Struggling to work out HashMap implementation with custom type.

[ChillFish8]

We have a custom type in this case JsSafeBigInt which is basically a wrapper around a i64 that serializes to a string but is handled internally like a i64.

If we have a field with say this example struct:

#[derive(Object)]
struct Foo {
   bar: JsSafeBigInt,
}

It works (All the required traits are implemented for it.

But if we make this a map e.g.

#[derive(Object)]
struct Foo {
   bar: HashMap<JsSafeBigInt, bool>,
}

we get the following error:

 --> models\src\users\bases.rs:29:10
   |
29 | #[derive(Object, FromRow, ValueList, FieldNamesAsArray, Debug, Clone)]
   |          ^^^^^^ `poem_openapi::types::ParseError<JsSafeBigInt>` cannot be formatted with the default formatter
   |
   = help: the trait `std::fmt::Display` is not implemented for `poem_openapi::types::ParseError<JsSafeBigInt>`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
   = note: required because of the requirements on the impl of `ParseFromJSON` for `HashMap<JsSafeBigInt, bool>`
   = note: this error originates in the derive macro `Object` (in Nightly builds, run with -Z macro-backtrace for more info)

Which is slightly confusing. And im not entirely sure how to fix this.

[sunli829]

You need to implement the following traits for JsSafeBigInt. 😁

impl FromStr for JsSafeBigInt {
    type Err: ParseJsSafeBigIntError;

   ....
}

impl Display for ParseJsSafeBigIntError {
}

impl Display for JsSafeBigInt {
}

[ChillFish8]

Ah i think o may be missing the custom parse error, right now we just have ParseError<T>

[sunli829]

It works. 😁

#[derive(Debug, Eq, PartialEq, Hash)]
struct JsSafeBigInt(i64);

impl FromStr for JsSafeBigInt {
    type Err = std::num::ParseIntError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        s.parse()
    }
}

impl Display for JsSafeBigInt {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        Display::fmt(&self.0, f)
    }
}

#[derive(Object)]
struct Foo {
    bar: HashMap<JsSafeBigInt, bool>,
}

[ChillFish8]

Sorry I got distracted and forgot to respond, yes the error was rather than using std::num::ParseIntError it was implemented as poem_openapi::types::ParseError<Self>; which now, was probably a bit jank.

[sunli829]

I can't reproduce this error message:

#[derive(Object, FromRow, ValueList, FieldNamesAsArray, Debug, Clone)]
         ^^^^^^ `poem_openapi::types::ParseError<JsSafeBigInt>` cannot be formatted with the default formatter

On rustc 1.58.0 the error message looks correct.

#[derive(Object)]
         ^^^^^^ `JsSafeBigIntParseError` cannot be formatted with the default formatter

What rust version are you using? 🙂

[ChillFish8]

I believe that's the same error I was getting 😅 It just didn't occur to me that ParseError wouldn't implement Display if the inner type implemented Display.