diff --git a/oslash/either.py b/oslash/either.py
index 47b5448..533639a 100644
--- a/oslash/either.py
+++ b/oslash/either.py
@@ -98,7 +98,7 @@ def __eq__(self, other) -> bool:
         return isinstance(other, Right) and self._value == other._value
 
     def __str__(self) -> str:
-        return "Right %s" % self._value
+        return f"Right {self._value}"
 
 
 class Left(Either[TSource, TError]):
@@ -129,7 +129,7 @@ def __eq__(self, other) -> bool:
         return isinstance(other, Left) and self._error == other._error
 
     def __str__(self) -> str:
-        return "Left: %s" % self._error
+        return f"Left: {self._error}"
 
 
 assert(isinstance(Either, Functor))
diff --git a/oslash/identity.py b/oslash/identity.py
index 33dfde5..4248b2d 100644
--- a/oslash/identity.py
+++ b/oslash/identity.py
@@ -52,7 +52,7 @@ def __eq__(self, other) -> bool:
         return self._value == other()
 
     def __str__(self) -> str:
-        return "Identity(%s)" % self._value
+        return f"Identity({self._value})"
 
     def __repr__(self) -> str:
         return str(self)
diff --git a/oslash/maybe.py b/oslash/maybe.py
index dbb65c9..a2f1158 100644
--- a/oslash/maybe.py
+++ b/oslash/maybe.py
@@ -166,7 +166,7 @@ def __eq__(self, other) -> bool:
         return bool(other.map(lambda other_value: other_value == self._value))
 
     def __str__(self) -> str:
-        return "Just %s" % self._value
+        return f"Just {self._value}"
 
     def __repr__(self) -> str:
         return str(self)