Rust chaining Results (Combinators)

Question

I've been following a Rust tutorial where two versions of a function are purported to generate the same results:

Version 1:

pub fn get_transactions(fname:&str) -> Result<Vec<Transaction>,String> {
    let s = match std::fs::read_to_string(fname){
        Ok(v)=>v,
        Err(e)=> return Err(e.to_string()),
    };
    let t:Vec<Transaction> = match serde_json::from_str(&s) {
        Ok(v)=>v,
        Err(e)=> return Err(e.to_string()),
    };
    Ok(t)
}

Version 2:

fn get_transactions_b(fname:&str) -> Result<Vec<Transaction>,String> {
    std::fs::read_to_string(fname)
    .map_err(|e| e.to_string())
    .and_then(|ld| serde_json::from_str(&ld) )
    .map_err(|e| e.to_string())
}

---

However, I get the following message for version 2:

mismatched types

expected struct std::string::String, found struct serde_json::error::Error

note: expected enum std::result::Result<_, std::string::String> found enum std::result::Result<_, serde_json::error::Error>

help: try using a variant of the expected enum: _::_serde::export::Ok(serde_json::from_str(&ld)),

---

which I been unable to make head or tail out of:

• Hovering over ld closure argument |ld| in VS Code it says it's of type std::string::String

• Hovering over the ld in the from_str(&ld) I get the message.

• Now I understand a Result is comprised of an Ok and an Err, but I thought the combinator chaining would have worked.

• The compiler suggested fix doesn't work either.

  • (or make sense): What is the _::_ all about?
  • Why would the Ok not be inside the from_str?

• What would you have to do to make version 2 work?

Here's the Result with the methods for combinators in the Rust docs.

---

Cargo.toml

[dependencies]
serde = "1.0.115"
serde_derive = "1.0.115"
serde_json = "1.0.57"

---

• cargo 1.45.1
• rustc 1.45.2

Answer 1

The problem comes from the and_then call.

You are not allowed to change the error type when calling and_then. So your function there should return a Result<Vec<Transaction>, String>. However, the error type returned by serde_json::from_str is a serde_json::error::Error.

You can fix it like this:

std::fs::read_to_string(fname)
    .map_err(|e| e.to_string())
    .and_then(|ld| serde_json::from_str(&ld).map_err(|e| e.to_string()))

Notice the call to map_err is now inside the function passed to and_then.