Misconstrued Language Similarity Considered Harmful
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Very frequently I come across posts of the form Language X for Language
Y programmers. It's not that there is anything wrong with them, but,
more often than not, the underlying tone of such posts is to highlight
some apparent (and often misconstrued) similarities between the two
Objects in Java and processes in Erlang have some similarity in the sense that both of them abstract some state of your application. But that's where the similarity ends - the rest is all gaping differences. Objects in a class oriented OO language like Java and processes in a concurrency oriented language like Erlang are miles apart in philosophy, usage, implementation and granularity. Read Joe Armstrong's Why OO sucks for details. But for the purpose of this post, the following statement from Joe suffices to nip in the bud any logic that claims objects in Java are similar to processes in Erlang ..
"As Erlang became popular we were often asked "Is Erlang OO" - well, of course the true answer was "No of course not" - but we didn't to say this out loud - so we invented a serious of ingenious ways of answering the question that were designed to give the impression that Erlang was (sort of) OO (If you waved your hands a lot) but not really (If you listened to what we actually said, and read the small print carefully)."
Similarity breeds Contentment
It's always comforting to have a base of similarity. It's human nature to force find a similar base and transform one's thought process with respect to it. With programming languages, it works only if the two languages share the same philosophy and implement similar ideas. But you can never learn a new language by pitching it against your own favorite language and identifying apparent similarities. It is these apparent similarities that tend to influence how you think of the idioms of the new language and you will be misled into believing and practising something that will lead you to the path of antipatterns. Consider static typing and dynamic typing - a debate that has possibly been beaten to death. But when you are making the change, learn to think in the new paradigm. It's foolish to think in terms of concrete types in a dynamically typed setting. Think in terms of the contracts that the abstraction implemnents and organize your tests around them. Ola Bini wrote a fantastic post on this in response to a twitter discussion that originated from me.
The starting point should always be the philosophy and the philosophical differences that the two languages imbibe. Haskell typeclasses may seem similar in many respects to polymorphism in object-oriented languages. In fact it is more similar to parametric polymorphism, while the most dominant form of polymorphism in OO is subtype polymorphism. And Haskell being a functional language does not support subtyping.
This post relates pattern matching in Erlang and conditionals in Java in the same vein. It's true both of them offer some form of dispatch in program control. But the more significant and idiomatic difference that matters in this context is between the conditional statements in Java and expressions in the functional setting of Erlang. It's this expression based programming that influences the way you structure your code in Erlang. Instead of highlighting upfront the similarity of both constructs as means of flow control, emphasize on the difference in thought process that gives your program a different geometry. And finally, the most important use of pattern matching is in programming with algebraic data types, a whole new idiom that gets unveiled.
Ok, if you want to write a post on language X for prospective programmers of language Y, go ahead and highlight the differences in philosophy and idioms. And then try to implement your favorite feature from language Y in X. Misconstrued similarities often bias programmers new to language X the wrong way.
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