Most languages are designed and changes are carefully constructed to avoid breaking older programs. This is a tradition from the earliest days of programming. New versions of FORTRAN and COBOL were introduced with new features, yet the newer compilers accepted the older programs. (Probably because the customers of the expensive computers would be very mad to learn that an "upgrade" had broken their existing programs.)
Since then, almost every language has followed this tradition. BASIC, Pascal, dBase (and Clipper and XBase), Java, Perl, ... they all strove for (and still strive for) backwards compatibility.
The record is not perfect. A few exceptions do come to mind:
- In the 1990s, multiple releases of Visual Basic broke compatibility with older versions as Microsoft decided to improve the syntax.
- Early versions of Perl changed syntax. Those changes were Larry Wall deciding on improvements to the syntax.
- The C language changed syntax for the addition-assignment and related operators (from =+ to +=) which resolved an ambiguity in the syntax.
- C++ broke compatibility with a scoping change in "for" statements. That was its only such change, to my knowledge.
But there is one language that is an exception.
That language is Python.
Python has seen a number of changes over time. I should say "Pythons", as there are two paths for Python development: Python 2 and Python 3. Each path has multiple versions (Python 2.4, 2.5, 2.6, and Python 3.4, 3.5, 3.6, etc.).
The Python 3 path was started as the "next generation" of Python interpreters, and it was started with the explicit statement that it would not be compatible with the Python 2 path.
Not only are the two paths different (and incompatible), versions within each path (or at least the Python 3 path) are sometimes incompatible. That is, some things in Python 3.6 are different in Python 3.7.
I should point out that the changes between versions (Python 3.6 and 3.7, or even Python 2 and 3) are small. Most of the language remains the same across versions. If you know Python 2, you will find Python 3 familiar. (The familiarity may cause frustration as you stumble across one of the compatibility-breaking changes, though.)
Should we care? What does it mean for Python? What does it mean for programming in general?
One could argue that changes to a programming language are necessary. The underlying technology changes, and programming languages must "keep up". Thus, changes will happen, either in many small changes or one big change. The latter often is a shift away from one programming language to another. (One could cite the transition from FORTRAN to BASIC as computing changed from batch to interactive, for example.)
But that argument doesn't hold up against other evidence. COBOL, for example, has been popular for transaction processing and remains so. C and C++ have been popular for operating systems, device drivers, and other low-level applications, and remain so. Their backwards-compatible growth has not appreciably diminished their roles in development.
Other languages have gained popularity and remain popular too. Java and C# have strong followings. They, too, have not been hurt by backwards-compatibility.
Python is an opportunity to observe the behavior of the market. We have been working on the assumption that backwards-compatibility is desired by the user base. This assumption may be a false one, and the Python approach may be a good start to observe the true desires of the market. If successful (and Python is successful, so far) then we may see other languages adopt the "break a few things" philosophy for changes.
Of course, there may be some demand for languages that keep compatibility across versions. It may be a subset of the market, something that isn't visible with only one language breaking compatibility, but only visible when more languages change their approach. If that is the case, we may see some languages advertising their backwards-compatibility as a feature.
Who knows? It may be that the market demand for backwards-compatibility may come from Python users. As Python gains popularity (and it is gaining popularity), more and more individuals and organizations build Python projects, they may find Python's approach unappealing.
Let's see what happens!
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