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Writing first interpreter with Python - choice of OCaml language

I am reading code for Scheme interpreters with Python by P. Norvig, and I would like to try to write an interpreter with Python. This is properly the subject of post: What language can a junior programmer implement an interpreter for it?. However, I am strongly interested in learning OCaml for a long time, and it could be the right occasion if I make up my mind for OCaml.
Should I expect much effort to implement a Python interpreter of OCaml language? This is just a personal project and my aim (besides learning OCaml), is mainly to practice the things I have read on compilers and interpreters.
If OCaml is not suited, what would you personally advice?
Could you hint at good ref for OCaml (or even: writting an OCampl parser/interpreter).
## EDIT
What about Lua / Ruby / BASIC ? (because I want to learn those as well)
Thanks and regards

You seem to be saying you want to create an OCaml interpreter in Python (not a Python interpreter in OCaml, right?). OCaml per se is too large a language to choose for an educational project, in my opinion. I would choose a much smaller language. That's why Scheme is a good choice--the core language is quite small.
With OCaml you also have type inference, which is excellent to learn about but again is a rather large topic.
For something small and somewhat ML-ish you might start with the untyped lambda calculus. It's an extremely common test case.

If you want a good introduction to the semantics underpinning of OCaml (a bit of the theory), you should have a look at the book Using, Understanding, and Unraveling
The OCaml Language by Didier Rémy.
Writing an implementation of the full OCaml language would be quite some work, but writing an implementation of a reasonable subset of it is doable, for example, as a semester project.
Given than ML languages are very good at symbolic manipulations (interpreters, compilers, analysers, proof checkers...), it is probably more fun to write a Python implementation in OCaml than an OCaml implementation in Python -- which may explain some confusion around your question.

You can start reading SICP of Gerald Sussman from MIT, and in the 4th chapter you will build a few interpreters for different purposes. In the 5th chapter you will build compilers.
Also, try reading the source code of GNU/Emacs, which has a nice interpreter of elisp.
Also, you can subscribe to Compilers course of coursera.org, ad build there a quite complex interpreter.
I suggest you to write an interpreter of scheme/lisp, because you do not have to cope with parsing, and python works exactly the same as these.

How to write an interpreter (in Python)
This is a video I made a while ago, after giving a similar talk at a local Python meetup. In the video I implement a small functional language in Python in under an hour. This might be a good place to start .

Learning Ruby from Python; Differences and Similarities

Inspired by Learning Python from Ruby; Differences and Similarities.
I'm in the exact opposite boat - I'm pretty well-versed in Python, but I need to start learning Ruby soon (and Rails later, but that's another topic). The question I'm linking to addresses this a bit, but are there any other resources out there that teach Ruby for someone who already has Python experience? (In other words, someone with a background in programming, but who doesn't yet know the difference between code that IRB will execute properly and proper Ruby code.)
EDIT: I'm not just looking for a list of the differences between the two languages - I'm wondering if there are any guides/walkthroughs that are directed towards teaching Python programmers to write idiomatic Ruby. I know similar guides exist for migrating to Python from other languages, but I don't know about Ruby.

While not targeted at Python programmers, you might find this idomatic Ruby talk useful.
(Related, but for Python: Code Like a Pythonista: Idiomatic Python)

The Ruby website has a page exactly on this topic. It's a summary list, so other answers with more best-practice style comments are surely a good idea.

Learn a scripting language besides Python [closed]

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Closed 10 years ago.
Someone told me once, that programmers tend to learn one scripting language properly and ignore or dislike other scripting languages. Do you have similar experiences?
I'm using Python as my choice for scripting for few years, however, I'm sure that there are many existing and emerging languages that could impress the Pythonistas. Can you recommend scripting languages that would be interesting and useful to learn besides of Python?

Look, Python pretty much has all you need (in my opinion) for application programming. You can write anything from a protocol stack to YouTube, from media players to 3D games and graphics and you get excellent performance.
It occupies the same niche as some of these other mentioned languages:
C, you have access to almost all of the useful C/C++ libraries. The only reason I would pick to write something in C over Python is because I needed the performance gain. Even then, I would probably prototype it in Python first; it's much easier to revise your design when your application is written in Python.
Ruby, there is no good reason to ever use Ruby instead of Python.
Perl, it's great for some particular kinds of tasks, but if you're a fan of consistent, readable and sane programming styles you will hate looking at about 95% of existing Perl code. I don't know if this is because the people who program in Perl tend to be (in my experience) sys admins first and programmers second, or because Perl has a design philosophy that allows for multiple distinct ways to achieve the same effect.
Given that, I would say that if you are going to learn another language, make sure it gives you the ability to do something new. There are two scripting languages that I would recommend for you to learn:
Bash, what a joy it is to manipulate your filesystem with a combination of for loops and pipes. Bash programming doesn't give you more than what you can already do with Python, but if you are a *nix user you will experience great gains in your daily productivity.
Javascript, being able to write browser-based applications is a useful skill and almost definitely the way most applications will be done in the future. The Javascript/browser environment is set to gain a whole host of capabilities in the coming few years, from audio manipulation to OpenGL graphics, and some very fast engines are either in the works or already available (like V8, which powers the Chrome browser and compiles Javascript to native byte code.) Have you seen Quake2 ported to WebGL?
My answer basically boils down to this: first, learn languages that are useful.

Ruby - what it enables and does with blocks is really interesting, and quite foreign to python based programming
Erlang - the functional language has a lot of interesting examples and it will definitely make your head work differently afterwards (in a good way)
Javascript - yes, I'm serious. ALthough there's a fair number of grips to be had with this prototype language, it does some really interesting things with that prototyping and just slightly differently than Ruby and/or Python. And a ton of folks are pouring big money into making Javascript a outstandingly fast scripting language.

I would recommend learning Haskell and a dialect of Lisp such as Scheme or Common Lisp, if you master either of those you'll gain insight into how things are accomplished with the functional paradigm and it'll help out your Python as well.
Here are some languages categorized by paradigms I'd learn:
Imperative/Procedural languages:
C
Functional paradigm languages:
Haskell
Common Lisp/Scheme
Similar object oriented languages:
Ruby
ECMAScript
Other:
Perl
I would advise you to stay away from PHP unless you really need the work. You would probably want to run back to Python.

Scripting languages are so similar that the marginal benefit of moving from one scripting language to another is usually low. So it's unsurprising that people wouldn't bother to learn more than one. Nevertheless, in my career I have passed through times when my main scripting language (in roughly chronological order) was
Awk
Tcl
Icon
Ksh
Lua
I also used Perl and Python but never found them enough better to be worth switching to.
If you want to check out another scripting language, I recommend Lua, because
It's powerful and remarkably simple, having the best power-to-weight ratio of all languages named here.
Like Tcl it was designed from the beginning to incorporate C code seamlessly. This facility works extremely well and greatly extends the range of problems for which it is useful (see Adobe Lightroom, World of Warcraft, Garry's Mod, CHDK).
The implementation is highly performant and brilliantly engineered. If you want to learn something about how languages are implemented, it will repay careful study.
If, however, your goal is to learn a new language to expand your mind, learn something else besides a scripting language. For example, learn Haskell and pick up some mind-blowing ideas (many stolen from the same sources that Guido stole from), or learn C and really understand exactly what's happening on the hardware.

The only relatively unbiased answer you can really look for is probably statistical, and you would still have to account for the natural tendency of people to follow the path of least resistance once one is found or carved.
How many people learnt Python to a decent level, found the language resonates with the way they want to work, then move to something else because the language or the ecosystem, or both, don't support their needs?
I'd say probably a single digit percentage of the educated userbase, wouldn't be surprised if it amounted to less than 5%.
Unless you have work related prospects that involve a different language, or you need to move sideways for similar reasons, I'd say you're probably best off learning something complimentary to Python rather than similar or equivalent.
C++ for low-level or computationally intensive tasks, CUDA if your field can take advantage of it (med-viz, CGI etc.), whatever flavour of shell/sysadmin oriented scripting and hacks float where you work (bash, tcl, awk or whatever else) and so on.
Personally the reason I haven't bothered past a first glance with ruby, php, or a number of other languages is simply that it's better ROI to keep working on my python skills than picking up something that offers mostly the same qualities just in different forms.
If you really want to learn something else for the sake of opening your mind up a bit, and want to stick to "scripting", then LUA was an interesting toy for me for a while, mostly for the ridiculous performance you can squeeze out of a relatively easy integration process, and because it is a rather different set of tracks compared to Python. That, and the fact WoW plugins had to be written in LUA ;)

I'll give an honest answer from my perspective.
No.
Having started scripting using batch, bash, and Perl, discovering Python was discovering precisely what I'd want from a scripting language (and more, but that's off topic). It integrates with familiar Unix interfaces, is modular, doesn't force any particular paradigm, cross platform and under active development. The same can be said of no other scripting language I know of.
The only other scripting languages I'd consider using is Lua or Scheme, for their smaller footprints and suitability for embedding, Python can be a little hefty. However they're hardly suitable for the more general purpose shell and other forms of scripting.
Update0
I just noticed mentions of Ruby and PHP in other answers, these both slipped my mind, because I'd never consider using them. Ruby is slower and not quite as popular, and PHP is more C/Perl like, with flatter interfaces, which comes with performance boons of its own. Using these alternatives to Python is a matter of taste.

To answer your first question: Do people learn one language and then ignore or dislike others?
Well, if you know one language well, you will need to see great advantages to move to another.
I started out using perl and eventually thought that there must be easier way to do some things. I picked up python and stopped using perl almost at once.
A little while later I thought I'd try ruby and learned a bit about that. The advantages over using python weren't big enough to switch, so I decided to stick with python. If I had started out using ruby, I'd probably be using that still.
If you are using python, I don't think you will easily find another scripting language that will win you over.
On the other hand, if you learn functional programming, you will probably learn a few new things, some of them will even be useful in your python programming, since a few things in python seems to be inspired by functional programming and knowing how to use them will make you a better programmer in general and a better python programmer too.

Learn a Lisp. Whether it's "scripting" or not, Eric Raymond had the right of it when he wrote:
"Lisp is worth learning for the
profound enlightenment experience you
will have when you finally get it;
that experience will make you a better
programmer for the rest of your days,
even if you never actually use Lisp
itself a lot."
The programming paradigm needed to be highly effective in Lisp is sufficiently unlike what you use with Python day-to-day that the perspective it gives is very, very much worth it.
And within Lisps, my choice? Clojure; like other Lisps, its macro system gives you capabilities comparable (actually superior) to the excellent metaprogramming in Python, but Clojure in particular has a focus on batteries-included practicality (and an intelligent, opinionated design) which will be familiar to anyone fond of GvR's instincts. Moreover, Clojure's strengths are extremely disjoint from Python's -- in particular, it shines at highly-multithreaded, CPU-bound concurrent programming, which is one of Python's weaknesses -- so having both in your toolbox increases the chance you'll have the right tool when a tricky job comes along.
(Is it scripting? In my view, that's pretty academic these days; if you have a REPL where you can type code and get an immediate response, modify the state of a running program, or experiment with an API, I see a language as "scripting" enough).

I would learn a statically typed language with very powerful type expression capabilities and awesome concurrency.
One of the following would be a good choice (in order of my preference):
Scala
F#
Haskell
Ocaml
Erlang
Typed languages like the above make you think different. Also these languages have REPLs so they can be used as a scripting language although truthfully I'm not really sure what the definition is of "scripting" language is.
Python is missing good concurrency builtin to the language so knowing how to deal with concurrency for many python programmers is a challenge.
I have found that strongly typed languages scale better for big projects for many reasons:
Because types are so important they become an invaluable way to communicate the problem
Refactoring in these languages is much much easier.
Automatic Serialization is sometimes easier too (although for Haskell thats less true).
A lot less time spent on writing assertions on type checking.
Browsing the code is easier because most IDEs will allow you click on and go to different types

I'm actually learning Scala after Python. From "Programming in Scala":
The name Scala stands for “scalable language.” The language is so named because it was designed to grow with the demands of its users. You can apply Scala to a wide range of programming tasks, from writing small scripts to building large systems.
Integration of object-oriented and functional programming inside the language with expressive strong static type system is interesting by itself. And yes, you can use Scala as scripting language. I feel uncomfortable coding in languages with dynamic typing discipline so Scala seems to be a good alternative. Besides its complexity at the initial learning stage.
If you satisfied with dynamic typing discipline take a look at the roots. Smalltalkof course. Try Squeak with Squeak by Example companion book or its open-source fork Pharo with Pharo by Example book for the start.

Ruby/Groovy/Perl if you'd like to stick to traditional scripting practices.
Otherwise I'd heartily recommend you Clojure and Scala - two of the more innovative programing languages of the past few years.

If you are already familiar with Python, you are unlikely to find something compelling in the same niche, although Ruby does have a very strong and vocal following that seems to like it very much. Perhaps you should consider a scripting language that fills a different role, such as BASH shell script for quick, simple scripts that don't need the complexity of Python or JavaScript which runs in the browser.

I can't say that I agree with wiping Ruby off the map... Ruby fixed every problem that perl had as far as syntax goes... I loved Python first but let ruby get a little more mature and it will get in the the fray more and more... Why do I support Ruby strongly? just step away from python for a few months and then give Ruby a chance... I was a Ruby hater when I was a python guy. But I can't hardly stand to use python at this point. One day someone is gonna clean up the GC and toss in some native threads and everybody better watch out.
off the rant, Python is a full featured, not just good, Great Language... Perl... what a mess... I don't know how Perl can look at itself in the mirror standing next to any other mainstream scripting language... PHP is much prettier... At least Perl is fast, right...(CPAN never hurt it either) if Speed is the real issue there are other interpreters that juice it up a bit... Jython, jRuby, PyPy... the list goes one, screw Bash...

General questions regarding Python language

I'm a newbie to programming and I've decided to start with Python. Just curious though, is it enough/recommended to learn Python from online tutorials or from books? I want to go further than simple "Hello World!" programs. I'm not sure if books will actually teach you how to make more advanced programs.
One example is Exif-py. How do you even start programming a program like this? Do you just sit down and start writing the code, or do you have to search for APIs or anything? I went through the code briefly and it all looked alien to me.
I'm not really sure how to express my questions into words, so do check back because I'll edit my question if the right words come to me.

Well, I learnt all my Python from online sources (not just tutorials, but reference documentation, blog posts and other texts). It's certainly possible, although some people prefer the "guided" way a book teaches you, particularly people new to programming (at that point I had already been programming for years).
To create a program such as Exif.py, you would first have to know what you must do in broad terms BEFORE starting to program. You would study the EXIF format, then figure out how that is put into the image files, then you would have to formulate that in terms of the language you're using (in this case, Python). This usually requires that you're already familiar with it, otherwise it'll be a fairly slow process.
I'd suggest starting with simpler programs to begin with, or maybe follow a book such as Dive Into Python (free online), seeing as you're new to programming, and need to not only learn the language, but to think like a programmer.

The tutorials are there to teach you the language syntax and the standard library, not really on how to solve a particular programming problem. Think of what you learn there as your toolbox.
I think I understand what you are saying. You want to break above and beyond the simple applications and write your own stuff, right? Well, first you need to figure out what it is you want to make. Then comes the hard part; how are you going to make it? I'd suggest starting by trying to break it down into a number of small simple problems instead of tackling it as one large problem.
If you are trying to find examples of larger projects, I'd suggest looking at the Python Cheeseshop (package index) and download a few packages you've heard of to see how they did it. Also, people often post handy pieces of code on their personal blogs and that shows up on PlanetPython.

If stuff like recursion, modules, classes, iteration, exceptions, dictionaries, are indeed new to you, I suggest How to Think Like a Computer Scientist in Python (lovingly abbreviated as ThinkCSPy in the community ;-).
It is a bit outdated - for best results, use Python 2.6 or lower, but then again 3.0 is not that different.
EDIT: If ThinkCSPy has little to teach you, try Dive Into Python. It's advanced and pretty "real-world", but step-by-step.

If by 'newbie to programming' you mean that you just started it last week or something along the lines, then maybe you might want to give the MIT OpenCourseWare Introduction to Computer Science videos (and homework!) a bit of your time.
The opencourse covers the syntax of the Python language, some helpful hints and general do's and don'ts that apply to any programming paradigm.
Though, if by 'newbie to programming', that you already know the basic concepts you need to start writing basic programs (like adding, subtracting, multiplying, logical operations, functions etc).
If so, Project Euler is particularly good for exercising that cranial muscle in problem solving via programming, though most of the problems on the site require you at also have knowledge of mathematics (and if you don't, at least know how to google).
Just remember, if you get stuck on anything don't get put down! It's all part of learning! If you're really stuck, search SO!

There are a few very good online resources:
The Python Tutorial.
Dive Into Python. Python from novice to pro.
Code Like a Pythonista: Idiomatic Python
Another list of resources: Essential Python Reading List.
Most Python books I've seen so far are "not that good" - but that depends
on your background / prior knowledge about Python and programming.

You study the EXIF format, study the GIF/JPEG format, open the binary file, scan it and get the data out.

What language should I learn as a bridge to C (and derivatives) [closed]

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The first language I learnt was PHP, but I have more recently picked up Python. As these are all 'high-level' languages, I have found them a bit difficult to pick up. I also tried to learn Objective-C but I gave up.
So, what language should I learn to bridge between Python to C

It's not clear why you need a bridge language. Why don't you start working with C directly? C is a very simple language itself. I think that hardest part for C learner is pointers and everything else related to memory management. Also C lang is oriented on structured programming, so you will need to learn how to implement data structures and algorithms without OOP goodness. Actually, your question is pretty hard, usually people go from low level langs to high level and I can understand frustration of those who goes in other direction.

The best place to start learning C is the book "The C Programming Language" by Kernighan and Ritchie.
You will recognise a lot of things from PHP, and you will be surprised how much PHP (and Perl, Python etc) do for you.
Oh and you also will need a C compiler, but i guess you knew that.

I generally agree with most of the others - There's not really a good stepping stone language.
It is, however, useful to understand what is difficult about learning C, which might help you understand what's making it difficult for you.
I'd say the things that would prove difficult in C for someone coming from PHP would be :
Pointers and memory management This is pretty much the reason you're learning C I imagine, so there's not really any getting around it. Learning lower level assembly type languages might make this easier, but C is probably a bridge to do that, not the other way around.
Lack of built in data structures PHP and co all have native String types, and useful things like hash tables built in, which is not the case in C. In C, a String is just an array of characters, which means you'll need to do a lot more work, or look seriously at libraries which add the features you're used to.
Lack of built in libraries Languages like PHP nowadays almost always come with stacks of libraries for things like database connections, image manipulation and stacks of other things. In C, this is not the case other than a very thin standard library which revolves mostly around file reading, writing and basic string manipulation. There are almost always good choices available to fill these needs, but you need to include them yourself.
Suitability for high level tasks If you try to implement the same type of application in C as you might in PHP, you'll find it very slow going. Generating a web page, for example, isn't really something plain C is suited for, so if you're trying to do that, you'll find it very slow going.
Preprocessor and compilation Most languages these days don't have a preprocessor, and if you're coming from PHP, the compilation cycle will seem painful. Both of these are performance trade offs in a way - Scripting languages make the trade off in terms of developer efficiency, where as C prefers performance.
I'm sure there are more that aren't springing to mind for me right now. The moral of the story is that trying to understand what you're finding difficult in C may help you proceed. If you're trying to generate web pages with it, try doing something lower level. If you're missing hash tables, try writing your own, or find a library. If you're struggling with pointers, stick with it :)

Learning any language takes time, I always ensure I have a measurable goal; I set myself an objective, then start learning the language to achieve this objective, as opposed to trying to learn every nook and cranny of the language and syntax.
C is not easy, pointers can be hard to comprehend if you’re not coming assembler roots. I first learned C++, then retro fit C to my repertoire but I started with x86 and 68000 assembler.

Python is about as close to C as you're going to get. It is in fact a very thin wrapper around C in a lot of places. However, C does require that you know a little more about how the computer works on a low level. Thus, you may benefit from trying an assembly language.
LC-3 is a simple assembly language with a simulated machine.
Alternatively, you could try playing with an interactive C interpreter like CINT.
Finally, toughing it out and reading K&R's book is usually the best approach.

Forget Java - it is not going to bring you anywhere closer to C (you have allready proved that you don't have a problem learning new syntax).
Either read K&R or go one lower: Learn about the machine itself. The only tricky part in C is pointers and memory management (which is closely related to pointers, but also has a bit to do with how functions are called). Learning a (simple, maybe even "fake" assembly) language should help you out here.
Then, start reading up on the standard library provided by C. It will be your daily bread and butter.
Oh: another tip! If you really do want to bridge, try FORTH. It helped me get into pointers. Also, using the win32 api from Visual Basic 6.0 can teach you some stuff about pointers ;)

C is a bridge onto itself.
K&R is the only programming language book you can read in one sitting and almost never pick it up again ...

My suggestion is to get a good C-book that is relevant to what you want to do. I agree that K & R is considered to be "The book" on C, but I found "UNIX Systems Programming" by Kay A. Robbins and Steven Robbins to be more practical and hands on. The book is full of clean and short code snippets you can type in, compile and try in just a few minutes each.
There is a preview at http://books.google.com/books?id=tdsZHyH9bQEC&printsec=frontcover (Hyperlinking it didn't work.)

I'm feeling your pain, I also learned PHP first and I'm trying to learn C++, it's not easy, and I am really struggling, It's been 2 years since I started on c++ and Still the extent of what I can do is cout, cin, and math.
If anyone reads this and wonders where to start, START LOWER.

Java might actually be a good option here, believe it or not. It is strongly based on C/C++, so if you can get the syntax and the strong typing, picking up C might be easier. The benefit is you can learn the lower level syntax without having to learn pointers (since memory is managed for you just like in Python and PHP). You will, however, learn a similar concept... references (or objects in general).
Also, it is strongly Object Oriented, so it may be difficult to pick up on that if you haven't dealt with OOP yet.... you might be better off just digging in with C like others suggested, but it is an option.

I think C++ is a good "bridge" to C. I learned C++ first at University, and since it's based on C you'll learn a lot of the same concepts - perhaps most notably pointers - but also Object Oriented Design. OO can be applied to all kinds of modern languages, so it's worth learning.
After learning C++, I found it wasn't too hard to pick up the differences between C++ and C as required (for example, when working on devices that didn't support C++).

try to learn a language which you are comfortable with, try different approach and the basics.

Languages are easy to learn (especially one like C)... the hard part is learning the libraries and/or coding style of the language. For instance, I know C++ fairly well, but most C/C++ code I see confuses me because the naming conventions are so different from what I work with on a daily basis.
Anyway, I guess what I'm trying to say is don't worry too much about the syntax, focus on said language's library. This isn't specific to C, you can say the same about c#, vb.net, java and just about every other language out there.

Pascal! Close enough syntax, still requires you to do some memory management, but not as rough for beginners.