5 ms·

What is "the arrow" in this context?

• tremon 6y ago
  "the arrow" is an expression of a function operation, or rather, how a function transforms its arguments. In Haskell, the following signature: func :: A -> B declares a function func that takes one argument of type A and returns a value of type B. I'm guessing a bit, I feel this goes over my head, but here's my layman's interpretation: In Rust, in order to declare a linear type, you must do so when initializing the variable: let x = vec![1, 2, 3]; Now x is declared as move-only, so that the vector cannot be duplicated by assigning it to a second variable; after assignment, accessing x will result in a compiler error. In Haskell, you declare a function func :: A %1 -> B to indicate that after this function call, accessing A should result in a compiler error.
  • paavohtl 6y ago
    I don't find this answer satisfactory, because you also need to specify in Rust whether or not a function moves its argument(s). // This moves the argument fn func(a: A) -> B { .. } func(x); // x is not accessible after this line // This borrows the argument, and therefore won't move it fn func(a: &A) -> B { .. } func(&x); // x remains accessible after this line
    • Ericson2314 6y ago
      From the perspective of Rust, Linear Haskell is going to seem quite weird, and I say that as Haskeller wishing that language extension was more like Rust. I'm not really sure how to explain it, but basically only functions that are themselves linear can have linear parameters, It's vaguely like "`Fn(A) -> B` only if `A: Copy`", and `FnOnce(A) -> B` only if `A: !Copy`, except there is no `Copy` or `!Copy` and the only say to determine it is `Fn` vs `FnOnce`.