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Beau­tiful Racket / tuto­rials

Imagine a language: wires

In jsonic, we saw how we could make a simple but useful DSL in under 50 lines of code. If anyone thought that creating a DSL was auto­mat­i­cally a giant under­taking—Racket makes it easy.

In turn, when DSLs are easy to make, they become a prac­tical option for solving a larger set of program­ming prob­lems—even small ones. In this tuto­rial, we’ll take that prin­ciple to its extreme. We’ll make a language called wires that’s designed to be used once and discarded.

And let’s cleanse the rice-cake flavor of JSON from our palate with some­thing more enter­taining.

The puzzle

In 2015, programmer Eric Wastl created a terrific series of program­ming puzzles called Advent of Code. Though I recom­mend the whole set—and I’m looking forward to this year’s edition—Eric let me build this tuto­rial around my favorite puzzle from 2015, Some Assembly Required. The descrip­tion below is by Eric.

This year, little Bobby Tables got a set of wires and bitwise logic gates! Unfor­tu­nately, little Bobby is under the recom­mended age range, and he needs help assem­bling the circuit.

Each wire has an iden­ti­fier (some lower­case letters) and can carry a 16-bit signal (a number from 0 to 65535). A signal is provided to each wire by a gate, another wire, or some specific value. Each wire can only get a signal from one source, but can provide its signal to multiple desti­na­tions. A gate provides no signal until all of its inputs have a signal.

The included instruc­tion booklet describes how to connect the parts:

For example, here’s a simple circuit:

x AND y -> d
x OR y -> e
x LSHIFT 2 -> f
y RSHIFT 2 -> g
NOT x -> h
NOT y -> i
123 -> x
456 -> y
1
2
3
4
5
6
7
8
x AND y -> d
x OR y -> e
x LSHIFT 2 -> f
y RSHIFT 2 -> g
NOT x -> h
NOT y -> i
123 -> x
456 -> y
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After it’s run, these are the signals on the wires:

d: 72
e: 507
f: 492
g: 114
h: 65412
i: 65079
x: 123
y: 456
1
2
3
4
5
6
7
8
d: 72
e: 507
f: 492
g: 114
h: 65412
i: 65079
x: 123
y: 456
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The ques­tion

The instruc­tion booklet for the circuit kit provides a descrip­tion of a circuit (which will be our puzzle input).

We have to deter­mine: What signal ends up on wire a?

Of course, we’re going to answer this ques­tion by making a DSL called wires.

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