diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 62eac59..d90f5c7 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -39,6 +39,8 @@ jobs:
       run: poetry install
       # if: steps.cache.outputs.cache-hit != 'true'
      
+    - name: lint with mypy
+      run: poetry run mypy --disallow-untyped-defs rrl.py
     - name: lint with flake8
       run: poetry run flake8 . --show-source --statistics --ignore=E203,E501
     - name: pytest
diff --git a/poetry.lock b/poetry.lock
index 5f6e526..a61a806 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -70,6 +70,30 @@ optional = false
 python-versions = "*"
 version = "0.6.1"
 
+[[package]]
+category = "dev"
+description = "Optional static typing for Python"
+name = "mypy"
+optional = false
+python-versions = ">=3.5"
+version = "0.790"
+
+[package.dependencies]
+mypy-extensions = ">=0.4.3,<0.5.0"
+typed-ast = ">=1.4.0,<1.5.0"
+typing-extensions = ">=3.7.4"
+
+[package.extras]
+dmypy = ["psutil (>=4.0)"]
+
+[[package]]
+category = "dev"
+description = "Experimental type system extensions for programs checked with the mypy typechecker."
+name = "mypy-extensions"
+optional = false
+python-versions = "*"
+version = "0.4.3"
+
 [[package]]
 category = "dev"
 description = "Core utilities for Python packages"
@@ -185,8 +209,24 @@ optional = false
 python-versions = "*"
 version = "0.10.1"
 
+[[package]]
+category = "dev"
+description = "a fork of Python 2 and 3 ast modules with type comment support"
+name = "typed-ast"
+optional = false
+python-versions = "*"
+version = "1.4.1"
+
+[[package]]
+category = "dev"
+description = "Backported and Experimental Type Hints for Python 3.5+"
+name = "typing-extensions"
+optional = false
+python-versions = "*"
+version = "3.7.4.3"
+
 [metadata]
-content-hash = "c23179aae0ca10f0951b0e00730b561c6eaf0663bebac037d282e25bbbde2ebe"
+content-hash = "a28e23e32762c0133dbba42d3e00a20e2ae0666f6d3ee7e9ea2452635dffccf3"
 python-versions = "^3.8"
 
 [metadata.files]
@@ -218,6 +258,26 @@ mccabe = [
     {file = "mccabe-0.6.1-py2.py3-none-any.whl", hash = "sha256:ab8a6258860da4b6677da4bd2fe5dc2c659cff31b3ee4f7f5d64e79735b80d42"},
     {file = "mccabe-0.6.1.tar.gz", hash = "sha256:dd8d182285a0fe56bace7f45b5e7d1a6ebcbf524e8f3bd87eb0f125271b8831f"},
 ]
+mypy = [
+    {file = "mypy-0.790-cp35-cp35m-macosx_10_6_x86_64.whl", hash = "sha256:bd03b3cf666bff8d710d633d1c56ab7facbdc204d567715cb3b9f85c6e94f669"},
+    {file = "mypy-0.790-cp35-cp35m-manylinux1_x86_64.whl", hash = "sha256:2170492030f6faa537647d29945786d297e4862765f0b4ac5930ff62e300d802"},
+    {file = "mypy-0.790-cp35-cp35m-win_amd64.whl", hash = "sha256:e86bdace26c5fe9cf8cb735e7cedfe7850ad92b327ac5d797c656717d2ca66de"},
+    {file = "mypy-0.790-cp36-cp36m-macosx_10_9_x86_64.whl", hash = "sha256:e97e9c13d67fbe524be17e4d8025d51a7dca38f90de2e462243ab8ed8a9178d1"},
+    {file = "mypy-0.790-cp36-cp36m-manylinux1_x86_64.whl", hash = "sha256:0d34d6b122597d48a36d6c59e35341f410d4abfa771d96d04ae2c468dd201abc"},
+    {file = "mypy-0.790-cp36-cp36m-win_amd64.whl", hash = "sha256:72060bf64f290fb629bd4a67c707a66fd88ca26e413a91384b18db3876e57ed7"},
+    {file = "mypy-0.790-cp37-cp37m-macosx_10_9_x86_64.whl", hash = "sha256:eea260feb1830a627fb526d22fbb426b750d9f5a47b624e8d5e7e004359b219c"},
+    {file = "mypy-0.790-cp37-cp37m-manylinux1_x86_64.whl", hash = "sha256:c614194e01c85bb2e551c421397e49afb2872c88b5830e3554f0519f9fb1c178"},
+    {file = "mypy-0.790-cp37-cp37m-win_amd64.whl", hash = "sha256:0a0d102247c16ce93c97066443d11e2d36e6cc2a32d8ccc1f705268970479324"},
+    {file = "mypy-0.790-cp38-cp38-macosx_10_9_x86_64.whl", hash = "sha256:cf4e7bf7f1214826cf7333627cb2547c0db7e3078723227820d0a2490f117a01"},
+    {file = "mypy-0.790-cp38-cp38-manylinux1_x86_64.whl", hash = "sha256:af4e9ff1834e565f1baa74ccf7ae2564ae38c8df2a85b057af1dbbc958eb6666"},
+    {file = "mypy-0.790-cp38-cp38-win_amd64.whl", hash = "sha256:da56dedcd7cd502ccd3c5dddc656cb36113dd793ad466e894574125945653cea"},
+    {file = "mypy-0.790-py3-none-any.whl", hash = "sha256:2842d4fbd1b12ab422346376aad03ff5d0805b706102e475e962370f874a5122"},
+    {file = "mypy-0.790.tar.gz", hash = "sha256:2b21ba45ad9ef2e2eb88ce4aeadd0112d0f5026418324176fd494a6824b74975"},
+]
+mypy-extensions = [
+    {file = "mypy_extensions-0.4.3-py2.py3-none-any.whl", hash = "sha256:090fedd75945a69ae91ce1303b5824f428daf5a028d2f6ab8a299250a846f15d"},
+    {file = "mypy_extensions-0.4.3.tar.gz", hash = "sha256:2d82818f5bb3e369420cb3c4060a7970edba416647068eb4c5343488a6c604a8"},
+]
 packaging = [
     {file = "packaging-20.4-py2.py3-none-any.whl", hash = "sha256:998416ba6962ae7fbd6596850b80e17859a5753ba17c32284f67bfff33784181"},
     {file = "packaging-20.4.tar.gz", hash = "sha256:4357f74f47b9c12db93624a82154e9b120fa8293699949152b22065d556079f8"},
@@ -262,3 +322,31 @@ toml = [
     {file = "toml-0.10.1-py2.py3-none-any.whl", hash = "sha256:bda89d5935c2eac546d648028b9901107a595863cb36bae0c73ac804a9b4ce88"},
     {file = "toml-0.10.1.tar.gz", hash = "sha256:926b612be1e5ce0634a2ca03470f95169cf16f939018233a670519cb4ac58b0f"},
 ]
+typed-ast = [
+    {file = "typed_ast-1.4.1-cp35-cp35m-manylinux1_i686.whl", hash = "sha256:73d785a950fc82dd2a25897d525d003f6378d1cb23ab305578394694202a58c3"},
+    {file = "typed_ast-1.4.1-cp35-cp35m-manylinux1_x86_64.whl", hash = "sha256:aaee9905aee35ba5905cfb3c62f3e83b3bec7b39413f0a7f19be4e547ea01ebb"},
+    {file = "typed_ast-1.4.1-cp35-cp35m-win32.whl", hash = "sha256:0c2c07682d61a629b68433afb159376e24e5b2fd4641d35424e462169c0a7919"},
+    {file = "typed_ast-1.4.1-cp35-cp35m-win_amd64.whl", hash = "sha256:4083861b0aa07990b619bd7ddc365eb7fa4b817e99cf5f8d9cf21a42780f6e01"},
+    {file = "typed_ast-1.4.1-cp36-cp36m-macosx_10_9_x86_64.whl", hash = "sha256:269151951236b0f9a6f04015a9004084a5ab0d5f19b57de779f908621e7d8b75"},
+    {file = "typed_ast-1.4.1-cp36-cp36m-manylinux1_i686.whl", hash = "sha256:24995c843eb0ad11a4527b026b4dde3da70e1f2d8806c99b7b4a7cf491612652"},
+    {file = "typed_ast-1.4.1-cp36-cp36m-manylinux1_x86_64.whl", hash = "sha256:fe460b922ec15dd205595c9b5b99e2f056fd98ae8f9f56b888e7a17dc2b757e7"},
+    {file = "typed_ast-1.4.1-cp36-cp36m-win32.whl", hash = "sha256:4e3e5da80ccbebfff202a67bf900d081906c358ccc3d5e3c8aea42fdfdfd51c1"},
+    {file = "typed_ast-1.4.1-cp36-cp36m-win_amd64.whl", hash = "sha256:249862707802d40f7f29f6e1aad8d84b5aa9e44552d2cc17384b209f091276aa"},
+    {file = "typed_ast-1.4.1-cp37-cp37m-macosx_10_9_x86_64.whl", hash = "sha256:8ce678dbaf790dbdb3eba24056d5364fb45944f33553dd5869b7580cdbb83614"},
+    {file = "typed_ast-1.4.1-cp37-cp37m-manylinux1_i686.whl", hash = "sha256:c9e348e02e4d2b4a8b2eedb48210430658df6951fa484e59de33ff773fbd4b41"},
+    {file = "typed_ast-1.4.1-cp37-cp37m-manylinux1_x86_64.whl", hash = "sha256:bcd3b13b56ea479b3650b82cabd6b5343a625b0ced5429e4ccad28a8973f301b"},
+    {file = "typed_ast-1.4.1-cp37-cp37m-win32.whl", hash = "sha256:d5d33e9e7af3b34a40dc05f498939f0ebf187f07c385fd58d591c533ad8562fe"},
+    {file = "typed_ast-1.4.1-cp37-cp37m-win_amd64.whl", hash = "sha256:0666aa36131496aed8f7be0410ff974562ab7eeac11ef351def9ea6fa28f6355"},
+    {file = "typed_ast-1.4.1-cp38-cp38-macosx_10_15_x86_64.whl", hash = "sha256:d205b1b46085271b4e15f670058ce182bd1199e56b317bf2ec004b6a44f911f6"},
+    {file = "typed_ast-1.4.1-cp38-cp38-manylinux1_i686.whl", hash = "sha256:6daac9731f172c2a22ade6ed0c00197ee7cc1221aa84cfdf9c31defeb059a907"},
+    {file = "typed_ast-1.4.1-cp38-cp38-manylinux1_x86_64.whl", hash = "sha256:498b0f36cc7054c1fead3d7fc59d2150f4d5c6c56ba7fb150c013fbc683a8d2d"},
+    {file = "typed_ast-1.4.1-cp38-cp38-win32.whl", hash = "sha256:715ff2f2df46121071622063fc7543d9b1fd19ebfc4f5c8895af64a77a8c852c"},
+    {file = "typed_ast-1.4.1-cp38-cp38-win_amd64.whl", hash = "sha256:fc0fea399acb12edbf8a628ba8d2312f583bdbdb3335635db062fa98cf71fca4"},
+    {file = "typed_ast-1.4.1-cp39-cp39-macosx_10_15_x86_64.whl", hash = "sha256:d43943ef777f9a1c42bf4e552ba23ac77a6351de620aa9acf64ad54933ad4d34"},
+    {file = "typed_ast-1.4.1.tar.gz", hash = "sha256:8c8aaad94455178e3187ab22c8b01a3837f8ee50e09cf31f1ba129eb293ec30b"},
+]
+typing-extensions = [
+    {file = "typing_extensions-3.7.4.3-py2-none-any.whl", hash = "sha256:dafc7639cde7f1b6e1acc0f457842a83e722ccca8eef5270af2d74792619a89f"},
+    {file = "typing_extensions-3.7.4.3-py3-none-any.whl", hash = "sha256:7cb407020f00f7bfc3cb3e7881628838e69d8f3fcab2f64742a5e76b2f841918"},
+    {file = "typing_extensions-3.7.4.3.tar.gz", hash = "sha256:99d4073b617d30288f569d3f13d2bd7548c3a7e4c8de87db09a9d29bb3a4a60c"},
+]
diff --git a/pyproject.toml b/pyproject.toml
index 470a86c..4e65de7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -13,6 +13,7 @@ redis = "^3.5.3"
 pytest = "^6.1.1"
 freezegun = "^1.0.0"
 flake8 = "^3.8.4"
+mypy = "^0.790"
 
 [build-system]
 requires = ["poetry>=0.12"]