diff --git a/pddl/parser/domain.py b/pddl/parser/domain.py
index a26084d..2658e4d 100644
--- a/pddl/parser/domain.py
+++ b/pddl/parser/domain.py
@@ -23,15 +23,20 @@
 from pddl.helpers.base import assert_
 from pddl.logic.base import And, ExistsCondition, ForallCondition, Imply, Not, OneOf, Or
 from pddl.logic.effects import AndEffect, Forall, When
-from pddl.logic.functions import Decrease
+from pddl.logic.functions import Decrease, Divide
 from pddl.logic.functions import EqualTo as FunctionEqualTo
 from pddl.logic.functions import (
-    Function,
+    FunctionExpression,
     GreaterEqualThan,
     GreaterThan,
     Increase,
     LesserEqualThan,
     LesserThan,
+    Minus,
+    NumericFunction,
+    NumericValue,
+    Plus,
+    Times,
     TotalCost,
 )
 from pddl.logic.predicates import DerivedPredicate, EqualTo, Predicate
@@ -51,7 +56,7 @@ def __init__(self, *args, **kwargs):
 
         self._constants_by_name: Dict[str, Constant] = {}
         self._predicates_by_name: Dict[str, Predicate] = {}
-        self._functions_by_name: Dict[str, Function] = {}
+        self._functions_by_name: Dict[str, FunctionExpression] = {}
         self._current_parameters_by_name: Dict[str, Variable] = {}
         self._requirements: Set[str] = set()
         self._extended_requirements: Set[str] = set()
@@ -347,15 +352,36 @@ def atomic_function_skeleton(self, args):
             return TotalCost()
         function_name = args[1]
         variables = self._formula_skeleton(args)
-        return Function(function_name, *variables)
+        return NumericFunction(function_name, *variables)
+
+    def f_exp(self, args):
+        """Process the 'f_exp' rule."""
+        if len(args) == 1:
+            if isinstance(args[0], (int, float)):
+                return NumericValue(args[0])
+            return args[0]
+        op = None
+        if args[1] == Symbols.MINUS.value:
+            op = Minus
+        if args[1] == Symbols.PLUS.value:
+            op = Plus
+        if args[1] == Symbols.TIMES.value:
+            op = Times
+        if args[1] == Symbols.DIVIDE.value:
+            op = Divide
+        return (
+            op(*args[2:-1])
+            if op is not None
+            else PDDLParsingError("Operator not recognized")
+        )
 
     def f_head(self, args):
         """Process the 'f_head' rule."""
         if len(args) == 1:
-            return args[0]
+            return NumericFunction(args[0])
         function_name = args[1]
         variables = [Variable(x, {}) for x in args[2:-1]]
-        return Function(function_name, *variables)
+        return NumericFunction(function_name, *variables)
 
     def typed_list_name(self, args) -> Dict[name, Optional[name]]:
         """Process the 'typed_list_name' rule."""
diff --git a/pddl/parser/problem.py b/pddl/parser/problem.py
index 5d01f81..cb1a836 100644
--- a/pddl/parser/problem.py
+++ b/pddl/parser/problem.py
@@ -20,14 +20,19 @@
 from pddl.exceptions import PDDLParsingError
 from pddl.helpers.base import assert_
 from pddl.logic.base import And, Not
+from pddl.logic.functions import Divide
 from pddl.logic.functions import EqualTo as FunctionEqualTo
 from pddl.logic.functions import (
-    Function,
     GreaterEqualThan,
     GreaterThan,
     LesserEqualThan,
     LesserThan,
     Metric,
+    Minus,
+    NumericFunction,
+    NumericValue,
+    Plus,
+    Times,
 )
 from pddl.logic.predicates import EqualTo, Predicate
 from pddl.logic.terms import Constant, Variable
@@ -111,11 +116,11 @@ def init_el(self, args):
             return args[0]
         elif args[1] == Symbols.EQUAL.value:
             if isinstance(args[2], list) and len(args[2]) == 1:
-                return FunctionEqualTo(*args[2], args[3])
+                return FunctionEqualTo(*args[2], NumericValue(args[3]))
             elif not isinstance(args[2], list):
-                return FunctionEqualTo(args[2], args[3])
+                return FunctionEqualTo(args[2], NumericValue(args[3]))
             else:
-                funcs = [FunctionEqualTo(x, args[3]) for x in args[2]]
+                funcs = [FunctionEqualTo(x, NumericValue(args[3])) for x in args[2]]
                 return funcs
 
     def literal_name(self, args):
@@ -130,8 +135,10 @@ def literal_name(self, args):
     def basic_function_term(self, args):
         """Process the 'basic_function_term' rule."""
         if len(args) == 1:
-            return Function(args[0])
-        return [Function(x) for x in args[1:-1]]
+            return NumericFunction(args[0])
+        function_name = args[1]
+        objects = [Constant(x) for x in args[2:-1]]
+        return NumericFunction(function_name, *objects)
 
     def goal(self, args):
         """Process the 'goal' rule."""
@@ -186,19 +193,40 @@ def atomic_formula_name(self, args):
     def f_head(self, args):
         """Process the 'f_head' rule."""
         if len(args) == 1:
-            return args[0]
+            return NumericFunction(args[0])
         function_name = args[1]
         variables = [Variable(x, {}) for x in args[2:-1]]
-        return Function(function_name, *variables)
+        return NumericFunction(function_name, *variables)
 
     def metric_spec(self, args):
         """Process the 'metric_spec' rule."""
-        if isinstance(args[3], list) and len(args[3]) == 1:
-            return "metric", Metric(*args[3], args[2])
-        elif not isinstance(args[2], list):
+        if args[2] == Symbols.MINIMIZE.value:
+            return "metric", Metric(args[3], args[2])
+        elif args[2] == Symbols.MAXIMIZE.value:
             return "metric", Metric(args[3], args[2])
         else:
-            raise ParseError
+            raise PDDLParsingError(f"Unknown metric operator: {args[2]}")
+
+    def metric_f_exp(self, args):
+        """Process the 'metric_f_exp' rule."""
+        if len(args) == 1:
+            if isinstance(args[0], (int, float)):
+                return NumericValue(args[0])
+            return args[0]
+        op = None
+        if args[1] == Symbols.MINUS.value:
+            op = Minus
+        if args[1] == Symbols.PLUS.value:
+            op = Plus
+        if args[1] == Symbols.TIMES.value:
+            op = Times
+        if args[1] == Symbols.DIVIDE.value:
+            op = Divide
+        return (
+            op(*args[2:-1])
+            if op is not None
+            else PDDLParsingError("Operator not recognized")
+        )
 
 
 _problem_parser_lark = PROBLEM_GRAMMAR_FILE.read_text()