From f45c311001162a46112e247ab6b51051bc978f80 Mon Sep 17 00:00:00 2001
From: Charles Stephens <cfs@purestorage.com>
Date: Wed, 1 Nov 2023 16:51:18 -0600
Subject: [PATCH] Fix str() bug for tuple values

Replaced the use of old-style formatting strings in __str__ methods.

At first, that seems like a dated but harmless practice, but unfortunately

While dated and seemingly a harmless practice, OSlash actually uses it
in an incorrect way.

If a program using OSlash deals with types other than strings, then the
__str__ methods for classes like `Left`, `Right`, and `Either`, OSlash
directly writes:

    return '%s' % value

where `value` is an arbitrary type.

Unfortunately, '%s' % value is only guaranteed to work if the value
is a string, since Python will not call `str()` on it.

(Well, more precisely, `'%s' % value` is guaranteed to *fail* if the
value happens to be a tuple of a certain kind, since Python will try
to unpack the tuple into the string formatting operation)

The upshot is that such programs will get TypeErrors.  Instead, use
Python 3.8+ f-strings to guarantee that the value is converted to a
string.
---
 oslash/either.py   | 4 ++--
 oslash/identity.py | 2 +-
 oslash/maybe.py    | 2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)

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)