Belgium introduced a system of structured communication for bank transfers (Dutch: gestructureerde mededeling (GM); French: Communication structurée (CS)). The format is three digits, four digits and five digits separated by slashes (/), and preceded and succeeded by three plusses (+) or asterisks (*). The two last digits are a checksum of all the previous digits. An example and the lowest representable structured communication is +++000/0000/00097+++.

This package provides a data type StructuredCommunication to parse, validate, manipulate and render structured communication. The package aims to prevent constructing invalid structured communication, although it is not impossible when parsing, such that one can later fix the checksum of the parsed communication, or check its validity.

The main aspect of the package is the StructuredCommunication which encapsulates two Word16s and one Word32 for the three components of the structured communication. It might however be advisable not to use the data constructor, since it can construct StructuredCommunication that is simply invalid. It is better to use the structuredCommunication which only constructs a StructuredCommunication wrapped in a Just data constructor, if the given Integral parameters are out of range, or the checksum does not match. This is thus more safe.

StructuredCommunication is an instance of Show, but does not render it in the given format, it renders it as how you can use it as a quasiquoter, so - as is often done in Haskell - to use it in code.

One can use communicationToString :: StructuredCommunication -> String and communicationToText :: StructuredCommunication -> Text to convert it to a String or a Text which uses three plusses, followed by the three segments separated by a slash and three additional plusses.

One can also parse a StructuredCommunication with four parsers: communicationParser, communicationParser' communicationEParser, and communicationEParser'. The parsers without a prime perform also validation that the checksum matches. The parsers with an E also require that the stream ends. For each parser, there is also a conter-part function such as parseCommuncation, parseCommuncation', parseCommuncationE, and parseCommuncationE' which runs the parser with a stream. These functions either return the StructuredCommunication or a ParseError.

The package also has a QuasiQuoter beCommuncation that can be parsed to an expression or a pattern. This thus can be used to use structured communication for example to render it somewhere (on an invoice for example), but also to do pattern matching in a function to check if a certain StructuredCommunication matches.

The algorithm of the checksum sees the first ten digits as a whole number and determines the result of that number modulo 97. If that result is zero, the checksum is 97, otherwise it is the result of the modulo operation. So for +++123/4567/890……+++, the number is 1234567890, the result of the modulo operation is 2, so that means that the result with checksum is +++123/4567/89002+++.

Belgian bank account numbers also have twelve digits, except that one does not write slashes and plusses or asterisks, but the same checksum algorithm is used. The digits are grouped in three groups of four digits. Therefore the lowest Belgian banking account number is 0000 0000 0097. In order to convert this to an International Bank Account Number (IBAN), a prefix BE is added and two extra checksum digits. The IBAN variant of the lowest Belgian account number is thus BE54 0000 0000 0097.

Belgian VAT numbers use ten numbers, often defined in one group of four digits, and two groups of three digits. For transactions related this VAT number, one often adds the checksum to the VAT number to turn it into a structured communication. If the VAT number is thus 0000.000.000, then the structured communication to pay VAT in advance to the federal government is +++000/0000/00097+++. Beware that this is not the case for all banking transfers regarding the (VAT of) a company.

Some portals, like Bolero for example also use the client number as prefix, and then add the checksum to construct structured messages for bank transfers to the corresponding account(s).