diff --git a/src/ytdl_sub/script/functions.py b/src/ytdl_sub/script/functions.py
index 93842cd1..7c058a98 100644
--- a/src/ytdl_sub/script/functions.py
+++ b/src/ytdl_sub/script/functions.py
@@ -1,27 +1,67 @@
+from abc import ABC
+from dataclasses import dataclass
+from typing import Generic
+from typing import TypeVar
+
+T = TypeVar("T")
+
+
+@dataclass(frozen=True)
+class Resolvable(ABC, Generic[T]):
+    value: T
+
+    def resolve(self) -> str:
+        return str(self.value)
+
+
+@dataclass(frozen=True)
+class Integer(Resolvable[int]):
+    pass
+
+
+@dataclass(frozen=True)
+class Float(Resolvable[float]):
+    pass
+
+
+@dataclass(frozen=True)
+class Boolean(Resolvable[bool]):
+    pass
+
+
+@dataclass(frozen=True)
+class String(Resolvable[str]):
+    pass
+
+
 class Functions:
     @staticmethod
-    def lower(string: str) -> str:
+    def lower(string: String) -> String:
         """
         Returns
         -------
         Lower-cased string
         """
-        return string.lower()
+        return String(string.value.lower())
 
     @staticmethod
-    def upper(string: str) -> str:
+    def upper(string: String) -> String:
         """
         Returns
         -------
         Upper-cased string
         """
-        return string.upper()
+        return String(string.value.upper())
 
     @staticmethod
-    def capitalize(string: str) -> str:
+    def capitalize(string: String) -> String:
         """
         Returns
         -------
         Capitalized string
         """
-        return string.capitalize()
+        return String(string.value.capitalize())
+
+    @staticmethod
+    def concat(l_string: String, r_string: String) -> String:
+        return String(f"{l_string}{r_string}")
diff --git a/src/ytdl_sub/script/parser.py b/src/ytdl_sub/script/parser.py
index daf75010..7d13005c 100644
--- a/src/ytdl_sub/script/parser.py
+++ b/src/ytdl_sub/script/parser.py
@@ -1,14 +1,14 @@
 from typing import List
 from typing import Optional
 
+from ytdl_sub.script.functions import Boolean
+from ytdl_sub.script.functions import Float
+from ytdl_sub.script.functions import Integer
+from ytdl_sub.script.functions import String
 from ytdl_sub.script.types import ArgumentType
-from ytdl_sub.script.types import Boolean
-from ytdl_sub.script.types import Float
 from ytdl_sub.script.types import Function
-from ytdl_sub.script.types import Integer
 from ytdl_sub.script.types import LiteralString
 from ytdl_sub.script.types import NumericType
-from ytdl_sub.script.types import String
 from ytdl_sub.script.types import SyntaxTree
 from ytdl_sub.script.types import Variable
 from ytdl_sub.utils.exceptions import StringFormattingException
diff --git a/src/ytdl_sub/script/types.py b/src/ytdl_sub/script/types.py
index 1a0afc53..0e031c81 100644
--- a/src/ytdl_sub/script/types.py
+++ b/src/ytdl_sub/script/types.py
@@ -1,31 +1,18 @@
 from dataclasses import dataclass
-from typing import List, Dict, Optional
+from typing import Dict
+from typing import List
+from typing import Optional
 from typing import Set
 from typing import Union
 
+from ytdl_sub.script.functions import Boolean
+from ytdl_sub.script.functions import Float
+from ytdl_sub.script.functions import Functions
+from ytdl_sub.script.functions import Integer
+from ytdl_sub.script.functions import String
 from ytdl_sub.utils.exceptions import StringFormattingException
 
 
-@dataclass(frozen=True)
-class Integer:
-    value: int
-
-
-@dataclass(frozen=True)
-class Float:
-    value: float
-
-
-@dataclass(frozen=True)
-class Boolean:
-    value: bool
-
-
-@dataclass(frozen=True)
-class String:
-    value: str
-
-
 @dataclass(frozen=True)
 class Variable:
     name: str
@@ -40,6 +27,12 @@ class Function:
     name: str
     args: List[ArgumentType]
 
+    def __post_init__(self):
+        try:
+            getattr(Functions, self.name)
+        except AttributeError:
+            raise StringFormattingException(f"Function name {self.name} does not exist")
+
     @property
     def variables(self) -> Set[Variable]:
         """
@@ -68,6 +61,11 @@ class SyntaxTree:
 
     @property
     def variables(self) -> Set[Variable]:
+        """
+        Returns
+        -------
+        All variables used within the SyntaxTree
+        """
         variables: Set[Variable] = set()
         for token in self.ast:
             if isinstance(token, Variable):
@@ -79,11 +77,19 @@ class SyntaxTree:
 
     @classmethod
     def detect_cycles(cls, parsed_overrides: Dict[str, "SyntaxTree"]) -> None:
+        """
+        Parameters
+        ----------
+        parsed_overrides
+            ``overrides`` in a subscription, parsed into a SyntaxTree
+        """
         variable_dependencies: Dict[Variable, Set[Variable]] = {
             Variable(name): ast.variables for name, ast in parsed_overrides.items()
         }
 
-        def _traverse(to_variable: Variable, visited_variables: Optional[List[Variable]] = None) -> None:
+        def _traverse(
+            to_variable: Variable, visited_variables: Optional[List[Variable]] = None
+        ) -> None:
             if visited_variables is None:
                 visited_variables = []
 
@@ -97,4 +103,6 @@ class SyntaxTree:
         for variable in variable_dependencies.keys():
             _traverse(variable)
 
-
+    @classmethod
+    def resolve(cls, parsed_overrides: Dict[str, "SyntaxTree"]) -> Dict[str, str]:
+        raise NotImplemented()
diff --git a/src/ytdl_sub/utils/thumbnail.py b/src/ytdl_sub/utils/thumbnail.py
index 9d342f64..a999f01c 100644
--- a/src/ytdl_sub/utils/thumbnail.py
+++ b/src/ytdl_sub/utils/thumbnail.py
@@ -75,7 +75,7 @@ def download_and_convert_url_thumbnail(
     # timeout after 8 seconds
     with urlopen(thumbnail_url, timeout=1.0) as file:
         with tempfile.NamedTemporaryFile(delete=False) as thumbnail:
-            thumbnail.write(file._read())
+            thumbnail.write(file.read())
 
         try:
             os.makedirs(os.path.dirname(output_thumbnail_path), exist_ok=True)
diff --git a/tests/unit/script/test_parser.py b/tests/unit/script/test_parser.py
index 9775767e..26b6ec78 100644
--- a/tests/unit/script/test_parser.py
+++ b/tests/unit/script/test_parser.py
@@ -1,12 +1,12 @@
 import pytest
 
+from ytdl_sub.script.functions import Boolean
+from ytdl_sub.script.functions import Float
+from ytdl_sub.script.functions import Integer
+from ytdl_sub.script.functions import String
 from ytdl_sub.script.parser import parse
-from ytdl_sub.script.types import Boolean
-from ytdl_sub.script.types import Float
 from ytdl_sub.script.types import Function
-from ytdl_sub.script.types import Integer
 from ytdl_sub.script.types import LiteralString
-from ytdl_sub.script.types import String
 from ytdl_sub.script.types import SyntaxTree
 from ytdl_sub.script.types import Variable
 from ytdl_sub.utils.exceptions import StringFormattingException