lr-lalr parser generator

Introduction

What is a parser generator?

It is a program wich has a text file containing a description of a grammar as input. A grammar is a way to describe structure of some language. But i will not go into details because English is not my native language and there are far better explanations available on the internet.

Input file

Input file has a rather complicated syntax, and i don't have the time to explain it now :(

But looking at the example provided will get you started.

Example

The example provided in the test subfolder of the zip file is in a file called translate-gram.lrgr and it is a grammar showing an example how to use this parser generator to create a translator from infix expressions to postfix expressions. The entire file is 75 lines long and shouldn't be to difficult to understand.

It starts with a {{ ... }} block that contains code that will be placed at the start of the generated parser file.

Afther that there are a few grammar rules.

• Right side of the rule is between {{ and }}.

• Nonterminal symbols are placed between < and > (without any spaces)

• Terminal symbols are placed between “ and “ (no spaces)

• Everything else is interpreted as D code

Be carefull when changin something not to have D-code that could be mistaken for something else. For example don't write “writefln( “hello” );” with spaces around “hello” because this is a terminal symbol. Write it together.

Compiling the parser generator and the example

I use Derek Panell's great and usefull Build utillity to make things easier.

1. Extract the folder somewhere.

2. Build main.d -clean -full will compile the parser generator to main.exe

3. Move translate-gram.lrgr into the folder where main.exe is and run it like this:

4. main.exe -i translate-gram.lrgr +forseRecalc

5. This will generate two files:

   • translate-gram.lrgr.zLRtbl – a compressed parsing table used by parsing engine

   • translate-gram.d – D code for the parser

6. Build translate-gram.d -clean -full will build the example

7. Run the executable program with some expression as parameter

   • translate-gram.exe 1+sin(2)*cos(3)-1/(1+exp(1/cos(3.14159)))

     and you will get the following outpu:

     1 2 sin 3 cos * + 1 1 1 3.14159 cos / exp + / -

The Future

• Better documentation

• More examples

• Nicer code (more checks)