Sunday, May 9, 2021

Talking to pins

The last few weeks, I spent a little time getting Objective-S working well on the Raspberry Pi, specifically my Pi400. It's a really wonderful little machine, and the form factor and price remind me very much of the early personal computers.

What's missing, IMHO, is an experience akin to the early BASICs. And I really mean "akin", not a nostalgia project, but recovering a real quality that has been lost: not really "simplicity", more "straightforwardness".

Of course, one of the really cool thing about the Pi is its GPIO interface that lets you do all sorts of electronics experiments, and I hear that the equivalent of "Hello World" for the Raspi is making an LED blink.

import RPi.GPIO as GPIO 
from time import sleep 
GPIO.setup(17, GPIO.OUT) 
while True: 
    GPIO.output(17, True) 
    GPIO.output(17, False) 

Hmm. That's a a lot of semantic noise for something so conceptually simple. All we want to is set the value of a pin. As soon as I saw this, I knew it would be ideal for Polymorphic Identifiers, because a pin is the ultimate state, and PIs and their stores are made for abstracting over state.

Of course, I first had to to get Objective-S running on the Pi, which meant getting GNUstep to run. While there is a wonderful set of install scripts, the one for the Raspi only worked with an ancient clang version and libobjc 1.9. Alas, that version has some bugs on the Raspi, for example with the imp_implentationWithBlock() runtime function that Objective-S uses to define methods.

Long story short, after learning about GNUstep installs and waiting for the wonderful David Chisnall to remove some obsolete 32 bit exception-version detection code from libobjc, we now have a script that installs current GNUstep with a reasonably current clang: With that in hand, a few bug fixes in MPWFoundation and Objective-S, I could add a really rudimentary Store that manages talking to the pins. And this allows me to write the following in an interactive shell to drive the customary GPIO pin 17 that I connected to the LED via resistor:

gpio:17 ← 1.

Now that's what I am talking about!

Of course, we're supposed to make it blink, not just turn it on. We could use the same looping approach as the Python script, or convenience methods like the ones provided, but the breadboard and pins make me think of wanting to connect components to do the job instead.

So let's connect some components, software architecture style! The following script creates an instance of a Blinker object (using an object literal), which emits alternating ones and zeros and connects it to the pin.

blinker ← #Blinker{ #seconds: 1 }. 
blinker → ref:gpio:17. 
blinker run.
gpio:17 ← 0.

Once connected it tells the blinker to start running, which creates an NSTimer adds it to the current runloop and then runs the run loop. That run is interruptible, so Ctrl-C breaks and runs the cleanup code.

What about setting up the pin for output? Happens automatically when you first output to it, but I will add code so you can do it manually.

Where does the Blinker come from? That's actually an object-template based on an MPWFixedValueSource.

object Blinker : #MPWFixedValueSource{ #values: #(0,1) }

You can, of course, hook up a fixed-value source to any kind of stream.

While getting here took a lot of work, and resulted in me (re-)learning a lot about GNUstep, the result, even this intermediate one, is completely worth it and makes me very happy. This stuff really works even better than I thought it would.

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