Error parsing response when returning a Principal from a canister method written in Rust

[Paul Young]

When calling the following canister method:

#[query]
fn whoami() -> ic_cdk::export::Principal {
    ic_cdk::caller()
}

or:

#[query]
fn whoami() -> ic_cdk::export::candid::Principal {
    ic_cdk::caller()
}

I receive the following error:

ÙÙ÷£fstatushrejectedkreject_code�nreject_messagex¥IC0503: Canister rrkah-fqaaa-aaaaa-aaaaq-cai trapped explicitly: Custom(Cannot parse header 4449444c0001

Caused by:
    binary parser error: index at byte offset 6)

dfx canister call <my-project> whoami shows (principal "<my-principal>") so I expected that to work in a browser too. Other methods like the "hello/greet" example work in the browser.

• @dfinity/agent 0.11.1
• dfx 0.9.3
• ic-cdk 0.5.0
• macOS Monterey, Version 12.2.1 (21D62)
• Brave, Version 1.36.119 Chromium: 99.0.4844.83
• Safari, Version 15.3 (17612.4.9.1.8)

[Yan Chen]

The error seems to come from the Rust side trying to decode input argument. The client is sending DIDL\00\01, which is invalid. We should be sending DIDL\00\00 instead.

[Paul Young]

For what it's worth, it appears to work in the Candid UI but I get a different value than I do on the command line with dfx canister call

[Yan Chen]

Can you share the code for calling the canister in JS?

Browser identity and dfx identity are different. And for Candid UI, it only uses the anonymous identity for now.

[Paul Young]

This appears to have been due to some bindings I wrote that were conceptually typed as Unit -> Principal but called such functions as whoami({}) since {} can represent unit.