Casting and Type vs. Set theory

A few days ago I saw an awesome movie where Brian Beckaman talks about monads on channel9:

In this movie he explains that one can reason about “Types” just like a “Set”

Brian never goes into deeper details about this in the movie, so this post will deal with my own thought on this matter only.
So don’t hold him responsible for anything crazy that I might write :-)

I found this topic quite interesting, I have never thought about it this way, but it does makes perfect sense.

e.g. Imagine the Type “Byte”, it represents an integer number from 0 to 255.
And thus, all the valid forms of a byte would fit in a set with 256 elements.

The type “Int32” also represents an integer number, but with a larger precision.
But it still models the exact same concept, the concept of an integer value.
Every valid form of an Int32 will also fit into a set, however much larger than the byte set.

Since both types models the same concept, but with different precision, the smaller set can be seen as a subset of the larger set.
That is; the byte set is a sub set of the Int32 set.

Type set diagram:
One intereting thing that this brings to the table, is that it allows us to reason about type casts.
e.g. Since the byte set is a subset of Int32, we can know that any element found in the byte set, will also exist in the Int32 set.
So we can assume that we can pick any element from a bag of Bytes and put it into a bag of Int32s.

And this is exactly what we can do when we write programs (at least in most languages), we can make an implicit type cast from byte to Int32.
Simply because bytes belongs to a subset of Int32, even though the byte is not a sub type of Int32.

The inheritance graph for these types looks quite different than the Set diagram:


The types that represent integer values in the inheritance diagram shows that they do not have a parent / child relation.
They are unrelated except for shared base of ValueType, but you can still do implicit casts from one type to another when the source type belongs to a subset of the destination type.

This is quite obvious facts when dealing with integers, and I guess that most developers already knew when it is possible to do implicit casts from one type to another.
But if we ignore the über obvious for a second and think about what this can be used for:

This does allow us to reason about our OWN types, when and where it makes sense to allow implicit or explicit casts and not.

I have never really had any good approach for this before, I pretty much only threw in implicit cast operators here and there without much thought.
But by thinking of types as sets, it is easy to see if your custom types are a sub/super set of another type, and then allow implicit or explicit casting based on that.
And it might even show you that your types are not overlapping sets at all, and in such case you might want to create transformer / factory methods instead of allowing any type of casting.

e.g. a type that is a string representation of a week day might look like should allow implicit casts to string.
But since they model different concepts; weekday vs. string, those sets are not overlapping.
And thus, you should not allow any sort of casting, but rather supply a “WeekDay.Parse” method or something similar.

While if you were to invent your own Nullable of T type, you could easily see that any instance of T would also fit into the set of Nullable T.
And thus you should allow implicit casts from T to Nullable of T.

Well, that’s all..
I hope this wasn’t far too crazy :-)



  1. You will probably find this post: on SPIN (SPARQL Inferencing Notation) interesting:

    In this example the type inference is actually based on predicates. So it is possible for a property change to change type. I.e. if the width and height of a rectangle match, it becomes a square and visa versa.

    Interesting possibilities.

  2. The question begs to be asked: Can one throw a spanner in the works by recursion and self-reference, which Set-theory is famous for? Most famously, Russel’s paradox, involving “a set containing sets that are not members of themselves”.

    For instance, In Java I may work with a superclass which I use to say something general about a bunch of classes. It is a list of classes more specific than itself so the superclass cannot contain itself. In fact it’s part of the definiton of superclass.

    It’s useful but I want to categorize my superclasses for easy retrieval, so I make a new class which has the description “The class of all superclasses” which by the above definition I can expand into “the class of all classes that do not contain themselves”.
    Hence arriving at Russels paradox.

    Now if only I could come up with an example with practical significance :-P

    1. This paradox comes from unrestricted comprehension, i.e., the idea that you can define a set as the collection of all the elements satisfying a given property, say, of not containing itself. There is a mathematical notation for expressing set comprehensions; using it, the set of all sets containing themselves is defined as {x | x ∉ x}.

      Now in computer science we do not have a construct equivalent to set comprehension, so we do not have the paradox (some languages have list comprehensions, but it constructs values, not types). Using reflexion, you do have the type java.lang.Class of all classes, but you cannot define a subclass by saying “all the elements of type Class satisfying such and such property shall also have the type Class_Not_Containing_Itself”. You can only create a subclass Sub with more fields and methods, and deem all the values created using the Sub() constructor to have type Sub.

      Therefore, we’re safe. Phew!

  3. Pretty sure that CS is more derived from type theory than set theory anyway (although OOP is heavily influenced by set theory). Haven’t read into type theory yet but from what I understand, it’s an alternative TO naive set theory to avoid things such as Russell’s Paradox.

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