Add Clash.Sized.Vector.ToTuple

[martijnbastiaan]

Newer GHCs (mistakenly) emit warnings when pattern matching on Vecs, with neither :> nor Cons being able to prevent it. vecToTuple allows users to convert Vecs to tuples, sidestepping GHCs exhaustiveness checker while not compromising on safety.

Nested tuples were considered as an alternative implementation, simplifying implementation and removing the need for TemplateHaskell. This would, however, compromise on user comfort while offering no expected real-world benefits.

---

This is clearly a work around. Perhaps we should write a GHC plugin to circumvent these warnings (and depending on the solution upstream)?

Documentation

Still TODO:

• Write a changelog entry (see changelog/README.md)
• Check copyright notices are up to date in edited files

[christiaanb]

Was there a reason you use :> instead of Cons for the pattern match?

[martijnbastiaan]

Was there a reason you use :> instead of Cons for the pattern match?

Discussed in the office: there is a difference in behavior. E.g. in the following:

vecToTuplePat :: (Vec 2 a) -> (a, a)
vecToTuplePat (a1 :> a2 :> _) = (a1, a2)

-- True
pat :: Bool
pat = case vecToTuplePat (1 :> undefined) of
  ~(a, _) -> a `seq` True

vecToTupleCon :: (Vec 2 a) -> (a, a)
vecToTupleCon (a1 `Cons` a2 `Cons` _) = (a1, a2)

-- Bottom
con :: Bool
con = case vecToTupleCon (1 :> undefined) of
  ~(a, _) -> a `seq` True

then pat ~ True, while con ~ _|_. We're not sure whether this will be an issue in practice, but we've decided to err on the safe side and be lazy.

[martijnbastiaan]

@leonschoorl I applied your suggestions, thanks!