when

fnSeries when(Series mask, Series a, Series b)

Eager conditional selection (ADR-2026-10-23-08): element-wise mask ? a : b under the approved semantics - a BOOLEAN mask of the branches' length, supertype-promoted branches, and a null mask slot yielding a null output slot (unlike filter, where null drops, nothing is dropped here, so the null stays visible). A null slot in the selected branch propagates its null; values of unselected slots are never read.

Parameters

`mask`	the BOOLEAN condition
`a`	the branch selected where the mask is `true`
`b`	the branch selected where the mask is `false`

Returns

the selected Series, named after a.

Throws

DataFrameException when the mask is not BOOLEAN or the

lengths differ.

fnSeries when(T)(Series mask, Series a, T b) if (!is(T == Series))

Ditto, with a scalar b branch broadcast across the mask.

fnSeries when(T)(Series mask, T a, Series b) if (!is(T == Series))

Ditto, with a scalar a branch broadcast across the mask.

fnSeries when(T, U)(Series mask, T a, U b) if (!is(T == Series) && !is(U == Series))

Ditto, with both branches scalar.