Tuesday, April 14, 2020

The COBOL-to-SomeBetterLang Translator

Here's a popular idea.
... a COBOL-to-X translator, where X is a more-modern programming language ...
This is a noble aspiration.

In principle -- down deep -- all programming can be reduced to an idealized Turing Machine.

This means that we *should* be able to locate all the state changes in a given spaghetti-bowl of COBOL. Given the abstract state transitions, we can emit a version of that machine in any language.

Emphasis on the *should*.

There are road-blocks.

The first two are rarities. But. When confronted with these, we'll have significant problems.

  • The ALTER statement means the code can be changed at run-time. There are constraints, but still... When the code is not static, the possible domain of state changes moves outside working storage and into the procedure division itself.
  • A data structure with a RENAMES clause. This adds a layer of alternative naming, making the data states quite a bit more complex.
The next one is a huge complication: the GOTO statement. This makes state transitions extremely difficult to analyze. It's possible to untangle any GOTO of arbitrary complexity into properly tested IF and WHILE statements. 

However. The tangle of GOTO's may have been actually meaningful. It may have carried some suggestion of a business owner's intent. A COBOL elimination algorithm may turn tangled code into opaque code. (It's also possible that it clarifies age-old bad programming.)

The ordinary REDEFINES clause. This was heavily used as a storage optimization for the tiny, slow file systems we had back in the olden days. It's a union of distinct types. And. It's a "free" union. We do not know how to distinguish the various types that are being redefined. It's intimately tied to processing logic in the procedure division.

Just to make it even more horrifying...

File layouts evolve over time. It's entirely possible for a *working*, *valid*, *in-production* file to have content that does not match any working program's DDE. The data has flags or indicators or something that lets the app glide past the bad data. But the data is bad. It used to be good. Then something changed, and now it's almost uninterpretable. But the apps work because there are enough paths through the logic to make the row "work" without it matching any file layout in any obvious way.

I'm not sure an automated translation from COBOL is of any value. 

I think it's far better to start with file layouts, review the code, and then write new code from scratch in a modern language. This manual rewrite leads directly to small programs that -- in a modern language -- are little more than class definitions. In some cases, each legacy COBOL app would like becomes a Python module.

Given snapshots of legacy files, the Python can be tested to be sure it does the same things. The processing is not nuanced, or tricky, or even particularly opaque.

The biggest problem is the knowledge captured in COBOL code tends to be disorganized. The real work is disentangling it. A language that supports ruthless refactoring will be helpful.

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