This is Stuff

Placeholdered map is able to associate keys with values. User can add key value pairs into it and ask for data associated with keys. It works exactly as it would work in an ordinary map.

The additional feature is ability to use placeholders. Placeholder knows when it was created and is useful to cheat on order in which data were added into the map. If the user adds data into placeholder, the datastructure behaves as if the data were added when the placeholder was created.

We will create two such structures. First keeps key value pairs and can return last value associated with the key. Value added into placeholder is returned only if user did not added the same key after the placeholder was created. Second returns all values associated with the key in order they were put in. Placeholders can be used to add data in the middle of those lists.

Parsing simple grammar with antlr is simple. All you have to do is to use regular expressions to describe your language and let antlr generate lexer and parser. Parsing big or complicated languages occasionally require more, because describing them in regular expressions only can be hard or even impossible.

Semantic predicates are java (or C++, or JavaScript, or ...) conditions written inside the grammar. Antlr uses them either to choose between multiple alternatives or as additional assertions to be checked. They can be placed inside both lexer and parser, but this post focus only on their usage within the parser. They add a lot of power to antlr.

Most compilers treat comments as just another meaningless whitespaces. They identify them in the source code and then throw them away. On the other hand, things are a bit harder if you need to know comments content and their position.

Such requirement is not too common and official ANTLR documentation does not says much about how to do it. The compiler we have been working on needed to preserve comments, so we had to come up with our own solution. This post documents it.

First chapter introduces our compiler. It describes what it does, where it is located and how it is written. Second chapter explains our comment preserving requirements. The rest of the post describes datastructures and algorithms used in our solution.

ANTLR tool is useful any time you need to create compiler, interpreter or parser of your own language. It takes so called grammar file as an input and generates two classes: lexer and parser.

This post explains what is lexer, what is parser and how to write grammar files to generate them. In the end of the post, you will be able to create a compiler into abstract syntax tree for any simple programming language.

Our example project, a boolean expression language, is written in Java and available on Github. Besides that, everything explained in this post is language independent. Grammar files are the same in all languages.

Antlr stands for ANother Tool for Language Recognition. The tool is able to generate compiler or interpreter for any computer language. Besides obvious use, e.g. need to parse a real 'big' programming language such as Java, PHP or SQL, it can help with smaller, more common tasks.

It is useful any time you need to evaluate expressions unknown at compile-time or to parse non-trivial user input or files in a weird format. Of course, it is possible to create custom hand made parser for any of these tasks. However, it usually takes much more time and effort. A little knowledge of a good parser generator may turn these time-consuming tasks into easy and fast exercises.