05_solution.md

5 Solution

Quiz

Notice the last line of this function. What will the compiler complain is wrong with this function? And why?

fn read_version(transactionhex: &str) -> u32 {
    // convert hex to bytes
    let transaction_bytes = hex::decode(transactionhex);
    1;
}

Solution

Notice how there's no return statement at the bottom of the function. Rust will automatically return the last expression without a semicolon. However, if you look closely, there is a semicolon after the 1. This means there is nothing being returned from our function. In this case, Rust assumes that you mean to return an empty tuple type, (). So our function body is expected to return an empty tuple, but our function signature states that it must return a u32 type. This mismatch is what produces a compile error.

error[E0308]: mismatched types
 --> src/main.rs:1:43
  |
1 | fn read_version(transaction_hex: &str) -> u32 {
  |    ------------                           ^^^ expected `u32`, found `()`
  |    |
  |    implicitly returns `()` as its body has no tail or `return` expression
...
4 |     1;
  |      - help: remove this semicolon to return this value