@Override
    public DataType setInputType(DataType inputType, TypeContext context) {
        super.setInputType(inputType, context);
        if (inputType == null) return null;
        return getFieldType(inputType, context);
    }
Returns the value of a struct field.

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