FIR example on the Chisel 3 website with more idiomatic Scala functional programming concepts

[Øyvind Harboe]

I wondered why the example on the Chisel website isn't using more idomatic functional programming concepts...

https://github.com/chipsalliance/chisel3

https://scastie.scala-lang.org/4HOSVJLPTv68Gn6J2ih0TQ

So rather than:

  zs(0) := io.in

  for (i <- 1 until coeffs.length) { zs(i) := zs(i-1) } 

  // Do the multiplies

  val products = VecInit.tabulate(coeffs.length)(i => zs(i) * coeffs(i))

  

Use: 

  zs := Seq(io.in) ++ zs.tail

  // Do the multiplies

  val products = (zs zip coeffs).map{case (a, b) => a * b}

[michael etzkorn]

There's an example of this module doing exactly that in some tutorial slides somewhere (I know cause my coworker found it and asked me about map). I think the motivation for not doing so (and probably why the version with map was difficult for me to find on my own) is that the example needs to be accessible to novices wanting to pick up Chisel. We need to have examples targeted to beginners and people who've never seen software techniques on hardware data structures.

[Jack Koenig]

Michael is right that the main motivation there is trying to appeal to a wider (less experienced in functional programming audience). That being said, I wouldn't be opposed to having a tutorial taking that original example and turning it into the more functional version.

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[Øyvind Harboe]

Where in the source code would such a pull request go?

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Øyvind Harboe
+4791786146

[Jack Koenig]

All docs for the chisel3 section on the Chisel website (https://www.chisel-lang.org/chisel3) is located in the chisel3 repo in the docs directory: https://github.com/chipsalliance/chisel3/tree/master/docs.

If you wanted to add a new file it also needs to be mentioned in the menu.yml in the website repo: https://github.com/freechipsproject/www.chisel-lang.org/blob/c6651c5d4f35fd8c033f0a654513c646fc53ed19/docs/src/main/resources/microsite/data/menu.yml

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[Michael Etzkorn]

I did find one of the original versions of the FIR example in the Berkeley Chisel Tutorial paper from 2017. 

def Delays[T <: Data](x: T, n: Int): List[T] = if (n <= 1) List(x) else x :: Delays(RegNext(x), n-1) 

def FIR[T <: Data with Num[T]](ws: Seq[T], x: T): T = (ws, Delays(x, ws.length)).zipped. map( _ * _ ).reduce( _ + _ )  

Definitely like this style of using functions to instantiate the logic since it's like building a library of hardware that can then be worked into the modules. 

Paper can be found here: 

https://inst.eecs.berkeley.edu/~cs250/sp17/handouts/chisel-tutorial.pdf