Sunday, November 4, 2007

"Strong" typing not so strong?

I've been in debates about the wisdom of defining web services with "strongly" typed messages (with specific fields of "specific" types) versus more general messages with, say, fields containing XML content. I have argued that so-called strong types like integer and string are no more "strong" than a segment of XML. This is because the string (and to a lesser extent integer, float, etc) data types are so broad that they can't really be called "strong". Strings are almost always further interpreted by the software and people on both ends of a message.

This notion is embodied by the testing technique known as Equivalence Partitioning. [A good summary is in the article "Make Child's Play of Equivalence Class Partitioning".] It says that there is more detailed structure to data than string and integer and to only test at that level is to miss many crucial test cases. It defines subranges and such to guide what to test, in effect, defining more granular data types. Languages like Pascal and Ada had more refined data types like subranges but those ideas were lost when C-like languages (including C++ & Java) came into vogue.

Existential Programming would change the way data types are defined to get away from simply defining a structure that is an aggregation of "primitive data types". It would let data types define a function that computes whether (or how closely) a putative value meets the requirements for that data type. While one could put that logic in a "setter" method of a class, that doesn't work with direct assignments (via assignment operators) of values (at least in Java-like languages).

To see where even subrange definitions like Pascal/Ada allowed (e.g. 3..27) are not sufficient, look at the logic required to validate the month/day/year values of a date as detailed in the ECP article. Only an algorithm can encode what is required for legal values, not simple declarations of string and integer properties.

So, "strong" typing is a relative term.

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