alt Hacker NewsNothing wrong with addressing it conceptually! We will, in the upcoming versions, probably via codata / coroutines. For V1, I'm keeping the language set smell. When it is stable, we'll add more features. Lean had 10+ years to mature; Bend is on day 1.
`-` means "erased argument". You can use an erased argument as many times as you want, in erased positions. That's also how QTT works (Idris2 is based on it). This example is there precisely to introduce Kinds, which are universes indexed on quantities.
- Kind(&2) is inhabited by clonable values. - Kind(&1) is inhabited by linear values. - Kind(&0) is like Rocq's Prop.
`A & B` is just sugar for the pair type former (which is sugar for a sigma).
Thanks for your questions and patience!
So why does the length function take the ‘a’ parameter (the type of the elements?) and its Kind? Wouldn’t the type imply the kind? Why does the kind matter? Is the - a constraint on the kind? How would the program be different without the -?
When you say “pair type former” do you mean that Array<U32> & U32 is what Rust would call (Array<U32>, U32)? If so, why does that example function actually return a value of this type? It sure looks like it returns plain U32.
> You can use an erased argument as many times as you want, in erased positions.
What’s the rationale for this? Why is an “erased” position special? What is an erased position, anyway?
ISTM if I want to use an affine term that has zero size at runtime as a token that may be used at most once, I think I wouldn’t want an exception for using it in an “erased” position. Can I have a function like a -> a & a where the input is “erased”?