, `DAVE`},
+		{`now.IsZero()`, false},
+	}
+
+	for _, tt := range tests {
+		program, err := expr.Compile(tt.Input, expr.Env(envFunction))
+		require.NoError(t, err)
+
+		result, err := expr.Run(program, envFunction)
+		require.NoError(t, err, program.Disassemble())
+		require.Equal(t, tt.Result, result)
+	}
+}
+
+func TestEval_funcTypeCheck(t *testing.T) {
+	tests := []string{
+		`i + name`,
+		`name()`,
+		`upper(123)`,
+		`now.IsUnknown()`,
+		`unknown`,
+	}
+
+	for _, tt := range tests {
+		_, err := expr.Compile(tt, expr.Env(envFunction))
+		require.Error(t, err, tt)
+	}
+}
diff --git a/internal/conf/config.go b/internal/conf/config.go
index 56991b1f0..1c641ea28 100644
--- a/internal/conf/config.go
+++ b/internal/conf/config.go
@@ -7,23 +7,27 @@ import (
 
 type Config struct {
 	MapEnv    bool
-	Types     TypesTable
+	Types     TypeFinder
 	Operators OperatorsTable
 	Expect    reflect.Kind
 	Optimize  bool
 }
 
 func New(i interface{}) *Config {
-	var mapEnv bool
-	if _, ok := i.(map[string]interface{}); ok {
-		mapEnv = true
+	config := &Config{
+		Optimize: true,
 	}
 
-	return &Config{
-		MapEnv:   mapEnv,
-		Types:    CreateTypesTable(i),
-		Optimize: true,
+	if _, ok := i.(map[string]interface{}); ok {
+		config.MapEnv = true
+		config.Types = CreateTypesTable(i)
+	} else if fn, ok := i.(func(string) interface{}); ok {
+		config.Types = TypesFunction(fn)
+	} else {
+		config.Types = CreateTypesTable(i)
 	}
+
+	return config
 }
 
 // Check validates the compiler configuration.
@@ -32,7 +36,7 @@ func (c *Config) Check() error {
 	// exist in environment and have correct signatures.
 	for op, fns := range c.Operators {
 		for _, fn := range fns {
-			fnType, ok := c.Types[fn]
+			fnType, ok := c.Types.LookupType(fn)
 			if !ok || fnType.Type.Kind() != reflect.Func {
 				return fmt.Errorf("function %s for %s operator does not exist in environment", fn, op)
 			}
diff --git a/internal/conf/operators_table.go b/internal/conf/operators_table.go
index 5368366db..d2ffacd9e 100644
--- a/internal/conf/operators_table.go
+++ b/internal/conf/operators_table.go
@@ -6,9 +6,9 @@ import "reflect"
 // Functions should be provided in the environment to allow operator overloading.
 type OperatorsTable map[string][]string
 
-func FindSuitableOperatorOverload(fns []string, types TypesTable, l, r reflect.Type) (reflect.Type, string, bool) {
+func FindSuitableOperatorOverload(fns []string, types TypeFinder, l, r reflect.Type) (reflect.Type, string, bool) {
 	for _, fn := range fns {
-		fnType := types[fn]
+		fnType, _ := types.LookupType(fn)
 		firstInIndex := 0
 		if fnType.Method {
 			firstInIndex = 1 // As first argument to method is receiver.
diff --git a/internal/conf/types_table.go b/internal/conf/types_table.go
index 760b180f1..607c86bfb 100644
--- a/internal/conf/types_table.go
+++ b/internal/conf/types_table.go
@@ -7,8 +7,30 @@ type Tag struct {
 	Method bool
 }
 
+type TypeFinder interface {
+	LookupType(identifier string) (Tag, bool)
+}
+
 type TypesTable map[string]Tag
 
+func (t TypesTable) LookupType(identifier string) (Tag, bool) {
+	tag, ok := t[identifier]
+	return tag, ok
+}
+
+type TypesFunction func(identifier string) interface{}
+
+func (fn TypesFunction) LookupType(identifier string) (Tag, bool) {
+	i := fn(identifier)
+	if i == nil {
+		return Tag{}, false
+	}
+
+	return Tag{
+		Type: reflect.TypeOf(i),
+	}, true
+}
+
 // CreateTypesTable creates types table for type checks during parsing.
 // If struct is passed, all fields will be treated as variables,
 // as well as all fields of embedded structs and struct itself.
diff --git a/vm/runtime.go b/vm/runtime.go
index e9e8874bc..c5aac89c3 100644
--- a/vm/runtime.go
+++ b/vm/runtime.go
@@ -15,7 +15,13 @@ type Call struct {
 
 type Scope map[string]interface{}
 
-func fetch(from interface{}, i interface{}) interface{} {
+func fetch(op byte, from interface{}, i interface{}) interface{} {
+	if op == OpFetch {
+		if fn, ok := from.(func(string) interface{}); ok {
+			return fn(i.(string))
+		}
+	}
+
 	v := reflect.ValueOf(from)
 	switch v.Kind() {
 
@@ -41,7 +47,7 @@ func fetch(from interface{}, i interface{}) interface{} {
 	case reflect.Ptr:
 		value := v.Elem()
 		if value.IsValid() && value.CanInterface() {
-			return fetch(value.Interface(), i)
+			return fetch(op, value.Interface(), i)
 		}
 
 	}
@@ -68,7 +74,13 @@ func slice(array, from, to interface{}) interface{} {
 	panic(fmt.Sprintf("cannot slice %v", from))
 }
 
-func fetchFn(from interface{}, name string) reflect.Value {
+func fetchFn(op byte, from interface{}, name string) reflect.Value {
+	if op == OpCall {
+		if fn, ok := from.(func(string) interface{}); ok {
+			return reflect.ValueOf(fn(name))
+		}
+	}
+
 	v := reflect.ValueOf(from)
 
 	// Methods can be defined on any type.
diff --git a/vm/vm.go b/vm/vm.go
index 1f817cc0f..bb0f51d8d 100644
--- a/vm/vm.go
+++ b/vm/vm.go
@@ -79,7 +79,7 @@ func (vm *VM) Run(program *Program, env interface{}) interface{} {
 			vm.push(a)
 
 		case OpFetch:
-			vm.push(fetch(env, vm.constants[vm.arg()]))
+			vm.push(fetch(op, env, vm.constants[vm.arg()]))
 
 		case OpFetchMap:
 			vm.push(env.(map[string]interface{})[vm.constants[vm.arg()].(string)])
@@ -229,7 +229,7 @@ func (vm *VM) Run(program *Program, env interface{}) interface{} {
 		case OpIndex:
 			b := vm.pop()
 			a := vm.pop()
-			vm.push(fetch(a, b))
+			vm.push(fetch(op, a, b))
 
 		case OpSlice:
 			from := vm.pop()
@@ -240,7 +240,7 @@ func (vm *VM) Run(program *Program, env interface{}) interface{} {
 		case OpProperty:
 			a := vm.pop()
 			b := vm.constants[vm.arg()]
-			vm.push(fetch(a, b))
+			vm.push(fetch(op, a, b))
 
 		case OpCall:
 			call := vm.constants[vm.arg()].(Call)
@@ -250,7 +250,7 @@ func (vm *VM) Run(program *Program, env interface{}) interface{} {
 				in[i] = reflect.ValueOf(vm.pop())
 			}
 
-			out := fetchFn(env, call.Name).Call(in)
+			out := fetchFn(op, env, call.Name).Call(in)
 			vm.push(out[0].Interface())
 
 		case OpMethod:
@@ -263,7 +263,7 @@ func (vm *VM) Run(program *Program, env interface{}) interface{} {
 
 			obj := vm.pop()
 
-			out := fetchFn(obj, call.Name).Call(in)
+			out := fetchFn(op, obj, call.Name).Call(in)
 			vm.push(out[0].Interface())
 
 		case OpArray: