In this post I will show you how to write a simple DSL in C# using ANTLR.
What is DSL?
A domain-specific language is a computer language specialized to a particular application domain. This is in contrast to a general-purpose language, which is broadly applicable across domains.
What is ANTLR?
In computer-based language recognition, ANTLR, or ANother Tool for Language Recognition, is a parser generator that uses LL for parsing. ANTLR is the successor to the Purdue Compiler Construction Tool Set, first developed in 1989, and is under active development
What are we going to build
In this article, I will show you how to create a DSL that converts a sentence into C# POCO
classes that you can serialize and save into the database.
book a hotel room for Santosh Singh and 3 guests on 23-SEP-2013 00:12
We are going to parse the above statement into C# POCO
classes by using ANLTR, and the output will be as shown below.
{"Time":"2013-09-23T00:12:00","Person":{"FirstName":"Santosh","LastName":"Singh","NumberOfGuests":3}}
Prerequisite
- ANTLR
- Visual Studio Code with ANTLR extensions
Before writing, grammar letβs first create our
POCO
classes that we want to populate from our DSL
Booking.cs
using System;
public partial class Booking
{
public DateTime Time { get; set; }
public Person Person { get; set; }
}
Person.cs
public class Person{
public string FirstName { get; set; }
public string LastName { get; set; }
public int NumberOfGuests { get; set; }
public override string ToString()
{
return $"{FirstName}-{LastName}-{NumberOfGuests}";
}
}
ANTLR Grammar
The first thing we will do that write the grammar file for the above statement. Below is the grammar for our DSL. ANTLR grammar consists of two parts
- Parser
- Lexer
In our grammar parser are
- booking
- time
- person
Booking is the start of the parsing.
Here I am using Actions and Attribute instead of Visitor. Actions are blocks of text written in the target language and enclosed in curly braces. The recognizer triggers them according to their locations within the grammar. For example, the following rule emits a
Booking
model after the parser has seen a valid declaration:
booking
returns[Booking b]:
'book' 'a' 'hotel' 'room' 'for' person 'guests' 'on' time {
$b=new Booking();
$b.Time=$time.t;
$b.Person=$person.p;
};
Grammar
grammar Booking;
@lexer::header {
using System;
}
booking
returns[Booking b]:
'book' 'a' 'hotel' 'room' 'for' person 'guests' 'on' time {
$b=new Booking();
$b.Time=$time.t;
$b.Person=$person.p;
};
time
returns[DateTime t]:
d = datetime {
Console.WriteLine($d.text);
$t=DateTime.Parse($d.text);
};
person
returns[Person p]:
f = firstName l = lastName 'and' n = numberOfGuests {
$p=new Person();
$p.FirstName=$f.text;
$p.LastName=$l.text;
$p.NumberOfGuests=int.Parse($n.text);
};
firstName: STRING;
lastName: STRING;
numberOfGuests: NUMBER;
datetime:
NUMBER NUMBER SEPARATOR MONTH SEPARATOR YEAR NUMBER NUMBER COLON NUMBER NUMBER;
YEAR: NUMBER NUMBER NUMBER NUMBER;
NUMBER: [0-9];
MONTH:
JAN
| FEB
| MAR
| APR
| MAY
| JUN
| JUL
| AUG
| SEP
| OCT
| NOV
| DEC;
STRING: [a-zA-Z][a-zA-Z]+;
JAN: [Jj][Aa][Nn];
FEB: [Ff][Ee][Bb];
MAR: [Mm][Aa][Rr];
APR: [Aa][Pp][Rr];
MAY: [Mm][Aa][Yy];
JUN: [Jj][Uu][Nn];
JUL: [Jj][Uu][Ll];
AUG: [Aa][Uu][Gg];
SEP: [Ss][Ee][Pp];
OCT: [Oo][Cc][Tt];
NOV: [Nn][Oo][Vv];
DEC: [Dd][Ee][Cc];
SEPARATOR: '-';
COLON: ':';
WS: (' ' | '\r' | '\n' | '\t') -> channel(HIDDEN);
Parse Tree
Now inside the main method, write the following code.
Assuming you have generated the parser and lexer code using ANTLR.
Program.cs
using System;
using Antlr4.Runtime;
using Newtonsoft.Json;
namespace antlraction
{
class Program
{
static void Main(string[] args)
{
var input = "book a hotel room for Santosh Singh and 3 guests on 23-SEP-2013 00:12";
var charStream = new AntlrInputStream(input);
var lexer = new BookingLexer(charStream);
var tokens = new CommonTokenStream(lexer);
var parser = new BookingParser(tokens);
var t = parser.booking().b;
System.Console.WriteLine(JsonConvert.SerializeObject(t));
}
}
}
{"Time":"2013-09-23T00:12:00","Person":{"FirstName":"Santosh","LastName":"Singh","NumberOfGuests":3}}
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