This is my current intuition on types. I heard the best way to get feedback on the internet is to publish something inaccurate, so there you have it.
What is a Type?
Types are sets of properties held by values. In a language, the more things you have that are values, the richer your types are.
Values can have many sorts of properties: all values that are numbers can be added together, all values that are strings can be concatenated, all values that are functions can be invoked.
A type could be for instance “all values that are printable”. Or even “all values that are values”. One could even devise a type that cannot contain any value, i.e “all values that are not values”.
A type is large if many values belong to it, and is small when few do. The larger the type, the smaller the sets of properties it represents.
The largest type a value can have is the type of having no particular property, so all values belong to it, whereas the smallest type is the type of having all the imaginable properties at once, which is impossible so no value can belong to it.
A value can have many properties: “3” is a value, a number, a representable value, and it has the value “3”.
So we could say that “3” is of type Value, Number, Representable, and BeingTheNumber3.
What do we use types for?
We need to dispatch on those types at some point if we want to manipulate values in our program.
One way to go is to keep all that information in the value itself, and do a so-called dynamic dispatch: at runtime, simply look at the value and see what properties it has. If it is missing the property you need, well that’s a so-called type error.
Another way to do that is to do a static dispatch. A program is made of variables that when running will hold values. They are called variables because over time they will hold different values. A variable has the type of the union of the types of all the values it can hold at runtime.
What this means is that a variable can have the properties of all the values it can hold, but not necessarily all at once.
We can annotate those variables before running our program (i.e statically) with their smallest type, that is the set of property we believe they will always hold at any given time later on when we actually run the program.
By doing so, we can use the proper implementation for a given variable without having to look at the value it is holding at runtime, so we no longer need to encode this property on the value itself.
If we have annotated a variable with a given type, but pass on a value with another type, we get a type error and unexpected behavior.
We can improve our static dispatch approach by actually proving that each variable will only hold values that share its assumed properties, i.e are of the same type. This requires implementing a type checker that can reflect on the invariances of our program and infer the types of each variable without running it.
If a unification fails, we get an error. By verifying a program with a type checker, we can get rid of all the type errors involving all the types that are expressible and used in our program. This could be for instance that no matter what, we only fetch the first element on non-empty lists, or that we always add vectors of the same length. Types can express arbitrary invariances, as long as the language can express them and the compiler can prove their veracity.
To be noted, using dynamic dispatch does not preclude having a type checker. By having proper annotations, we can still reflect on the invariances of our program, even if that knowledge is not necessarily used at runtime (see Gradual Typing, as with Clojure’s core.typed).
Not all types are easily expressible and/or easy to reason about. And proving that certain invariances are held can be computationally very expensive, if possible at all. But the more is expressed in the type system, the less can go wrong at runtime.