The imperative. When should you create a custom data type, as opposed to reusing pre-existing data types such as Either, Maybe or tuples? Here are some reasons you should reuse a generic type:
- It saves typing (both in declaration and in pattern matching), making it good for one-off affairs,
- It gives you a library of predefined functions that work with that type,
- Other developers have expectations about what the type does that make understanding quicker.
On the flip side of the coin:
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Punning is the lowest form of humor. And an endless source of bugs.
The imperative. In programming, semantically different data may have the same representation (type). Use of this data requires manually keeping track of what the extra information about the data that may be in a variable. This is dangerous when the alternative interpretation is right most of the time; programmers who do not fully understand all of the extra conditions are lulled into a sense of security and may write code that seems to work, but actually has subtle bugs. Here are some real world examples where it is particularly easy to confuse semantics.
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The imperative. Mutable data structures with many children frequently force any given child to be associated with one given parent data structure:
class DOMNode {
private DOMDocument $ownerDocument;
// ...
public void appendNode(DOMNode n) {
if (n.ownerDocument != this.ownerDocument) {
throw DOMException("Cannot append node that "
"does not belong to this document");
}
// ...
}
}
Client code must be careful not to mix up children that belong to different owners. An object can be copied from one owner to another via a special function. :
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