Why do I need to write let to declare a variable?

Question

In Haskell I don't need to write anything to declare a variable. In C++ I need to write auto, which as far as I know works in an analogous way to rust's let.

• Isn't it a step back to use let to declare a variable?:

  let hi = "hi";
  

• Type inference and the assignment operator should be enough, or aren't they?:

  hi = "hi";
  

I'm just asking because the first thing that caught my attention while skimming through Rust's tutorial were the let's everywhere. I felt like, I shouldn't be needing to type it! The compiler already knows that I'm declaring a variable! For declaring uninitialized variables, one could argue that it might be nice to declare them with a type. But again, it's optional, a matter of style. The compiler can deduce the type at first use, and don't compile if it's not used and hence can't deduce the type.

• What is the rationale behind forcing the users to write let? In particular, what is the rationale against making let optional?

Answer 1

I am not so sure about grammar considerations (I think omitting it would be fine, grammar-wise, just more complex), but let and variable assignment are not the same thing in Rust. let is 1. pattern matching, 2. binding introducing. You can only do x = 3 if x is a mutable slot, but you can always do let x = 3, it introduces a new binding of a possibly different type and mutability. Removing let would make the current binding semantics impossible. It would also make patterns much more difficult, if not impossible. For example, let (a, b) = some_fn_returning_tuple();.