Cover fe ature 47 january 2012 Published by the ieee computer Society 0018-9162/12/$31. 00 2012 ieee

We want to practice type-rich program-

ming, but we also want to minimize the 

size of the implementation by using 

only a few fundamental types.

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more, backward compatibility requirements for languages 

and protocols pose limitations. However, type safety is an 

unavoidable ideal. Encoding a program’s static structure in 

the type system (ensuring that every object has a type and 

can hold only the values of its type) can be a major tool for 

eliminating errors.

Programmers can and do vigorously disagree about the 

meaning of “type” and its purpose. I tend to emphasize a 

few significant benefits:

 

•

more specific interfaces (relying on named types), 

implying early error detection;

 

•

opportunities for terse and general notation (such as