Educational Language?

• List of educational languages on Wikipedia
  • shows 17 in the “Children” category

Karel?

• Invented by Richard E. Pattis at Stanford in the early 80’s.
  • Karel The Robot: A Gentle Introduction to the Art of Programming
• Named after Karel Čapek, the author of R.U.R.
• Its Czech version became quite popular in Czechoslovakia.

Why?

• It was the first programming language I learned.
• I implemented it years ago in Perl
  • as an interpreter,
  • as a monolithic unmaintainable script.
• I wanted to try better.

Karel the Robot—the Details

• The robot lives in a grid.
• The grid can contain walls and marks (originally beepers).
• The robot can’t walk through walls.
• The robot understands the following commands:

  left forward

  pick-mark drop-mark

  stop

Karel the Robot—the Details (2)

The robot can learn new commands

• using sequences of already known ones,
• and also the following control keywords and conditions:

  repeat N times while if ( + else ) done, command, end

  there is there is no ( not ) facing

  wall mark

  North East South West

• Perl-like comments also supported

Example - Turning Right

command right
    repeat 3 x
        left
    done  # <- Here ends the repeat.
end

 =

Calling a Function

command stairs
    while there's no wall
        forward
        left
        if there's no wall
            forward
            right  # User-defined.
        done
    done
end

• The robot remembers where a function was called from and returns back once the function has finished.

The Killer Feature

Recursion

command follow-right-hand
    while there's no mark
        right
        while there's a wall
            left
        done
        if there's no wall
            forward
            follow-right-hand  # Recursion!
        done
    done
end

Homework

• Implement increment and addition.
• Write a program that fills all the accessible grid squares with marks (+ demo).

Implementation

• Using Marpa::R2 for parsing
• Using Test::Spec for tests
• Using Moo for objects
• Stack

Tested on Humans

• My 8 years old son was excited.
• I was able to explain “off-by-one error” to him.

Current State

• Version 0.03 on its way to CPAN.
• Simple text UI is included in the distribution (demo).
• On GitHub, a web GUI written in Dancer2 and Template::Toolkit emerges.

Future

• Debugging: almost done
  • propagation of symbol positions and length: grammar → stack
  • propagation of the source code: parser → stack