Functional Reactive PythonOn Introducing CS to High School

Students

John PetersonWestern State College

Gunnison, CO

Goals

• Get high school students excited about CS with a 1 week summer camp

• Bring them to Gunnison in the summer Use video gaming as the bait

• Lots of outdoor activity• Integrate Math / Physics into the

program

Objectives

• Teach basic principles – avoid putting too much “in a can” or just doing drag & drop / menu pulling (Not Alice)

• Avoid big chunks of code – no time to teach software engineering

• Lots of small projects rather than one big one

• Don’t hide from basic math / physics• Use freely available software

Declarative Language Research

High school students are the best audience I’ve ever had for exploring new programming styles

• No pre-conceived notions about how to program

• Eager to learn• Will tell you what they think

The Game Engine

After considering a few possibilities, we settled on the Panda3D game engine.

• Good support from CMU• Python is a good instructional

language• All the basic features we needed

were covered

Modeling

We also chose to cover 3-D modeling a bit. We used Blender but this had some problems interacting with Panda:

• Texture loss (we never figured this out)• Lack of support for joints / animations in

the files exported to Panda• Poorly adapted to the Windows

environmentIn spite of this, many students spent a lot of

time building Blender models.

Student Work

While building these 3-D models isn’t programming, it did help us get accustomed to working in 3 space and allowed students to personalize their 3-D worlds.

Software Issues

Our goal was not to teach any specific language or software system

We wanted every program to be as declarative as possible: elegant and compact

FRP (Functional Reactive Programming) was used to hide time flow from the students

Plan A

Write a custom front end (in Haskell of course) to allow domain-specific semantics and notations, provide compile-time type safety, and give appropriate error messages.

• Probably the right thing to do• Not enough time!• Doesn’t solve the debugging problem• Need to build custom IDE

Plan B

Attempt to recreate FRP using Python libraries.

• Able to use existing IDE (Pydev in Eclipse), good debugger

• Python has lambda (just uglified lisp after all)

• Students are likely to run into Python later

• Much less work!!

Programming Style

Create factory functions for all game objects (models, lights, gui controls, camera) – these return an object “handle”

Use python keywords and good defaulting to avoid complexity in the factory

“Moving parts” are defined by reactive signals

Signals can be defined separately to allow circular reference

Use signals like “time” to get motion

Functional Reactive Programming

This is work I did back in the Yale (yuck!) days.

The big idea is to hide time flow – no event handles, explicit updating.

A “signal” (reactive value) is something that seems to mutate on its own as time progresses. Time flow becomes implicit.

Example: Moving Bear

from Panda import *

begin()

p = pandaBear()

p.position = P3(sin(time), 3, cos(time))

world.cameraPos = P3(0,-3,0)

start()

Teaching points: sin / cos, coordinate system, trajectories

Demo1.py

More Positioning

p1 = pandaBear()p2 = pandaBear()def f(t): return P3(sin(t), 3, cos(t))p1.position = f(time)p2.position = f(time + pi)p1.HPR = P3(time*2,0,0)p2.scale = 0.2 * (5 + sin(2*time))world.cameraPos = P3(0,-4,0)

Demo2.py

Adding Controls

Panda3d has lots of user interface devices. We wrapped these up in FRP style to make them easy to use.

h = hangglider()Text("Use slider to adjust heading")s = Slider(max = 2*pi)Text(s.value)h.position = P3(sin(time), 3, cos(time))h.HPR = P3(s.value, 0, 0)world.cameraPos = P3(0,-5,0)

Demo3.py

Teaching Math

A great thing about 3-D gaming is that you can’t hide from math! Students needed to pick this up fast.

We covered linear interpolation, basic trig, paths, periodic functions, and vector arithmetic (Thanks to Andy Keck for being a great math teacher!)

We didn’t shy away from calculus – here’s an example using derivatives.

Point Where You Go

Text("Plane circles while pointing forward")

p = boeing707()

b = P3C(3, time, sin(time*3))

p.position = b

db = deriv(P3(0,0,0), b)

hpr = P3toHPR(db) # Explained in lecture

p.HPR = P3(getX(hpr)-radians(90),-getY(hpr), 0)

world.cameraPos = P3(0,-10,1)

Demo4.py

Teaching Programming

We also wanted to teach basic ideas in programming

Function definition was easyWe used for loops which arranged

objects in various patterns to talk about conditional behavior, nested loops, arrays, and random numbers

Exploding Ballsmycolors = [red, blue, green, yellow, purple, brown, white, black]bagOfBalls = []endPos = []world.cameraPos = P3(0,-30,0)random.seed()explodeSpeed = 3 #.1 is very slow, 10 is very fastfor p in range(0,getRand(50,88)): theColor = mycolors[getRand(0,7)] if getRand(0,1) == 1: bagOfBalls.append(volleyBall(color = theColor)) else: bagOfBalls.append(soccerBall(color = theColor)) endPos.append(P3(getRand(-5,5), getRand(-5,5), getRand(-5,5)))startPos = P3(0,0,0)for index in range(0, len(endPos)): d = endPos[index] bagOfBalls[index].position = startPos + time*explodeSpeed*d

Demo5.py

Teaching PhysicsFinally, we were able to get to some simple physics.

Here’s a bouncing ball:def launch(p0, v0): a = P3(0,0,-g) v = v0 + integral(a) p = p0 + integral(v) return react(p, when(getZ(p) < 0, bounce, p, v))

def bounce(p0, v0): return launch(P3(getX(p0), getY(p0), -getZ(p0)), 0.9*P3(getX(v0), getY(v0), -getZ(v0)))

And the Roller Coasterdef f((t0, p0, v0), t1): deltaT = t1 - t0 pe = scaleP3(deltaT, v0) + p0 p1 = P3(getX(pe), getY(pe), coasterPath(getX(pe))) dir = p1 - p0 oldv = absP3(v0) deltaz = getZ(p1) - getZ(p0) s1 = oldv*oldv - 2 * g * deltaz if s1 < 0: newv = - sqrt(-s1) else: newv = sqrt(s1) * 0.999 v1 = scaleP3(newv, normP3(dir)) return ((t1, p1, v1), p1)

Demo6.py

Roller Coaster Notes

We covered the basic physics to make this understandable

Students can explore changes to the physical laws

Needed to “crack open” the underlying signal extrapolation function. That was tough since you see the signal state and delta t. But you can also use this to explain how integral and derivitave work

Panda Effects

Effects are fun for the students even though they don’t contribute to the core subject material.

We did lighting, particle effects (explosions, sparkles, flames, ..), and retexturing

Check out our discoDemo7.py

Character Animation

Panda has a number of built-in jointed characters. These are easy to control – they have additional signals to set joint angles.

What we didn’t do was introduce a way to create animations easily. No easy way to save a pose or sequence poses.

Games???

OK – I haven’t showed you any games. No paddleball or asteroids or shooting gallery or space invaders or doom.

Why? Because we didn’t get them working yet. There was plenty to do without gaming but next time this needs to be fixed.

There were FRP problems, notational problems, and time problems.

Game Engine Issues

• Any game engine has a huge API – it’s a lot of work to build on such a big base

• Need to “normalize” existing models. We wanted to use a common size / orientation so students could swap models easily.

• No 3-D design environment for positioning models or creating animations.

• Many high level abstractions are hard to encapsulate. We avoided many of these (collisions, tasks, animations).

Things the Worked

• Gaming as a way to unify math, cs, and physics

• Wide appeal – gamers and non gamers both had fun

• Lots of assistants – kept students from getting frustrated (thanks to Kate, Darek, and Stephan)

• FRP made programs small and easily understood

• Great source of projects for CS students• Summer employment for CS students

Things That Didn’t Work Well

• Blender (too complex, importing problems)

• FRP “in the large” – not sure how to do this yet

• Python – not really a great language for non-programmers. Notational issues kept coming up (no user-definable infix operators, couldn’t get field selection to lift, overloading problems, …)

For Next Year

• More gaming! • Better publicity – it’s hard to get students in the

door.• Better software – too much time in the debugger,

poor notations, no type checking• Better preparation – more examples, better

lesson planning (especially for math / physics integration into the CS stuff)

• Music videos – this is something that we definitely need to get to that leads nicely up to games. We used Haskore to teach music but didn’t get the sound to synch correctly with the action

Programming Language Issues

Making time flow implicit is a big win.Laziness is crucial. We spent way to much

time having to re-invent lazy evaluation. Forward reference was a big problem

Python’s O-O system kept getting in the wayPython lacks the syntactic flexibility that we

really neededGood type systems make things a LOT

easier. We did some “load time” type checking but didn’t get full H-M. Students didn’t understand the debugger.

Multi-Disciplinary EducationCS instruction suffers from tunnel vision – we don’t

do a good job with students that are not going all the way into CS.

Integrating the math and physics into our curriculum broadens the appeal and sends a message to future CS students

The creative side of CS is often hidden in CS1 – the game engine gives us a chance to work in a more creative environment

We were able to cover a wide range of math – not just one specific topic. The visual nature of the system helps a lot.

We hope to sell this beyond just CS instruction

Conclusions

Game engines are an appealing way to get students in the door to CS departments

The broad range of topics helped keep all of the students on board

3-D modeling was a nice contrast to programming – it let students blow off steam when programming got hard

CS needs to better serve students in other disciplines

We need to get students interested in CS earlierThe 2 girls at the camp enjoyed it as much as the

hard core gamers.

Scenes from Camp

The Last Conclusion

A good recreational program will make students forget the frustration of software development.