Pseudocode interpreter?

Question

Like lots of you guys on SO, I often write in several languages. And when it comes to planning stuff, (or even answering some SO questions), I actually think and write in some unspecified hybrid language. Although I used to be taught to do this using flow diagrams or UML-like diagrams, in retrospect, I find "my" pseudocode language has components of C, Python, Java, bash, Matlab, perl, Basic. I seem to unconsciously select the idiom best suited to expressing the concept/algorithm.

Common idioms might include Java-like braces for scope, pythonic list comprehensions or indentation, C++like inheritance, C#-style lambdas, matlab-like slices and matrix operations.

I noticed that it's actually quite easy for people to recognise exactly what I'm triying to do, and quite easy for people to intelligently translate into other languages. Of course, that step involves considering the corner cases, and the moments where each language behaves idiosyncratically.

But in reality, most of these languages share a subset of keywords and library functions which generally behave identically - maths functions, type names, while/for/if etc. Clearly I'd have to exclude many 'odd' languages like lisp, APL derivatives, but...

So my questions are,

1. Does code already exist that recognises the programming language of a text file? (Surely this must be a less complicated task than eclipse's syntax trees or than google translate's language guessing feature, right?) In fact, does the SO syntax highlighter do anything like this?

2. Is it theoretically possible to create a single interpreter or compiler that recognises what language idiom you're using at any moment and (maybe "intelligently") executes or translates to a runnable form. And flags the corner cases where my syntax is ambiguous with regards to behaviour. Immediate difficulties I see include: knowing when to switch between indentation-dependent and brace-dependent modes, recognising funny operators (like *pointer vs *kwargs) and knowing when to use list vs array-like representations.

3. Is there any language or interpreter in existence, that can manage this kind of flexible interpreting?

4. Have I missed an obvious obstacle to this being possible?

edit

Thanks all for your answers and ideas. I am planning to write a constraint-based heuristic translator that could, potentially, "solve" code for the intended meaning and translate into real python code. It will notice keywords from many common languages, and will use syntactic clues to disambiguate the human's intentions - like spacing, brackets, optional helper words like let or then, context of how variables are previously used etc, plus knowledge of common conventions (like capital names, i for iteration, and some simplistic limited understanding of naming of variables/methods e.g containing the word get, asynchronous, count, last, previous, my etc). In real pseudocode, variable naming is as informative as the operations themselves!

Using these clues it will create assumptions as to the implementation of each operation (like 0/1 based indexing, when should exceptions be caught or ignored, what variables ought to be const/global/local, where to start and end execution, and what bits should be in separate threads, notice when numerical units match / need converting). Each assumption will have a given certainty - and the program will list the assumptions on each statement, as it coaxes what you write into something executable!

For each assumption, you can 'clarify' your code if you don't like the initial interpretation. The libraries issue is very interesting. My translator, like some IDE's, will read all definitions available from all modules, use some statistics about which classes/methods are used most frequently and in what contexts, and just guess! (adding a note to the program to say why it guessed as such...) I guess it should attempt to execute everything, and warn you about what it doesn't like. It should allow anything, but let you know what the several alternative interpretations are, if you're being ambiguous.

It will certainly be some time before it can manage such unusual examples like @Albin Sunnanbo's ImportantCustomer example. But I'll let you know how I get on!

Answer 1

I think that is quite useless for everything but toy examples and strict mathematical algorithms. For everything else the language is not just the language. There are lots of standard libraries and whole environments around the languages. I think I write almost as many lines of library calls as I write "actual code".

In C# you have .NET Framework, in C++ you have STL, in Java you have some Java libraries, etc.

The difference between those libraries are too big to be just syntactic nuances.

<subjective>
There has been attempts at unifying language constructs of different languages to a "unified syntax". That was called 4GL language and never really took of.
</subjective>

As a side note I have seen a code example about a page long that was valid as c#, Java and Java script code. That can serve as an example of where it is impossible to determine the actual language used.

Edit:

Besides, the whole purpose of pseudocode is that it does not need to compile in any way. The reason you write pseudocode is to create a "sketch", however sloppy you like.

foreach c in ImportantCustomers{== OrderValue >=$1M}
    SendMailInviteToSpecialEvent(c)

Now tell me what language it is and write an interpreter for that.

Answer 2

1. To detect what programming language is used: http://stackoverflow.com/questions/475033/detecting-programming-language-from-a-snippet
2. I think it should be possible. The approach in 1. could be leveraged to do this, I think. I would try to do it iteratively: detect the syntax used in the first line/clause of code, "compile" it to intermediate form based on that detection, along with any important syntax (e.g. begin/end wrappers). Then the next line/clause etc. Basically write a parser that attempts to recognize each "chunk". Ambiguity could be flagged by the same algorithm.
3. I doubt that this has been done ... seems like the cognitive load of learning to write e.g. python-compatible pseudocode would be much easier than trying to debug the cases where your interpreter fails.
4. a. I think the biggest problem is that most pseudocode is invalid in any language. For example, I might completely skip object initialization in a block of pseudocode because for a human reader it is almost always straightforward to infer. But for your case it might be completely invalid in the language syntax of choice, and it might be impossible to automatically determine e.g. the class of the object (it might not even exist). Etc.
   b. I think the best you can hope for is an interpreter that "works" (subject to 4a) for your pseudocode only, no-one else's.

Note that I don't think that 4a,4b are necessarily obstacles to it being possible. I just think it won't be useful for any practical purpose.

Answer 3

Recognizing what language a program is in is really not that big a deal. Recognizing the language of a snippet is more difficult, and recognizing snippets that aren't clearly delimited (what do you do if four lines are Python and the next one is C or Java?) is going to be really difficult.

Assuming you got the lines assigned to the right language, doing any sort of compilation would require specialized compilers for all languages that would cooperate. This is a tremendous job in itself.

Moreover, when you write pseudo-code you aren't worrying about the syntax. (If you are, you're doing it wrong.) You'll wind up with code that simply can't be compiled because it's incomplete or even contradictory.

And, assuming you overcame all these obstacles, how certain would you be that the pseudo-code was being interpreted the way you were thinking?

What you would have would be a new computer language, that you would have to write correct programs in. It would be a sprawling and ambiguous language, very difficult to work with properly. It would require great care in its use. It would be almost exactly what you don't want in pseudo-code. The value of pseudo-code is that you can quickly sketch out your algorithms, without worrying about the details. That would be completely lost.

If you want an easy-to-write language, learn one. Python is a good choice. Use pseudo-code for sketching out how processing is supposed to occur, not as a compilable language.

Answer 4

I have a feeling that the answer to 2. is NO. All I need to prove it false is a code snippet that can be interpreted in more than one way by a competent programmer.

Answer 5

An interesting approach would be a "type-as-you-go" pseudocode interpreter. That is, you would set the language to be used up front, and then it would attempt to convert the pseudo code to real code, in real time, as you typed. An interactive facility could be used to clarify ambiguous stuff and allow corrections. Part of the mechanism could be a library of code which the converter tried to match. Over time, it could learn and adapt its translation based on the habits of a particular user.

People who program all the time will probably prefer to just use the language in most cases. However, I could see the above being a great boon to learners, "non-programmer programmers" such as scientists, and for use in brainstorming sessions with programmers of various languages and skill levels.

-Neil

Answer 6

Programs interpreting human input need to be given the option of saying "I don't know." The language PL/I is a famous example of a system designed to find a reasonable interpretation of anything resembling a computer program that could cause havoc when it guessed wrong: see http://horningtales.blogspot.com/2006/10/my-first-pli-program.html

Note that in the later language C++, when it resolves possible ambiguities it limits the scope of the type coercions it tries, and that it will flag an error if there is not a unique best interpretation.

Answer 7

Does code already exist that recognises the programming language of a text file?

Yes, the Unix file command.

(Surely this must be a less complicated task than eclipse's syntax trees or than google translate's language guessing feature, right?) In fact, does the SO syntax highlighter do anything like this?

As far as I can tell, SO has a one-size-fits-all syntax highlighter that tries to combine the keywords and comment syntax of every major language. Sometimes it gets it wrong:

def median(seq):
    """Returns the median of a list."""
    seq_sorted = sorted(seq)
    if len(seq) & 1:
        # For an odd-length list, return the middle item
        return seq_sorted[len(seq) // 2]
    else:
        # For an even-length list, return the mean of the 2 middle items
        return (seq_sorted[len(seq) // 2 - 1] + seq_sorted[len(seq) // 2]) / 2

Note that SO's highlighter assumes that // starts a C++-style comment, but in Python it's the integer division operator.

This is going to be a major problem if you try to combine multiple languages into one. What do you do if the same token has different meanings in different languages? Similar situations are:

• Is ^ exponentiation like in BASIC, or bitwise XOR like in C?
• Is || logical OR like in C, or string concatenation like in SQL?
• What is 1 + "2"? Is the number converted to a string (giving "12"), or is the string converted to a number (giving 3)?

Is there any language or interpreter in existence, that can manage this kind of flexible interpreting?

On another forum, I heard a story of a compiler (IIRC, for FORTRAN) that would compile any program regardless of syntax errors. If you had the line

= Y + Z

The compiler would recognize that a variable was missing and automatically convert the statement to X = Y + Z, regardless of whether you had an X in your program or not.

This programmer had a convention of starting comment blocks with a line of hyphens, like this:

C ----------------------------------------

But one day, they forgot the leading C, and the compiler choked trying to add dozens of variables between what it thought was subtraction operators.

"Flexible parsing" is not always a good thing.