dnsco commented on issue #3644:
URL: https://github.com/apache/arrow-rs/issues/3644#issuecomment-1412906085
So the builders implement extend but an &mut Builder does not have an
implementation. I'm bashing my head against the wall trying to figure out how
to make it work.
here's the new method:
```rust
pub fn set_val<T, R, V, F>(mut builder: T, value: Option<V>, getter: F) ->
Result<()>
where
T: Extend<Option<R>>,
V: Deref<Target = Value>,
F: FnOnce(&V::Target) -> Option<R>,
{
let v = value
.as_deref()
.map(|v| {
getter(v).ok_or_else(|| {
let msg = format!("Could not cast {} to correct type", v);
Error::new(InvalidData, msg)
})
})
.transpose()?;
builder.extend(vec![v]);
Ok(())
}
```
called as:
```rust
DataType::Float64 => {
let b =
struct_builder.field_builder::<Float64Builder>(i).unwrap();
set_val(b, value, Value::as_f64)?
}
```
Yielding the error:
```
96 | set_val(b, value, Value::as_f64)?
| ------- ^ the trait `Extend<std::option::Option<_>>` is not
implemented for `&mut PrimitiveBuilder<Float64Type>`
| |
| required by a bound introduced by this call
|
= help: the trait `Extend<std::option::Option<<P as
ArrowPrimitiveType>::Native>>` is implemented for `PrimitiveBuilder<P>`
note: required by a bound in `set_val`
--> project/src/record_conversion/builder_appending.rs:64:8
|
62 | pub fn set_val<T, R, V, F>(mut builder: T, value: Option<V>, getter: F)
-> Result<()>
| ------- required by a bound in this
63 | where
64 | T: Extend<Option<R>>,
| ^^^^^^^^^^^^^^^^^ required by this bound in `set_val`
```
Am I missing something simple? If I try and reference ```*b``` at the
callsite I get:
```
error[E0507]: cannot move out of `*b` which is behind a mutable reference
--> prost-arrow/src/record_conversion/builder_appending.rs:96:21
|
96 | set_val(*b, value, Value::as_f64)?
| ^^ move occurs because `*b` has type
`PrimitiveBuilder<Float64Type>`, which does not implement the `Copy` trait
```
Is there a reason there can't be an implementation of Extend for &mut
Builders?
Thanks!
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