Ascii Paint in J

Summary

All the things you can ask this program to do, in one line:

((((10 newCanvas 10) drawer 'o' 6 square 2 4) drawer 'x' 5 verticalLine 2 7) drawer 's' 7 horizontalLine 1 9) fill '#';5 3

Which produces this piece of art:

------------
|..........|
|....x.....|
|....x.....|
|.oooxoo...|
|.o##x.o...|
|.o##x.o...|
|sssssssss.|
|.o....o...|
|.oooooo...|
|..........|
------------

Make a blank canvas
Lines
Squares
4-connected boundary fill

Example of using a conjunction closure to exploit the constraint of infix positional arguments

Rather than implement 'draw' as a verb directly, using 3 parameters, a canvas, a colour and a point, I've got a conjunction called 'drawer' which takes a canvas and a colour as parameters to become an instance of a monadic function taking a set of points to draw on the internally held canvas with the internally held colour. It's basically just a closure, but I want it specifically for the way it helps to deal with the constraint of infix positional arguments, and perhaps even makes it an enabling constraint for composing a program from components of smaller parts.

Coding problem that this is based on

This program is loosely based on this problem from an interview section used by an unnamed scientific publisher which used to employ one of my sources close to the prime minister.

Preamble

Below is a coding problem that we would like you to solve. Please read through the description carefully and implement a solution for it. You're free to write your solution in any programming language. We don't want you to over-engineer the solution but be prepared to extend the functionality in the next step of the interview process. Finally, we ask you to submit a solution that you'd be happy to go live with and works "out of the box".

Problem Description

You are given the task of writing a simple console version of a drawing program. At this time, the functionality of the program is quite limited, but this might change in the future. In a nutshell, the program should work as follows:

create a new canvas
start drawing on the canvas by issuing various commands
quit

At the moment, the program should support the following commands:

Command	Description
`C w h`	Should create a new canvas of width w and height h.
`L x1 y1 x2 y2`	Should create a new line from (x1,y1) to (x2,y2). Currently only horizontal or vertical lines are supported. Horizontal and vertical lines will be drawn using the 'x' character.
`R x1 y1 x2 y2`	Should create a new rectangle, whose upper left corner is (x1,y1) and lower right corner is (x2,y2). Horizontal and vertical lines will be drawn using the 'x' character.
`B x y c`	Should fill the entire area connected to (x,y) with "colour" c. The behaviour of this is the same as that of the "bucket fill" tool in paint programs.
`Q`	Should quit the program.

Sample I/O

Below is a sample run of the program. User input is prefixed with enter command:.

enter command: C 20 4
----------------------
|                    |
|                    |
|                    |
|                    |
----------------------

enter command: L 1 2 6 2
----------------------
|                    |
|xxxxxx              |
|                    |
|                    |
----------------------

enter command: L 6 3 6 4
----------------------
|                    |
|xxxxxx              |
|     x              |
|     x              |
----------------------

enter command: R 16 1 20 3
----------------------
|               xxxxx|
|xxxxxx         x   x|
|     x         xxxxx|
|     x              |
----------------------

enter command: B 10 3 o
----------------------
|oooooooooooooooxxxxx|
|xxxxxxooooooooox   x|
|     xoooooooooxxxxx|
|     xoooooooooooooo|
----------------------

enter command: Q