[mrgriffin]

> `foo :: Semigroup a, Traversable t => t a -> a` I already know that whatever is passed to this function can be traversed and have it's sum computed. It's impossible to pass something which doesn't satisfy both of those constraints as this is a specification given at the type level which is checked at compile-time.

To add to your point, I don't think foo can even be implemented (more accurately: is not total) because neither `Semigroup a` or `Traversable t` guarantee a way to get an `a`.

I think you'd need either `Monoid a` which has `mempty`, or `(Foldable1 t, Traversable t)` which guarantees that there's at least one `a` available.

[tauoverpi]

Yep, I missed it as I don't often work in haskell anymore but with the correction the rest of the above still stands (haskell syntax is still the most efficient I'm aware of for talking about this). Also, it being unimplementable is also a nice property of having such a type system as you can prove that your specification is impossible to implement (more applicable in Lean, ATS2, Idris, Agda) or make invalid states impossible to even write (consider state transition rules at the type level).