I'm writing a scanner as part of a compiler.
I'm having a major headache trying to write this one portion:
I need to be able to parse a stream of tokens and push them one by one into a vector, ignoring whitespace and tokenizing special symbols (simple case, lets just consider parentheses and braces)
Example:
int main(){ }
should parse into 6 different tokens:
int
main
(
)
{
}
How would you go about solving this? I'm writing this in C++, but a java/C# solution would be appreciated as well.
Some points:
and no, I can't use Boost, I can't guarantee that the libraries will be
available to me. (don't ask...)
I don't want to use lex, or any other special tools. I've never done
this before and just want to try this once to say I've done it.
Stroustrup's book, The C++ Programming Language, has a great example in it about building a lexer/parser for a simple calculator program. It should serve as a good starting point to learn how to do what you want.
Buy a copy of Compilers: Principles, Techniques, and Tools (the Dragon Book). What you're attempting to write is a lexer, not a "scanner".
Why write your own - look at Lex.
If youmust have your own, you just read the input character by character and maintain some minimum state to accumulate identifiers.
The problem itself is not hard. If you can't solve it, you must be burned out, you just need a rest. Look at it again in the morning.
If you really want to learn something from this exercise, just start coding. It doesn't demand a lot of code, so you can fail repeatedly without blowing more than an afternoon.
At this point you'll have a good feel for the problem.
Then look in any random compilers book to see what the "usual" methods are, and you'll grok then immediately.
umm.. I'd just do a while loop with iterators testing each character for type, and only an alpha to non alpha change, dump the string if it's non empty. if it's a non alpha non white space character, I'd just push it onto the token stack, this is really a trivial parsing task. Shoot, I've been meaning to learn lexx/yacc, but the level of parsing you want is really easy. I wrote a html tokenizer once which is more complicated that this.. I mean you are just looking for names, white space and single non alphanumeric characters.. just do it.
If you want to write this from scratch, you could look into writing a finite state machine (states in an enum, a big switch/case block for state switching). You'd have to push the state to a stack since everything can be nested.
I know that this is not the ideal method; I'm just trying to directly address the question.
Related
I am wondering if are there any examples (googling I haven't found any) of TAB auto-complete solutions for Command Line Interface (console), that use ANTLR4 grammars for predicting the next term (like in a REPL model).
I've written a PL/SQL grammar for an open source database, and now I would like to implement a command line interface to the database that provides the user the feature of completing the statements according to the grammar, or eventually discover the proper database object name to use (eg. a table name, a trigger name, the name of a column, etc.).
Thanks for pointing me to the right direction.
Actually it is possible! (Of course, based on the complexity of your grammar.) Problem with auto-completion and ANTLR is that you do not have complete expression and you want to parse it. If you would have complete expression, it wont be any big problem to know what kind of element is at what place and to know what can be used at such a place. But you do not have complete expression and you cannot parse the incomplete one. So what you need to do is to wrap the input into some wrapper/helper that will complete the expression to create a parse-able one. Notice that nothing that is added only to complete the expression is important to you - you will only ask for members up to last really written character.
So:
A) Create the wrapper that will change this (excel formula) '=If(' into '=If()'
B) Parse the wrapped input
C) Realize that you are in the IF function at the first parameter
D) Return all that can go into that place.
It actually works, I have completed intellisense editor for several simple languages. There is much more infrastructure than this, but the basic idea is as I wrote it. Only be careful, writing the wrapper is not easy if not impossible if the grammar is really complex. In that case look at Papa Carlo project. http://lakhin.com/projects/papa-carlo/
As already mentioned auto completion is based on the follow set at a given position, simply because this is what we defined in the grammar to be valid language. But that's only a small part of the task. What you need is context (as Sam Harwell wrote: it's a semantic process, not a syntactic one). And this information is independent of the parser. And since a parser is made to parse valid input (and during auto completion you have most of the time invalid input), it's not the right tool for this task.
Knowing what token can follow at a given position is useful to control the entire process (e.g. you don't want to show suggestions if only a string can appear), but is most of the time not what you actually want to suggest (except for keywords). If an ID is possible at the current position, it doesn't tell you what ID is actually allowed (a variable name? a namespace? etc.). So what you need is essentially 3 things:
A symbol table that provides you with all possible names sorted by scope. Creating this depends heavily on the parsed language. But this is a task where a parser is very helpful. You may want to cache this info as it is time consuming to run this analysis step.
Determine in which scope you are when invoking auto completion. You could use a parser as well here (maybe in conjunction with step 1).
Determine what type of symbol(s) you want to show. Many people think this is where a parser can give you all necessary information (the follow set). But as mentioned above that's not true (keywords aside).
In my blog post Universal Code Completion using ANTLR3 I especially addressed the 3rd step. There I don't use a parser, but simulate one, only that I don't stop when a parser would, but when the caret position is reached (so it is essential that the input must be valid syntax up to that point). After reaching the caret the collection process starts, which not only collects terminal nodes (for keywords) but looks at the rule names to learn what needs to be collected too. Using specific rule names is my way there to put context into the grammar, so when the collection code finds a rule table_ref it knows that it doesn't need to go further down the rule chain (to the ultimate ID token), but instead can use this information to provide a list of tables as suggestion.
With ANTLR4 things might become even simpler. I haven't used it myself yet, but the parser interpreter could be a big help here, as it essentially doing what I do manually in my implementation (with the ANTLR3 backend).
This is probably pretty hard to do.
Fundamentally you want to use some parser to predict "what comes next" to display as auto-completion. This has to at least predict what the FIRST token is at the point where the user's input stops.
For ANTLR, I think this will be very difficult. The reason is that ANTLR generates essentially procedural, recursive descent parsers. So at runtime, when you need to figure out what FIRST tokens are, you have to inspect the procedural source code of the generated parser. That way lies madness.
This blog entry claims to achieve autocompletion by collecting error reports rather than inspecting the parser code. Its sort of an interesting idea, but I do not understand how his method really works, and I cannot see how it would offer all possible FIRST tokens; it might acquire some of them. This SO answer confirms my intuition.
Sam Harwell discusses how he has tackled this; he is one of the ANTLR4 implementers and if anybody can make this work, he can. It wouldn't surprise me if he reached inside ANTLR to extract the information he needs; as an ANTLR implementer he would certainly know where to tap in. You are not likely to be so well positioned. Even so, he doesn't really describe what he did in detail. Good luck replicating. You might ask him what he really did.
What you want is a parsing engine for which that FIRST token information is either directly available (the parser generator could produce it) or computable based on the parser state. This is actually possible to do with bottom up parsers such as LALR(k); you can build an algorithm that walks the state tables and computes this information. (We do this with our DMS Software Reengineering Toolkit for its GLR parser precisely to produce syntax error reports that say "missing token, could be any of these [set]")
I am creating an application in .NET.
I got a running application name http://www.spinnerchief.com/. It did what I needed it to do but but I did not get any help from Google.
I need functional results for my application, where users can give one sentence and then the user can get the same sentence, but have it worded differently.
Here is an example of want I want.
Suppose I put a sentence that is "Pankaj is a good man." The output should be similar to the following one:
Pankaj is a great person.
Pankaj is a superb man.
Pankaj is a acceptable guy.
Pankaj is a wonderful dude.
Pankaj is a superb male.
Pankaj is a good human.
Pankaj is a splendid gentleman
To do this correctly for any arbitrary sentence you would need to perform natural language analysis of the source sentence. You may want to look into the SharpNLP library - it's a free library of natural language processing tools for C#/.NET.
If you're looking for a simpler approach, you have to be willing to sacrifice correctness to some degree. For instance, you could create a dictionary of trigger words, which - when they appear in a sentence - are replaced with synonyms from a thesaurus. The problem with this approach is making sure that you replace a word with an equivalent part of speech. In English, it's possible for certain words to be different parts of speech (verb, adjective, adverb, etc) based on their contextual usage in a sentence.
An additional consideration you'll need to address (if you're not using an NLP library) is stemming. In most languages, certain parts of speech are conjugated/modified (verbs in English) based on the subject they apply to (or the object, speaker, or tense of the sentence).
If all you want to do is replace adjectives (as in your example) the approach of using trigger words may work - but it won't be readily extensible. Before you do anything, I would suggest that you clearly defined the requirements and rules for your problem domain ... and use that to decide which route to take.
For this, the best thing for you to use is WordNet and it's hyponym/hypernym relations. There is a WordNet .Net library. For each word you want to alternate, you can either get it's hypernym (i.e. for person, a hypernym means "person is a kind of...") or hyponym ("X is a kind of person"). Then just replace the word you are alternating.
You will want to make sure you have the correct part-of-speech (i.e. noun, adjective, verb...) and there is also the issue of senses, which may introduce some undesired alternations (sense #1 is the most common).
I don't know anything about .Net, but you should look into using a dictionary function (I'm sure there is one, or at least a library that streamlines the process if there isn't).
Then, you'd have to go through the string, and ommit words like "is" or "a". Only taking words you want to have synonyms for.
After this, its pretty simple to have a loop spit out your sentences.
Good luck.
I am creating a scripting language to be used to create web pages, but don't know exactly where to begin.
I have a file that looks like this:
mylanguagename(main) {
OnLoad(protected) {
Display(img, text, link);
}
Canvas(public) {
Image img: "Images\my_image.png";
img.Name: "img";
img.Border: "None";
img.BackgroundColor: "Transparent";
img.Position: 10, 10;
Text text: "This is a multiline str#ning. The #n creates a new line.";
text.Name: text;
text.Position: 10, 25;
Link link: "Click here to enlarge img.";
link.Name: "link";
link.Position: 10, 60;
link.Event: link.Clicked;
}
link.Clicked(sender, link, protected) {
Image img: from Canvas.FindElement(img);
img.Size: 300, 300;
}
}
... and I need to be able to make that text above target the Windows Scripting Host. I know this can be done, because there used to be a lot of Docs on it around the net a while back, but I cannot seem to find them now.
Can somebody please help, or get me started in the right direction?
Thanks
You're making a domain-specific language which does not exist. You want to translate to another language. You will need a proper scanner and parser. You've probably been told to look at antlr. yacc/bison, or gold. What went wrong with that?
And as an FYI, it's a fun exercise to make new languages, but before you do for something like this, you might ask a good solid "why? What does my new language provide that I couldn't get any other (reasonable) way?"
The thing to understand about parsing and language creation is that writing a compiler/interpreter is primarily about a set of data transformations done to an input text.
Generally, from an input text you will first translate it into a series of tokens, each token representing a concept in your language or a literal value.
From the token stream, you will generally then create an intermediate structure, typically some kind of tree structure describing the code that was written.
This tree structure can then be validated or modified for various reasons, including optimization.
Once that's done, you'll typically write the tree out to some other form - assembly instructions or even a program in another language - in fact, the earliest versions of C++ wrote out straight C code, which were then compiled by a regular C compiler that had no knowledge of C++ at all. So while skipping the assembly generation step might seem like cheating, it has a long and proud tradition behind it :)
I deliberately haven't gotten into any suggestions for specific libraries, as understanding the overall process is probably much more important than choosing a specific parser technology, for instance. Whether you use lex/yacc or ANTLR or something else is pretty unimportant in the long run. They'll all (basically) work, and have all been used successfully in various projects.
Even doing your own parsing by hand isn't a bad idea, as it will help you to learn the patterns of how parsing is done, and so then using a parser generator will tend to make more sense rather than being a black box of voodoo.
Languages similar to C# are not easy to parse - there are some naturally left-recursive rules. So you have to use a parser generator that can deal with them properly. ANTLR fits well.
If PEG fits better, try this: http://www.meta-alternative.net/mbase.html
So you want to translate C# programs to JavaScript? Script# can do this for you.
Rather than write your own language and then run a translator to convert it into Javascript, why not extend Javascript to do what you want it to do?
Take a look at jQuery - it extends Javascript in many powerful ways with a very natural and fluent syntax. It's almost as good as having your own language. Take a look at the many extensions people have created for it too, especially jQuery UI.
Assuming you are really dedicated to do this, here is the way to go. This is normally what you should do: source -> SCANNER -> tokens -> PARSER -> syntax tree
1) Create a scanner/ parser to parse your language. You need to write a grammar to generate a parser that can scan/parse your syntax, to tokenize/validate them.
I think the easiest way here is to go with Irony, that'll make creating a parser quick and easy. Here is a good starting point
http://www.codeproject.com/KB/recipes/Irony.aspx
2) Build a syntax tree - In this case, I suggest you to build a simple XML representation instead of an actual syntax tree, so that you can later walk the XML representation of your DOM to spit out VB/Java Script. If your requirements are complex (like you want to compile it or so), you can create a DLR Expression Tree or use the Code DOM - but here I guess we are talking about a translator, and not about a compiler.
But hey wait - if it is not for educational purposes, consider representing your 'script' as an xml right from the beginning, so that you can avoid a scanner/parser in between, before spitting out some VB/Java script/Html out of that.
I don't wan to be rude... but why are you doing this?
Creating a parser for a regular language is a non-trivial task. Just don't do it.
Why don't you just use html, javascript and css (and jquery as someone above suggested)
If you don't know where to begin, then you probably don't have any experience of this kind and probably you don't have a good reason, why to do this.
I want to save you the pain. Forget it. It's probably a BAD IDEA!
M.
Check out Constructing Language Processors for Little Languages. It's a very good intro I believe. In fact I just consulted my copy 2 days ago when I was having trouble with my template language parser.
Use XML if at all possible. You don't want to fiddle with a lexer and parser by hand if you want this thing in production. I've made this mistake a few times. You end up supporting code that you really shouldn't be. It seems that your language is mainly a templating language. XML would work great there. Just as ASPX files are XML. Your server side blocks can be written in Javascript, modified if necessary. If this is a learning exercise then do it all by hand, by all means.
I think writing your own language is a great exercise. So is taking a college level compiler writing class. Good luck.
You obviously need machinery designed to translate langauges: parsing, tree building, pattern matching, target-language tree building, target-language prettyprinting.
You can try to do all of this with YACC (or equivalents), but you'll discover that parsing
is only a small part of a full translator. This means there's a lot more work
to do than just parsing, and that takes time and effort.
Our DMS Software Reengineering Toolkit is a commercial solution to building full translators for relatively modest costs.
If you want to do it on your own from the ground up as an exercise, that's fine. Just be prepared for the effort it really takes.
One last remark: designing a complete language is hard if you want to get a nice result.
Personally I think that every self-imposed challenge is good. I do agree with the other opinions that if what you want is a real solution to a real life problem, it's probably better to stick with proved solutions. However, if as you said yourself, you have an academic interest into solving this problem, then I encourage you to keep on. If this is the case, I might point a couple of tips to get you on the track.
Parsing is not really an easy task, that is way we take at least a semester of it. However, it can be learned. I would recommend starting with Terrence Parr's book on language implementation patterns. There are many great books about compiling and parsing, probably the most loved and hated been the Dragon Book.
This is pretty heavy stuff, but if you are really into this, and have the time, you should definitely take a look. This would be the Robisson Crusoe's "i'll make it all by myself approach". I have recently written an LR parser generator and it took me no more than a long weekend, but that after reading a lot and taking a full two-semesters course on compilers.
If you don't have the time or simply don't want to learn to make a parser "like men do", then you can always try a commercial or academic parser generator. ANTLR is just fine, but you have to learn its meta-language. Personally I think that Irony is a great tool, specially because it stays inside C# and you can take a look at the source code and learn for yourself. Since we are here, and I'm not trying to make any advertisement at all, I have posted a tiny tool in CodePlex that could be useful for this task. Take a look for yourself, it's open-source and free.
As a final tip, don't get scared if someone tells you it cannot be done. Parsing is a difficult theoretical problem but it's nothing that can't be learned, and it really is a great tool to have in your portfolio. I think it speaks very good of a developer that he can write an descent-recursive parser by hand, even if he never has to. If you want to pursuit this goal to its end, take a college-level compilers course, you'll thank me in a year.
I'd like to parse formatted basic values and a few custom strings from a TextReader - essentially like scanf allows.
My input might not have line-breaks, so ReadLine+Regex isn't an option. I could use some other way of chunking text input; but the problem is that I don't know the delimiter at compile time (so that's tricky), and that that delimiter might be localization-dependant. For instance, a float followed by a comma might be "1.5," or "1,5," but in both cases attempting to parse the float should be "greedy".
To be safe, I'd like to assume my input is actively hostile (say, streaming in from a network stream): i.e. intentionally missing chunking delimiters.
I'd like to avoid custom Regex's: int.Parse and double.Parse work well and are localization-aware. Don't get me started on DateTime's - I might need a few custom patterns anyhow, but writing Regexes to cover that scenario doesn't sound like fun.
For a concrete example, let's say I have a TextReader and that I know the next value should be a double - how can I extract that double and possibly a limited amount of lookahead without reading the entire stream and without manually writing a localizable double-parser?
Similar Questions
There's a previous question "Looking for C# equivalent of scanf" which sounds similar but the Q+A focus on readline+regex (which I'd like to avoid). How can I use Regex against a TextReader? didn't find an answer (beyond chunking), and in any case I'd like to avoid writing my own Regexes.
Based on that lack of answers and still not having found anything myself, it seems that
There is no means to use localized parsing directly from Streams (or TextReaders) in .NET, nor is there a way to know how much of the stream corresponds to a parseable prefix in a systematic way.
There is no means to apply regular expressions to Streams (or TextReaders) in .NET, so there's no easy way of implementing something like this yourself.
If you really need something like this, the easiest option is a full-fledged parser generator. ANTLR works well for this; it has a lot of existing grammars you can copy-paste for the basics, and it comes with a GUI to help understand your grammar and makes parsers for .NET, java, C and a host of other languages. It's developer friendly, fast... ...but way too powerful and flexible for what I need; like shooting a bug with a shotgun - I'm not thrilled with this solution.
I'm looking for syntatic examples or common techniques for doing regular expression style transformations on words instead of characters, given a procedural language.
For example, to trace copying, one would want to create a document with similar meaning but with different word choices.
I'd like to be able to concisely define these possible transformations that I can apply to a text stream.
Eg. "fast noun" to "rapid noun", but "go fast." wouldn't get transformed (no noun afterwards.
Or: "Alice will sing song" to "song will be sung by Alice"
I'd expect this to be done in grammatical checkers, such as detecting passive voice.
A C# implementation for this sort of language-processing would be really neat, but I think the bulk of any effort is coming up with the right rules - Keeping the rules clear and understandable seems like a place to begin.
You could try Jason Rennie > WordNet-QueryData-1.47 > WordNet::QueryData
One good place to start researching would be "Word Net" - it's a dictionary of semantics, grouping words together by similar meaning, and also recording the relationships between words in useful ways.
There are a bunch of software projects leveraging the Word Net corpus, one of them may be what you need.
If you aren't tied to a particular language, Haskell has Aarne Ranta's Grammatical Framework:
http://www.grammaticalframework.org/
which is explicitly designed to generate parsers, etc for natural language processing of this sort.
A good place to start would be SIL's CARLAStudio for its "Computer Assisted Related Language Adaptation" suite. Alternatively SIL's Adapt It. SIL has a huge range of linguistic analysis software, which is the direction you appear to be going. It's certainly a big jump from regular expressions, which don't care about the meaning, to something that can handle linguistic analysis.
If you want something more robust for natural language parsing/transforming, you could try the C# port of OpenNLP.
I am not aware of any syntaxes that exist for English language processing like you discuss. You would need to create your own DSL using one of the toolsets (such as Word Net) out there.