public DataType requireFieldType(Expression expression, String fieldName) {
        Field field = documentType.getField(fieldName);
        if (field == null)
            throw new VerificationException(expression, "Input field '" + fieldName + "' not found");
        return field.getDataType();
    }
Returns information about the types of fields from a document type.

@author bratseth
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/DocumentFieldTypes.java