try_map

UPDATE 20th of Feb 2018 The Rust standard library is getting this functionality with name transpose, so this crate is going to fade away. The tracking issue is here: rust-lang/rust#47338

try_map method for Option and flip method for Option and Vec. These helper methods allow more ergonomic error handling when mapping functions that return Result, over collections.

How to use:

Add to Cargo.toml:

[dependencies]
try_map = "0.3"

Bring the extension traits to the scope in your code: (FallibleMapExt is for enabling try_map and FlipResultExt is for enabling flip.)

use try_map::{FallibleMapExt, FlipResultExt};

Use the try_map and flip methods like a boss!

    fn try_map_example() -> Result<Option<i32>, &'static str> {
        let x = Some(42)
            .try_map(|x| Ok(x + 1))?
            .try_map(|x| Ok(x + 1))?
            .try_map(|x| if true { Err("oh noes") } else { Ok(x + 1) })?
            .try_map(|x| Ok(x + 1))?;
    
        Ok(x)
    }
    assert_eq!(try_map_example(), Err("oh noes"));

    fn flip_example() -> Result<Option<i32>, &'static str> {
        let x = Some(42)
            .map(|x| Ok(x + 1)).flip()?
            .map(|x| Ok(x + 1)).flip()?
            .map(|x| if true { Err("oh noes") } else { Ok(x + 1) }).flip()?
            .map(|x| Ok(x + 1)).flip()?;
    
        Ok(x)
    }
    assert_eq!(flip_example(), Err("oh noes"));

What else?

There is an open issue in the Rust RFC repo suggesting bringing these helper methods to the standard library: rust-lang/rfcs#1815 Thanks for @killercup for suggesting implementing these in a 3rd party crate. This way they are immediately useful. (My claim is that they would be still useful in the standard library, though!)

It seems that to be able to abstract the trait providing try_map over different kinds of collections, we need a support for higher-kinded types or associated type constructors. However, the flip method seems to be implementable for all kinds of things just using associated types.