Clap can not parse YAML file: failed to convert YAML String("1") value to a string

Question

I use a YAML file to define my command line interface. I parse the file using Clap's load_yaml! macro, which worked out fine for some time:

#[macro_use]
extern crate clap;
use clap::{App, ArgMatches};

fn main() {
    let yml = load_yaml!("cl_arguments.yml");
    let matches = App::from_yaml(yml).get_matches();
    # some code goes here
}

Without me making any changes to the relevant code, this stopped working and I get the following error:

thread 'main' panicked at 'failed to convert YAML String("1") value to a string',
/home/me/.cargo/registry/src/github.com-1ecc6299db9ec823/clap-2.31.2/src/args/arg.rs:112:28

I am not sure if an update is to blame for this. I updated Rust and removed the Cargo.lock to rule out incompatibility issues, but that did not help.

This is a minimal version of my YAML file that produces the error:

name: "tool"
version: "0.1"
about: "description"
author: "m00am"

subcommands:
  - subcommand1:
      args:
        - path:
            index: "1"
            required: true
            default_value: "/tmp/"
            help: "Dummy Path"
  - subcommand2:
      args:
         - other_path:
             index: "1"
             required: true
             help: "A second dummy path"

Is this a known issue? What went wrong here? Is there anything I can do besides trying out combinations of older versions of Clap and the YAML crate?

Version info

Excerpt from Cargo.toml

[dependencies]
rand = "*"
cute = "0.3.0"
fasthash = "*"
bio = "*"
rulinalg = "*"
serde = "*"
serde_derive = "*"
bincode = "*"
statrs = "*"
separator = "*"
termion = "*"
sysinfo = "*"
clap = { version = "*", features = ["yaml"] }
needletail = "*"
gnuplot = "*"
time = "*"
cue = "*"

Excerpt from Cargo.lock

[[package]]
name = "clap"
version = "2.31.2"
source = "registry+https://github.com/rust-lang/crates.io-index"
dependencies = [
 "ansi_term 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)",
 "atty 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)",
 "bitflags 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
 "strsim 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)",
 "textwrap 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)",
 "unicode-width 0.1.4 (registry+https://github.com/rust-lang/crates.io-index)",
 "vec_map 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)",
 "yaml-rust 0.3.5 (registry+https://github.com/rust-lang/crates.io-index)",
]

...

[[package]]
name = "yaml-rust"
version = "0.3.5"
source = "registry+https://github.com/rust-lang/crates.io-index"

Can you show your Cargo.toml? Which crate versions are the project's dependencies locked to? — E_net4, Apr 20 '18 at 12:29

score 5 · Accepted Answer · answered Apr 20 '18 at 13:53

The index property of an argument specification should be an integer, not a string. One can find an example of a positional argument with an explicit index in the documentation. Remove the quotes from around that value and the configuration file will work again.

name: "tool"
version: "0.1"
about: "description"
author: "m00am"

subcommands:
  - subcommand1:
      args:
        - path:
            index: 1
            required: true
            default_value: "/tmp/"
            help: "Dummy Path"
  - subcommand2:
      args:
         - other_path:
             index: 1
             required: true
             help: "A second dummy path"

$ cargo run -- subcommand1 --help

USAGE:
    tool subcommand1 <path>

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

ARGS:
    <path>    Dummy Path [default: /tmp/]

I could not identify the reason why this used to work in previous versions. There is a chance that the implementation allowed it beyond the intended functionality. Otherwise, it could have been a breaking change from an early version of Clap with YAML configuration support. The dependencies in your Cargo.toml file are too flexible for long-term use, and makes these issues more likely to happen. Sticking to the default (caret) version ranges is usually the right thing to do (see specifying dependencies).

Thank you for your answer. I already learned a lot here. However, I might have obfuscated the actual error when crafting my minimal example. Initially the error was `thread 'main' panicked at 'failed to convert YAML Integer(1) value to a string`, so I made them strings. With the index values as integers the minimal example works for me, but my actual file doen't. I will change the question title to better suit the error I described here, do some more research and maybe ask a new question with a more detailed description of my problem. — m00am, Apr 21 '18 at 07:45
Turns out I added a `default_value` with an integer instead of a string, which turned out to be the reason for that error. Thank you for your help. — m00am, Apr 23 '18 at 09:13

Clap can not parse YAML file: failed to convert YAML String("1") value to a string

Version info

1 Answers1