#I have difficulties understanding rust docs.

44 messages · Page 1 of 1 (latest)

unique lance
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Just trying to use this package here https://docs.rs/ic-cdk/latest/ic_cdk/api/call/fn.call.html but I got way too many errors with it.
This is my first time using async with let v = futures::executor::block_on(example());
https://gist.github.com/rust-play/c6e3b5c305e571d459ef392e64efeafc

So, After implementing this just like the docs says I got this wired error

[E0277]: the trait bound `for<'a> std::string::String: ArgumentDecoder<'a>` is not satisfied
   --> frontend/src/main.rs:28:103
    |
28  | ..., String)> = ic_cdk::api::call::call(principal, "greet", ("Ali".to_string(), )).await;
    |                 ^^^^^^^^^^^^^^^^^^^^^^^ the trait `for<'a> ArgumentDecoder<'a>` is not implemented for `std::string::String`
    |
    = help: the following other types implement trait `ArgumentDecoder<'a>`:
              ()
              (A, B)
              (A, B, C)
              (A, B, C, D)
              (A, B, C, D, E)
              (A, B, C, D, E, F)
              (A, B, C, D, E, F, G)
              (A, B, C, D, E, F, G, H)
            and 9 others
light bolt
unique lance
light bolt
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no, what does "implementing this just like the docs says" mean?

unique lance
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So I did the same

light bolt
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there is no example there

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and the api is really weird

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writing ic_cdk::api::call::CallResult<(String,)> should work

unique lance
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Yeah 🥹

light bolt
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either that or the docs would be outdated

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which happens

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but not this time

unique lance
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I was thinking maybe I didn’t understand the docs cuz I am new to rust

light bolt
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yeah the only thing was the ArgumentDecoder bound on R

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ah I see why it's not implemented for String

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because it is on (A, B) etc.

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eh

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still weird

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btw (String,) is a 1-element tuple

unique lance
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Oh, the ArgumentDecoder and ArgumentEncider Was confusing as well

unique lance
light bolt
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the second generic parameter of fn call

unique lance
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Aha if it is generic instead of <R> i can say <String>

light bolt
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yeah kind of

unique lance
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one more help please

let greeting: ic_cdk::api::call::CallResult<&str, (ic_cdk::api::call::RejectionCode, &str)> = ic_cdk::api::call::call(principal, "greet", ("Ali", "")).await;

I removed the type here ic_cdk::api::call::CallResult<&str, (ic_cdk::api::call::RejectionCode, &str)>

let greeting = ic_cdk::api::call::call(principal, "greet", ("Ali", "")).await;

Now it cannot infer the type R
However here is my mistake
the output are not CallResult but it is impl Future<Output = CallResult<R>>

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error[E0698]: type inside `async fn` body must be known in this context
  --> frontend/src/main.rs:26:20
   |
26 |     let greeting = ic_cdk::api::call::call(principal, "greet", ("Ali", "")).await;
   |                    ^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `R` declared on the function `call`
light bolt
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once you call await you get the Output

unique lance
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yeah

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I should

light bolt
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you can specify R with call::<_, (&str,)>(principal, ...).await;

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although that won't work

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because you need String instead

unique lance
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Aha
what about somthing like

let greeting: Future<Output = CallResult<String>> = ic_cdk::api::call::call(principal, "greet", ("Ali", "")).await;

so it infer R as String

light bolt
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you can't write that in rust

unique lance
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so how can I tell rust R is String

light bolt
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also impl Future "disappears" when you call await

light bolt
unique lance
light bolt
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I told you how to specify it without that

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call::<_, (String,)>(principal, ...).await;

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if you use it later with something that expects that type you don't have to specify that at all