Bindings Are Cheap: Managing Rightward Drift

Ryan James Spencer

February 20 2020, 8:34PM

How do you avoid deeply nested if let or match statements when you're first coming to Rust? Rightward drift is a pain to decipher in any language, but the good news is you can manage rightward drift in Rust with a few techniques and some mental shifting. Maybe this is the code you're writing which has a lot of if-let chaining:

if let Some(x) = some_func() {
    // do stuff with x
    if let Some(y) = some_func2() {
        // do other stuff with y
        if let Some(z) = some_func3() {
          // and so on
        } else {
          reticulating_splines()
        }
    } else {
        engage_thrusters()
    }
} else {
    launch_the_missiles()
}

In Rust, everything is an expression, and every expression has a value. For control flow, that means all branches must return values of the same type. If you look at the code above you ought to see that whole thing as (), assuming the functions in the else blocks above return (). When I look at the above code snippet I think "this code is always meant to succeed but with different results on the types of success". This code is always mapping Some and None to (), which doesn't tell the caller much besides "I might have done something."

A None implies the absence of something. If we want more information for why the data we want isn't there we can use the Err variant on Result. The intent with the try (?) operator is to always allow a way to express this 'failure' back to the caller when it first happens; we should not assume we can go ahead safely with the subsequent code and return from the current function.

A style I like to recommend to people is known by some as "newspaper article" style. Since Rust is an expression-oriented language we can let-bind to almost anything! This means we can write our fix as:

let x = some_func()
  .or_else(|| { launch_the_missiles(); None } )?;
let y = some_func2()
  .or_else(|| { engage_thrusters(); None } )?;
let z = some_func3()
  .or_else(|| { reticulating_splines(); None } )?;
// and so on.

If we wanted to only give the caller the sense that nothing bad happened, we could wrap the whole thing in a block and discard the result (NB. the semicolon at the end of the block):

fn top_level() {
    fn go<T>() -> Option<T> {
        let x = some_func()
          .or_else(|| { launch_the_missiles(); None } )?;
        let y = some_func2()
          .or_else(|| { engage_thrusters(); None } )?;
        let z = some_func3()
          .or_else(|| { reticulating_splines(); None } )?;
        // and so on.
    }
    go(); // throw away the result for the caller.
}

But this is weird. Giving callers control is at the crux of good error handling, especially when it comes to something as powerful as errors as values!

What I absolutely love about the rampant let-bindings approach is that it provides a lot of flexibility for modification; with let bindings we can remove or modify the offending assignments exactly, rather than mangling a rather delicately constructed expression.

Rust also lets us shadow variables and with its move semantics we can avoid unexpected allocations when doing things like expressing data as it changes throughout various steps but under the same name:

struct Json {
  property: i64,
}

struct Error {
  SerdeError(serde::Error),
  IoError(std::fs::IoError),
}

fn update_json() -> Result<(), Error> {
  let json = include_str!("../some.json");
  let json: Json = serde_json::from_str(&json);
  json.property = 42;
  let json = serde_json::to_string(&json);
  fs::write("../some.json", json)?;
}

Use let bindings and the try operator liberally and you'll make your code easier to modify and read. If you have custom types you've written yourself you might,

  • one day be able to write an implementation for the Try trait yourself when it stabilizes (its currently experimental)
  • take a cue from Option and Result and write similar combinators that let you get at internal data for your type
  • merely wrap things in Option and Result and use the bevy of methods they expose

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