diff --git a/lark/lark.py b/lark/lark.py
index d4050552..8da98409 100644
--- a/lark/lark.py
+++ b/lark/lark.py
@@ -3,6 +3,7 @@
 import sys, os, pickle, hashlib
 import tempfile
 import types
+import re
 from typing import (
     TypeVar, Type, List, Dict, Iterator, Callable, Union, Optional, Sequence,
     Tuple, Iterable, IO, Any, TYPE_CHECKING, Collection
@@ -15,6 +16,7 @@
         from typing import Literal
     else:
         from typing_extensions import Literal
+    from .parser_frontends import ParsingFrontend
         
 from .exceptions import ConfigurationError, assert_config, UnexpectedInput
 from .utils import Serialize, SerializeMemoizer, FS, isascii, logger
@@ -27,7 +29,7 @@
 from .parser_frontends import _validate_frontend_args, _get_lexer_callbacks, _deserialize_parsing_frontend, _construct_parsing_frontend
 from .grammar import Rule
 
-import re
+
 try:
     import regex
     _has_regex = True
@@ -176,7 +178,7 @@ class LarkOptions(Serialize):
         '_plugins': {},
     }
 
-    def __init__(self, options_dict):
+    def __init__(self, options_dict: Dict[str, Any]) -> None:
         o = dict(options_dict)
 
         options = {}
@@ -205,21 +207,21 @@ def __init__(self, options_dict):
         if o:
             raise ConfigurationError("Unknown options: %s" % o.keys())
 
-    def __getattr__(self, name):
+    def __getattr__(self, name: str) -> Any:
         try:
             return self.__dict__['options'][name]
         except KeyError as e:
             raise AttributeError(e)
 
-    def __setattr__(self, name, value):
+    def __setattr__(self, name: str, value: str) -> None:
         assert_config(name, self.options.keys(), "%r isn't a valid option. Expected one of: %s")
         self.options[name] = value
 
-    def serialize(self, memo):
+    def serialize(self, memo = None) -> Dict[str, Any]:
         return self.options
 
     @classmethod
-    def deserialize(cls, data, memo):
+    def deserialize(cls, data: Dict[str, Any], memo: Dict[int, Union[TerminalDef, Rule]]) -> "LarkOptions":
         return cls(data)
 
 
@@ -252,7 +254,7 @@ class Lark(Serialize):
     grammar: 'Grammar'
     options: LarkOptions
     lexer: Lexer
-    terminals: List[TerminalDef]
+    terminals: Collection[TerminalDef]
 
     def __init__(self, grammar: 'Union[Grammar, str, IO[str]]', **options) -> None:
         self.options = LarkOptions(options)
@@ -446,7 +448,7 @@ def __init__(self, grammar: 'Union[Grammar, str, IO[str]]', **options) -> None:
 
     __serialize_fields__ = 'parser', 'rules', 'options'
 
-    def _build_lexer(self, dont_ignore=False):
+    def _build_lexer(self, dont_ignore: bool=False) -> BasicLexer:
         lexer_conf = self.lexer_conf
         if dont_ignore:
             from copy import copy
@@ -454,7 +456,7 @@ def _build_lexer(self, dont_ignore=False):
             lexer_conf.ignore = ()
         return BasicLexer(lexer_conf)
 
-    def _prepare_callbacks(self):
+    def _prepare_callbacks(self) -> None:
         self._callbacks = {}
         # we don't need these callbacks if we aren't building a tree
         if self.options.ambiguity != 'forest':
@@ -468,7 +470,7 @@ def _prepare_callbacks(self):
             self._callbacks = self._parse_tree_builder.create_callback(self.options.transformer)
         self._callbacks.update(_get_lexer_callbacks(self.options.transformer, self.terminals))
 
-    def _build_parser(self):
+    def _build_parser(self) -> "ParsingFrontend":
         self._prepare_callbacks()
         _validate_frontend_args(self.options.parser, self.options.lexer)
         parser_conf = ParserConf(self.rules, self._callbacks, self.options.start)
@@ -480,7 +482,7 @@ def _build_parser(self):
             options=self.options
         )
 
-    def save(self, f, exclude_options: Collection[str] = ()):
+    def save(self, f, exclude_options: Collection[str] = ()) -> None:
         """Saves the instance into the given file object
 
         Useful for caching and multiprocessing.
@@ -491,7 +493,7 @@ def save(self, f, exclude_options: Collection[str] = ()):
         pickle.dump({'data': data, 'memo': m}, f, protocol=pickle.HIGHEST_PROTOCOL)
 
     @classmethod
-    def load(cls, f):
+    def load(cls: Type[_T], f) -> _T:
         """Loads an instance from the given file object
 
         Useful for caching and multiprocessing.
@@ -499,7 +501,7 @@ def load(cls, f):
         inst = cls.__new__(cls)
         return inst._load(f)
 
-    def _deserialize_lexer_conf(self, data, memo, options):
+    def _deserialize_lexer_conf(self, data: Dict[str, Any], memo: Dict[int, Union[TerminalDef, Rule]], options: LarkOptions) -> LexerConf:
         lexer_conf = LexerConf.deserialize(data['lexer_conf'], memo)
         lexer_conf.callbacks = options.lexer_callbacks or {}
         lexer_conf.re_module = regex if options.regex else re
@@ -509,7 +511,7 @@ def _deserialize_lexer_conf(self, data, memo, options):
         lexer_conf.postlex = options.postlex
         return lexer_conf
 
-    def _load(self, f, **kwargs):
+    def _load(self: _T, f: Any, **kwargs) -> _T:
         if isinstance(f, dict):
             d = f
         else:
@@ -593,6 +595,7 @@ def lex(self, text: str, dont_ignore: bool=False) -> Iterator[Token]:
 
         :raises UnexpectedCharacters: In case the lexer cannot find a suitable match.
         """
+        lexer: Lexer
         if not hasattr(self, 'lexer') or dont_ignore:
             lexer = self._build_lexer(dont_ignore)
         else:
diff --git a/lark/parsers/lalr_parser.py b/lark/parsers/lalr_parser.py
index 292d5c24..48dac7b7 100644
--- a/lark/parsers/lalr_parser.py
+++ b/lark/parsers/lalr_parser.py
@@ -3,6 +3,7 @@
 # Author: Erez Shinan (2017)
 # Email : erezshin@gmail.com
 from copy import deepcopy, copy
+from typing import Dict, Any
 from ..lexer import Token
 from ..utils import Serialize
 
@@ -29,7 +30,7 @@ def deserialize(cls, data, memo, callbacks, debug=False):
         inst.parser = _Parser(inst._parse_table, callbacks, debug)
         return inst
 
-    def serialize(self, memo):
+    def serialize(self, memo: Any = None) -> Dict[str, Any]:
         return self._parse_table.serialize(memo)
 
     def parse_interactive(self, lexer, start):
diff --git a/lark/utils.py b/lark/utils.py
index f9c0fd01..6781e6fb 100644
--- a/lark/utils.py
+++ b/lark/utils.py
@@ -2,10 +2,12 @@
 import os
 from functools import reduce
 from collections import deque
+from typing import Callable, Iterator, List, Optional, Tuple, Type, TypeVar, Union, Dict, Any, Sequence
 
 ###{standalone
 import sys, re
 import logging
+
 logger: logging.Logger = logging.getLogger("lark")
 logger.addHandler(logging.StreamHandler())
 # Set to highest level, since we have some warnings amongst the code
@@ -15,9 +17,11 @@
 
 NO_VALUE = object()
 
+T = TypeVar("T")
+
 
-def classify(seq, key=None, value=None):
-    d = {}
+def classify(seq: Sequence, key: Optional[Callable] = None, value: Optional[Callable] = None) -> Dict:
+    d: Dict[Any, Any] = {}
     for item in seq:
         k = key(item) if (key is not None) else item
         v = value(item) if (value is not None) else item
@@ -28,7 +32,7 @@ def classify(seq, key=None, value=None):
     return d
 
 
-def _deserialize(data, namespace, memo):
+def _deserialize(data: Any, namespace: Dict[str, Any], memo: Dict) -> Any:
     if isinstance(data, dict):
         if '__type__' in data:  # Object
             class_ = namespace[data['__type__']]
@@ -41,6 +45,8 @@ def _deserialize(data, namespace, memo):
     return data
 
 
+_T = TypeVar("_T", bound="Serialize")
+
 class Serialize:
     """Safe-ish serialization interface that doesn't rely on Pickle
 
@@ -50,11 +56,11 @@ class Serialize:
                                         Should include all field types that aren't builtin types.
     """
 
-    def memo_serialize(self, types_to_memoize):
+    def memo_serialize(self, types_to_memoize: List) -> Any:
         memo = SerializeMemoizer(types_to_memoize)
         return self.serialize(memo), memo.serialize()
 
-    def serialize(self, memo=None):
+    def serialize(self, memo = None) -> Dict[str, Any]:
         if memo and memo.in_types(self):
             return {'@': memo.memoized.get(self)}
 
@@ -62,11 +68,11 @@ def serialize(self, memo=None):
         res = {f: _serialize(getattr(self, f), memo) for f in fields}
         res['__type__'] = type(self).__name__
         if hasattr(self, '_serialize'):
-            self._serialize(res, memo)
+            self._serialize(res, memo)  # type: ignore[attr-defined]
         return res
 
     @classmethod
-    def deserialize(cls, data, memo):
+    def deserialize(cls: Type[_T], data: Dict[str, Any], memo: Dict[int, Any]) -> _T:
         namespace = getattr(cls, '__serialize_namespace__', [])
         namespace = {c.__name__:c for c in namespace}
 
@@ -83,7 +89,7 @@ def deserialize(cls, data, memo):
                 raise KeyError("Cannot find key for class", cls, e)
 
         if hasattr(inst, '_deserialize'):
-            inst._deserialize()
+            inst._deserialize()  # type: ignore[attr-defined]
 
         return inst
 
@@ -93,18 +99,18 @@ class SerializeMemoizer(Serialize):
 
     __serialize_fields__ = 'memoized',
 
-    def __init__(self, types_to_memoize):
+    def __init__(self, types_to_memoize: List) -> None:
         self.types_to_memoize = tuple(types_to_memoize)
         self.memoized = Enumerator()
 
-    def in_types(self, value):
+    def in_types(self, value: Serialize) -> bool:
         return isinstance(value, self.types_to_memoize)
 
-    def serialize(self):
+    def serialize(self) -> Dict[int, Any]:  # type: ignore[override]
         return _serialize(self.memoized.reversed(), None)
 
     @classmethod
-    def deserialize(cls, data, namespace, memo):
+    def deserialize(cls, data: Dict[int, Any], namespace: Dict[str, Any], memo: Dict[Any, Any]) -> Dict[int, Any]:  # type: ignore[override]
         return _deserialize(data, namespace, memo)
 
 
@@ -123,7 +129,7 @@ def deserialize(cls, data, namespace, memo):
 
 categ_pattern = re.compile(r'\\p{[A-Za-z_]+}')
 
-def get_regexp_width(expr):
+def get_regexp_width(expr: str) -> Union[Tuple[int, int], List[int]]:
     if _has_regex:
         # Since `sre_parse` cannot deal with Unicode categories of the form `\p{Mn}`, we replace these with
         # a simple letter, which makes no difference as we are only trying to get the possible lengths of the regex
@@ -134,7 +140,8 @@ def get_regexp_width(expr):
             raise ImportError('`regex` module must be installed in order to use Unicode categories.', expr)
         regexp_final = expr
     try:
-        return [int(x) for x in sre_parse.parse(regexp_final).getwidth()]
+        # Fixed in next version (past 0.960) of typeshed
+        return [int(x) for x in sre_parse.parse(regexp_final).getwidth()]   # type: ignore[attr-defined]
     except sre_constants.error:
         if not _has_regex:
             raise ValueError(expr)
@@ -154,19 +161,19 @@ def get_regexp_width(expr):
 _ID_START =    'Lu', 'Ll', 'Lt', 'Lm', 'Lo', 'Mn', 'Mc', 'Pc'
 _ID_CONTINUE = _ID_START + ('Nd', 'Nl',)
 
-def _test_unicode_category(s, categories):
+def _test_unicode_category(s: str, categories: Sequence[str]) -> bool:
     if len(s) != 1:
         return all(_test_unicode_category(char, categories) for char in s)
     return s == '_' or unicodedata.category(s) in categories
 
-def is_id_continue(s):
+def is_id_continue(s: str) -> bool:
     """
     Checks if all characters in `s` are alphanumeric characters (Unicode standard, so diacritics, indian vowels, non-latin
     numbers, etc. all pass). Synonymous with a Python `ID_CONTINUE` identifier. See PEP 3131 for details.
     """
     return _test_unicode_category(s, _ID_CONTINUE)
 
-def is_id_start(s):
+def is_id_start(s: str) -> bool:
     """
     Checks if all characters in `s` are alphabetic characters (Unicode standard, so diacritics, indian vowels, non-latin
     numbers, etc. all pass). Synonymous with a Python `ID_START` identifier. See PEP 3131 for details.
@@ -174,19 +181,22 @@ def is_id_start(s):
     return _test_unicode_category(s, _ID_START)
 
 
-def dedup_list(l):
+def dedup_list(l: List[T]) -> List[T]:
     """Given a list (l) will removing duplicates from the list,
        preserving the original order of the list. Assumes that
        the list entries are hashable."""
     dedup = set()
-    return [x for x in l if not (x in dedup or dedup.add(x))]
+    # This returns None, but that's expected
+    return [x for x in l if not (x in dedup or dedup.add(x))]  # type: ignore[func-returns-value]
+    # 2x faster (ordered in PyPy and CPython 3.6+, gaurenteed to be ordered in Python 3.7+)
+    # return list(dict.fromkeys(l))
 
 
 class Enumerator(Serialize):
-    def __init__(self):
-        self.enums = {}
+    def __init__(self) -> None:
+        self.enums: Dict[Any, int] = {}
 
-    def get(self, item):
+    def get(self, item) -> int:
         if item not in self.enums:
             self.enums[item] = len(self.enums)
         return self.enums[item]
@@ -194,7 +204,7 @@ def get(self, item):
     def __len__(self):
         return len(self.enums)
 
-    def reversed(self):
+    def reversed(self) -> Dict[int, Any]:
         r = {v: k for k, v in self.enums.items()}
         assert len(r) == len(self.enums)
         return r
@@ -240,11 +250,11 @@ def open(name, mode="r", **kwargs):
 
 
 
-def isascii(s):
+def isascii(s: str) -> bool:
     """ str.isascii only exists in python3.7+ """
-    try:
+    if sys.version_info >= (3, 7):
         return s.isascii()
-    except AttributeError:
+    else:
         try:
             s.encode('ascii')
             return True
@@ -257,7 +267,7 @@ def __repr__(self):
         return '{%s}' % ', '.join(map(repr, self))
 
 
-def classify_bool(seq, pred):
+def classify_bool(seq: Sequence, pred: Callable) -> Any:
     true_elems = []
     false_elems = []
 
@@ -270,7 +280,7 @@ def classify_bool(seq, pred):
     return true_elems, false_elems
 
 
-def bfs(initial, expand):
+def bfs(initial: Sequence, expand: Callable) -> Iterator:
     open_q = deque(list(initial))
     visited = set(open_q)
     while open_q:
@@ -290,7 +300,7 @@ def bfs_all_unique(initial, expand):
         open_q += expand(node)
 
 
-def _serialize(value, memo):
+def _serialize(value: Any, memo: Optional[SerializeMemoizer]) -> Any:
     if isinstance(value, Serialize):
         return value.serialize(memo)
     elif isinstance(value, list):
@@ -305,7 +315,7 @@ def _serialize(value, memo):
 
 
 
-def small_factors(n, max_factor):
+def small_factors(n: int, max_factor: int) -> List[Tuple[int, int]]:
     """
     Splits n up into smaller factors and summands <= max_factor.
     Returns a list of [(a, b), ...]