private void executeMapEntry(MapEntryFieldValue entry, ExecutionContext context) {
        if (structFieldName.equals(keyName))
            context.setCurrentValue(entry.getKey());
        else if (structFieldName.equals(valueName))
            context.setCurrentValue(entry.getValue());
        else if (entry.getValue() instanceof StructuredFieldValue struct)
            executeStructField(struct, context);
        else
            throw new IllegalArgumentException("In " + this + ": Expected structured input, got " +
                                               entry.getValue().getDataType().getName());
    }
Returns the value of a struct field.

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