How to translate from a language to another?

I don'开发者_如何学编程t want an automated solution. When you have to translate a program from a language to another, what do you do? You prefer to rewrite it from the beginning or copy and paste it and change only what need to be changed? What's the best choice?

It depends on

• the goals (quick hack for one time use? long-lived production project for work?)

• the resources I have (how many man-hours? Test suit and/or functional spec for old code? familiarity with both languages?)

• most importantly, differences between the languages. Both the conceptual (OO? functional? reflection? control structures?) as well as available libraries.

  Please note that this bullet is not as trivial as it seems - this depends in large part on how idiomatic the original program is - as an example, some people write very "C-like" Perl code (e.g. using C control flow and very C++ like OO design) which can be trivially copied to C or C++, and some people write incredibly intricate idiomatic Perl using functional programming, closures and reflective capabilties; which can't be obviously translated into C/C++.

• Also, quality of the original code. E.g. good program will have separated business logic, usually expressed in standard configuration and control flow that's easier to directly clone.

E.g. translating from PHP to Perl for a hack job, you can often start out with copying, since many PHP constructs can be 1-to-1 mapped onto equivalent Perl constructs (just take your pick of Perl templating web library). The resulting code won't be GOOD Perl but will be Good Enough for some purposes.

On the other hand, translating, say, LISP code to Java, you're better off just translating the original code into a functionality specification and re-write from scratch. Your example of Python and JavaScript is probably in the same box.

Usually you have two languages that share at least some concepts (e.g. both have OO, and some imperative control structures) and thus you end up with some combination of the two approaches - parts of the code can be "thoughtlessly" translated, parts need to be re-written from scratch.

The more of the second (complete rewrite) approach, the better quality idiomatic and powerful code you end up with.

Normally, a rewrite using the original for inspiration and direction is the best choice.

The way you may do something in one language might very well not be the right way to do it in another.

When it comes to copy-paste, it is rare that you can just do that - languages are different and follow different syntax rules.

Of course, this all depends on the source and destination languages.

With your comment - javascript and python, I would say a rewrite is the best option.

That probably mostly depends on the similarity between the two languages and the meaning of "translation".

For instance, translating a bit of C89 code to C++ might not be so hard considering it should compile out of the box when you copy-paste (C++ is a compatible super-set to C89). I would hardly consider that a "translation", though.

On the other hand, translating Java to Haskell would certainly require a complete rewrite as the language paradygms, even worse than the syntax, are completely different.

Consider Wikipedia's list of programming languages. I'm too lazy to count how many of them are on this list, but let's assume there are 100.

If you want to translate one of them into another, that means that there are at least 100*99 = 9900 possible combinations for translation.

And that's an awful lot. Since most languages are unique, translation is very, very dependent on source and destination language.

Consider this Pascal to C converter. The author states it took him one and a half year to make a good translator for these particular languages. Obviously, this isn't a trivial task.

Depending on your ambitions, you might spend either one day, or many years translating a program from language A to language B.

How long this takes depends on your skill, size of your source code, complexity of languages A and B and their similarity.

As you can see, this isn't a trivial task and is highly dependent on your situation.