Nathan-Fenner opened a new issue, #5011:
URL: https://github.com/apache/arrow-rs/issues/5011
**Is your feature request related to a problem or challenge? Please describe
what you are trying to do.**
`zip` currently has the signature:
```rs
/// # Arguments
/// * `mask` - Boolean values used to determine from which array to take the
values.
/// * `truthy` - Values of this array are taken if mask evaluates `true`
/// * `falsy` - Values of this array are taken if mask evaluates `false`
pub fn zip(
mask: &BooleanArray,
truthy: &dyn Array,
falsy: &dyn Array,
) -> Result<ArrayRef, ArrowError> { ... }
```
However, it would be nice to be able to call `zip` where (usually just one
of) the arguments might could be scalar, i.e. via `&dyn Datum` (i.e. analogous
to `arrow::compute::kernels::numeric::add`):
```rs
/// # Arguments
/// * `mask` - Boolean values used to determine from which array to take the
values.
/// * `truthy` - Values of this array are taken if mask evaluates `true`
/// * `falsy` - Values of this array are taken if mask evaluates `false`
pub fn zip(
mask: &BooleanArray,
truthy: &dyn Datum, // a `Datum` instead of `Array`
falsy: &dyn Datum, // a `Datum` instead of `Array`
) -> Result<ArrayRef, ArrowError> { ... }
```
Currently, a caller with a (possibly) scalar value for one of the arguments
needs to essentially implement `zip` from scratch, and requires downcasting the
arrays in order to be able to obtain and copy the (singleton) scalar value.
**Describe the solution you'd like**
Update the `zip` function to accept `if_true: &dyn Datum` and `if_false:
&dyn Datum` parameters (just like `add`) with logic to copy the values to each
output row if needed, or make a new `zip` function which takes those
parameters as the type `&dyn Datum`.
(`mask` does not need to be a `&dyn Datum`, since when the `mask` is a
scalar, it is easy to avoid calling `zip` entirely)
**Describe alternatives you've considered**
Manually replicating the `if_true` or `if_false` values into an array prior
to calling `.zip` would work, but requires some tedious downcasting to support
every type, and is also probably less efficient because it requires
constructing (and then probably throwing away) a temporary array of the same
value repeated multiple times.
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