@Override
    public DataType setOutputType(DataType outputType, TypeContext context) {
        super.setOutputType(outputType, context);
        return getInputType(context);
    }
Returns the value of a struct field.

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