Awesome
Whitespace parser
Associated blog post: https://www.sderosiaux.com/2018/06/15/a-simple-way-to-write-parsers-using-the-state-monad/
Example of a parser implementation of the Whitespace programming language using only StateT
to convey the state, the stack and the output of the program.
Whitespace deals with 3 characters only: " "
, "\t"
, and "\n"
.
This parser relies massively upon StateT
and misc combinators, as we can see in the root parser:
def imp: StateT[F, (String, Stack), String] = for {
output <- stackCommands.all <+>
ioCommands.all <+>
arithmeticCommands.all <+>
heapCommands.all <+>
flowCommands.all
rest <- imp <+> StateT.pure[F, (String, Stack), String]("")
} yield output + rest
The state of our parser contains the program and the stack of execution. The result is a String
which is what is print to the screen.
Whitespace has 5 big chunks of commands to determine how to interpret the following characters.
For instance, one of the ioCommands
(triggered by "\t\n"
) is " "
which means: print the character on top of the stack.
Hello, world!
As a typical example, here we are parsing the Hello world from Wikipedia:
val helloWorld = " \t \t \n\t\n \t\t \t \t\n\t\n \t\t \t\t \n\t\n \t\t \t\t \n\t\n \t\t \t\t\t\t\n\t\n \t \t\t \n\t\n \t \n\t\n \t\t\t \t\t\t\n\t\n \t\t \t\t\t\t\n\t\n \t\t\t \t \n\t\n \t\t \t\t \n\t\n \t\t \t \n\t\n \t \t\n\t\n \n\n\n"
val Some(((rest, stack), output)) = new WhitespaceParser[Option].eval(helloWorld)
println(s"Stack: $stack")
println(s"Output: $output")
Stack: List(33, 100, 108, 114, 111, 119, 32, 44, 111, 108, 108, 101, 72)
Output: Hello, world!
Syntax
Details about the syntax here: https://hackage.haskell.org/package/whitespace-0.4/src/docs/tutorial.html
Notes
The parser is not complete, some commands are missing, but you get the idea.