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A very simple parser combinator

Parsing structured text (e.g. program source code) sounds more complicated than it actually is.

This repo exemplifies how to quickly write a parser combinator that can be used to do just that. See also this video for a quick overview of what we're trying to achieve.

Design goals:

  • Simple code, minimal comments (let the code do the talking)
  • Clear separation of concerns between classes
  • Easy to adapt to your own needs (core parser functionality is less than 100 lines, can be reused in you own projects)
  • Meant to be evocative, not exhaustive (things like error handling are greatly simplified)
  • 100% working, but intentionally incomplete (so you can have fun extending the code and tinkering with it).

What can it do

The code in this repo is able to parse somewhat complex BASIC programs such as:

10 LET SomeVariable = "John" + " " + "Smith"
20 PRINT "Hello, ", SomeVariable ,"! How are you?" : PRINT "This is another print statement." : LET SomeNumber123 = 37  + 1 * (4 * 5)
30 FOR ALoopVariable = 37 + SomeNumber123 To 1000
40 LET Temp = ALoopVariable MOD 5
50 IF ALoopVariable > 400 AND Temp = 2 THEN PRINT "This is a conditional print: ", ALoopVariable
60 NEXT

Feel free to play around and add support for more BASIC features, such as:

  • User input via theINPUT command (should be similar to how the PRINT command is already parsed)
  • User functions via DEF FN(how would you declare and parse user arguments in this case?)
  • Actually running the parsed BASIC programs (the parser returns a mostly-usable AST, but expression evaluation, not to be confused with expression parsing which is already handled, needs to be carefully thought out)

What's in the box

This repo takes the form of a single Visual Studio Solution. The solution itself is split in two parts:

  • A universal parser combinator. Simple, concise, and portable. Can be reused in your own projects if you want to. Not meant to be feature complete.
  • A BASIC program parser. Uses the universal parser functionality to parse actual BASIC programs.

The relevant files and their descriptions are as follows:

Universal parser functionality

File Description
Parser.cs Core parser functionality (While(),Until()) and combinator (Union(),Optional())
ParserExtras.cs Non-core but widely used parser helpers
TextWithPointer.cs Represents source code with a "current pointer"

BASIC language parser functionality

These files use the universal parser functionality mentioned previously to implement a simple BASIC program parser.

File Description
BasicParser.cs Used as a high-level wrapper/parser over a BASIC program
BasicProgramEntities.cs Entities used in BasicParser.cs
CommandArgumentsReader.cs Parser for BASIC command arguments (e.g. PRINT, FOR, LET, etc)
CoreEntitiesReader.cs Parsers for BASIC entities like string, int, etc
ExpressionReader.cs Parser for BASIC math-like expressions
ListReader.cs Parser for BASIC lists, in particular lists of command arguments
Misc.cs Various helper functions