Topological Unsort

To implement program with multiple languages, the languages must communicate. C is an interface language; most other languages have a C interface. We organize data in C programs with struct and union. To communicate between C and other languages data is converted to and from struct/union. Threads and processes send messages, each of which is a different struct/union. Most languages are more powerful than C, so they can restrict portions of their data organization to conform to C struct/union. When there are many different struct/union types to send, it can become arduous to change the corresponding data types in each language. Preferably, a script would automatically generate the struct/union types in each language. Each field of a struct/union can be a pointer to one or more other contiguous struct/union instances, an array, a basic type, an enumeration type, or a string. Each field can have validity dependent on other fields, called tags. Think of the tags as dimensions; tag values are a tag space. Each field is valid in a subset of the tag space. C is limited in that without an unwieldy number of identifiers, struct/union can not have a unique representation for every point in the tag space. Instead, consecutive fields should have similar tag space subsets. My script to generate struct/union types in several languages, takes as input lists of fields, each with a tag space subset. Consecutive fields or unions with equal tag space subsets collect into structs. Consecutive fields or structs with disjoint tag space subsets collect into unions. A tag space subset opens a union. A tag space superset closes structs and unions. A topological sort of the graph is the original list of fields.

State File Sets

It is common for an application to have state that persists between invocations. It is limiting to think of an application or its state in the singular. It is confusing for an application’s state to be hidden or unreadable. Instead, go with the Unix concepts of text files and processes, such that the processes state is completely determined by the content of text files given to it at invocation. At any one time, there can be multiple instances of an application, running in parallel. As a process’ state changes, the changes are captured by one or another of the text files given to it at invocation. Two processes with the same given text files would be in the same state.