Representing products

[bgamari]

One annoyance that I sometimes encounter is impedance mismatches due to the dual treatment of products in Clash. That is, a product can be represented as either a signal-of-a-product or as a product-of-signals (which are isomorphic as witnessed by Bundle). Under the status quo, product types will generally favor one of these representations, with the other being rather represented as a tuple. This asymmetry is awkward and compromises type safety.

A pattern that I have used in my projects is to represent products as higher-kinded type. For instance, one might have a simple product:

data Spi_M2S f
    = Spi_M2S { spiSclk :: f Bit
              , spiCs   :: f Bit
              , spiMosi :: f Bit
              }

where one can then provide a Bundle instance:

instance Bundle (Spi_M2S Identity) where
    type Unbundled dom (Spi_M2S Identity) = Spi_M2S (Signal dom)
    bundle = ...
    unbundle = ...

I have been tempted to write a TH splice to derive such instances in the past.

Of course, this is a slightly unfortunate tradeoff as we must now manually wrap/unwrap Identitys whenever working with the bundled type. I often find myself minimizing this syntactic overhead by locally binding mkI = Identity and unI = runIdentity.
However, another design would be to provide pattern synonyms to handle the common bundled case:

data Spi_M2S_F f
    = Spi_M2S_F { spiSclk :: f Bit
              , spiCs   :: f Bit
              , spiMosi :: f Bit
              }

type Spi_M2S = Spi_M2S_F Identity

pattern Spi_M2S :: Bit -> Bit -> Bit -> Spi_M2S
pattern Spi_M2S a b c = Spi_M2S_F { spiSclk = Identity a, spiCs = Identity b, spiMosi=Identity c }

This is another tricky set of trade-offs as now we rely on the DuplicateRecordFields and PatternSynonyms extensions. Is this worth it? I don't know.

Have others thought about this problem? Is there a nicer solution that I have missed?

[gergoerdi]

I've been using Barbies for this, so that the unbundled type can have no wrappers on the field types:

https://github.com/gergoerdi/retroclash-lib/blob/master/src%2FRetroClash%2FBarbies.hs

[bgamari]

That is a very useful reference. Thanks!

[gergoerdi]

That is a very useful reference. Thanks!

You might also like this usage example, then: https://github.com/gergoerdi/retroclash-lib/blob/master/src%2FRetroClash%2FCPU.hs