Is it possible to cast from Stream Float to Stream Word16?

[fer7una]

Hi everyone,

I'm working on Arduino.Copilot and my goal is to calculate the PWM signal interpreted as a Word8. I have to divide the value received from analog input (Int16) by 1023, multiplying the result by 0.5:

analogValue0 <- input a0 :: Sketch (Stream Int16) -- receive the value from A0 pin
analogValue1 <- input a1 :: Sketch (Stream Int16) -- receive the value from A1 pin
let valor = unsafeCast (abs ((analogValue0 - 511) `div` 2) :: Stream Float -- get the initial value
    analogValueF = unsafeCast analogValue1 :: Stream Float 
    factor = (analogValueF / 1023) * 0.5
    pwmValue = unsafeCast (valor * factor) :: Stream Word16 -- trying to cast from Stream Float to Stream Word16
pin3 =: pwm (unsafeCast pwmValue)

The problem is that there is no instance for UnsafeCast Float Word16. Someone who knows any alternative solution?

[ivanperez-keera]

@fdedden is this something we could add?

There's not even an instance for UnsafeCast Float Double, which is strange.

[RyanGlScott]

There isn't a way to do that at present, but we would like to add such a way to Copilot. Can you comment above on which semantics listed in #571 (reply in thread) would match your expectation on how a hypothetical Float-to-Word8 cast should work?

[ivanperez-keera]

Based on the code that I saw from @fer7una (received separately), I think he's looking for something like rounding. That being said, I think truncating would be enough.

Also, you can implement rounding if you have truncation:

round x
    | (x - x') >= 0.5 = x' + 1
    | otherwise       = x'
  where
    x' = truncate x

Haskell's prelude has an implementation that also works for negative numbers. It's a bit more convoluted. It could be extended to streams.

[fer7una]

Studying @ivanperez-keera's answer I have a provisional solution to the rounding problem, getting a Stream Float with rounded values and a function that receives a Stream Float and returns the same value in Stream Word8, it works by setting the list defined in the function with the proper range.

import Copilot.Language.Stream as Copilot
import Copilot.Arduino.Uno
import Data.Foldable

main :: IO ()
main = arduino $ do
  analogValue0 <- input a0 :: Sketch (Stream Int16) -- receive the value from A0 pin
  analogValue1 <- input a1 :: Sketch (Stream Int16) -- receive the value from A1 pin 
  let valor = unsafeCast abs((analogValue0 - 511)`div` 2) :: Stream Float
      analogValueF = unsafeCast analogValue1 :: Stream Float
      factor = (analogValueF/1023)*0.5
      pwmValueFloat = valor*factor
      pwmValueFloatRound = roundStream pwmValueFloat
      pwmValueW8 = eqValue pwmValueFloatRound 
  delay =: MilliSeconds 100

-- | Round using Copilot.Language.Stream
roundStream :: Stream Float -> Stream Float
roundStream x =
  if (x - x') >= 0.5
	then x' + 1
	else x'
  where
	x' = Copilot.floor x

-- | Return a Word8 value equal to the given Float (needed to round the Float value beforehand, if not exact, the function will return 0)
eqValue :: Stream Float -> Stream Word8
eqValue target = foldl1 (.|.) (map (\ s -> mux (unsafeCast s == target) s 0) streams)
  where
	streams = map constW8 [0..255]

[ivanperez-keera]

Does this compile and run (generate code)?

[fer7una]

The complete code with the WheelMotor sketches compiles! But I can't test it in the simulator because it generates a program that uses 96700 bytes of program memory (maximum is 32256 bytes).

[ivanperez-keera]

I've been looking around a bit. I think, to do this efficiently, we'll have to introduce new operations.

On a related note, I think we could implement the following two classes for Streams: https://hackage.haskell.org/package/base-4.21.0.0/docs/Prelude.html#t:Fractional and https://hackage.haskell.org/package/base-4.21.0.0/docs/Prelude.html#t:Floating.
We cannot do RealFrac or RealFloat.

[RyanGlScott]

Correct me if I'm wrong, but isn't Stream already an instance of Fractional and Floating?

[ivanperez-keera]

https://i.imgur.com/ktyIhi8.gif

[ivanperez-keera]

This will sit right there in history next to when I look for the answer to something and I end up finding my own response to the same question on StackOverflow #facepalm