@Override
    protected void doExecute(ExecutionContext context) {
        FieldValue input = context.getCurrentValue();
        if (input instanceof StructuredFieldValue struct)
            executeStructField(struct, context);
        else if (input instanceof MapEntryFieldValue entry)
            executeMapEntry(entry, context);
        else
            throw new IllegalArgumentException("In " + this + ": Expected structured input, got " + input.getDataType().getName());
    }
Returns the value of a struct field.

@author Simon Thoresen Hult
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/GetFieldExpression.java