No, you're really adding a constraint. You're adding a description of what exactly a piece of data represents, allowing the compiler or interpreter to limit the

No, you're really adding a constraint. You're adding a description of what exactly a piece of data represents, allowing the compiler or interpreter to limit the ways in which you can use it. The form of data in a computer is simply a sequence of binary information. It is up to you as a programmer to decide what it represents. In some cases, you can use a type system to have that automatically enforced in part. It's important to realize that the types you can use in practice as part of a formal type system are very rarely correct in the sense of fully describing what some data represents. For example, if a string of bytes represents a HTTP request, then "valid HTTP request bytes" is the correct type, but the type known to the compiler might just be string of bytes. Similarly, if you represent a graph by a hash table of int to list of int (with the ints labeling nodes), then the type of that is a map from int to list of int, where the int values in the latter must exist as keys as well - but the type system might not actually know that. In practice, there will therefore often be functions that validate the contents of the data - these are basically manual run time type checking functions.