What is dependent typing?

Dependent types enable larger set of logic errors to be eliminated at compile time. To illustrate this consider the following specification on function f:

Function f must take only even integers as input.

Without dependent types you might do something like this:

def f(n: Integer) := {
  if  n mod 2 != 0 then 
    throw RuntimeException
  else
    // do something with n
}

Here the compiler cannot detect if n is indeed even, that is, from the compiler's perspective the following expression is ok:

f(1)    // compiles OK despite being a logic error!

This program would run and then throw exception at runtime, that is, your program has a logic error.

Now, dependent types enable you to be much more expressive and would enable you to write something like this:

def f(n: {n: Integer | n mod 2 == 0}) := {
  // do something with n
}

Here n is of dependent type {n: Integer | n mod 2 == 0}. It might help to read this out loud as

n is a member of a set of integers such that each integer is divisible by 2.

In this case the compiler would detect at compile time a logic error where you have passed an odd number to f and would prevent the program to be executed in the first place:

f(1)    // compiler error

---

Here is an illustrative example using Scala path-dependent types of how we might attempt implementing function f satisfying such a requirement:

case class Integer(v: Int) {
  object IsEven { require(v % 2 == 0) }
  object IsOdd { require(v % 2 != 0) }
}

def f(n: Integer)(implicit proof: n.IsEven.type) =  { 
  // do something with n safe in the knowledge it is even
}

val `42` = Integer(42)
implicit val proof42IsEven = `42`.IsEven

val `1` = Integer(1)
implicit val proof1IsOdd = `1`.IsOdd

f(`42`) // OK
f(`1`)  // compile-time error

The key is to notice how value n appears in the type of value proof namely n.IsEven.type:

def f(n: Integer)(implicit proof: n.IsEven.type)
      ^                           ^
      |                           |
    value                       value

We say type n.IsEven.type depends on the value n hence the term dependent-types.

---

As a further example let us define a dependently typed function where the return type will depend on the value argument.

Using Scala 3 facilities, consider the following heterogeneous list which maintains the precise type of each of its elements (as opposed to deducing a common least upper bound)

val hlist: (Int, List[Int], String)  = 42 *: List(42) *: "foo" *: Tuple()

The objective is that indexing should not lose any compile-time type information, for example, after indexing the third element the compiler should know it is exactly a String

val i: Int = index(hlist)(0)           // type Int depends on value 0
val l: List[Int] = index(hlist)(1)     // type List[Int] depends on value 1 
val s: String = index(hlist)(2)        // type String depends on value 2

Here is the signature of dependently typed function index

type DTF = [L <: Tuple] => L => (idx: Int) => Elem[L, idx.type] 
                                  |                     |        
                                 value           return type depends on value

or in other words

for all heterogeneous lists of type L, and for all (value) indices idx of type Int, the return type is Elem[L, idx.type]

where again we note how the return type depends on the value idx.

Here is the full implementation for reference, which makes use of literal-based singleton types, Peano implementation of integers at type-level, match types, and dependent functions types, however the exact details on how this Scala-specific implementation works are not important for the purposes of this answer which is mearly trying to illustrate two key concepts regarding dependent types

1. a type can depend on a value
2. which allows a wider set of errors to be eliminated at compile-time

// Bring in scope Peano numbers which are integers lifted to type-level
import compiletime.ops.int._

// Match type which reduces to the exact type of an HList element at index IDX
type Elem[L <: Tuple, IDX <: Int] = L match {
  case head *: tail =>
    IDX match {
      case 0 => head
      case S[nextIdx] => Elem[tail, nextIdx]
    }
}

// type of dependently typed function index
type DTF = [L <: Tuple] => L => (idx: Int) => Elem[L, idx.type] 

// implementation of DTF index
val index: DTF = [L <: Tuple] => (hlist: L) => (idx: Int) => {
  hlist.productElement(idx).asInstanceOf[Elem[L, idx.type]]
}

Given dependent type DFT compiler is now able to catch at compile-time the following error

val a: String = (42 :: "foo" :: Nil)(0).asInstanceOf[String] // run-time error
val b: String = index(42 *: "foo" *: Tuple())(0)             // compile-time error

scastie

If you happen to know C++ it's easy to provide a motivating example:

Let's say we have some container type and two instances thereof

typedef std::map<int,int> IIMap;
IIMap foo;
IIMap bar;

and consider this code fragment (you may assume foo is non-empty):

IIMap::iterator i = foo.begin();
bar.erase(i);

This is obvious garbage (and probably corrupts the data structures), but it'll type-check just fine since "iterator into foo" and "iterator into bar" are the same type, IIMap::iterator, even though they are wholly incompatible semantically.

The issue is that an iterator type shouldn't depend just on the container type but in fact on the container object, i.e. it ought to be a "non-static member type":

foo.iterator i = foo.begin();
bar.erase(i);  // ERROR: bar.iterator argument expected

Such a feature, the ability to express a type (foo.iterator) which depends on a term (foo), is exactly what dependent typing means.

The reason you don't often see this feature is because it opens up a big can of worms: you suddenly end up in situations where, to check at compile-time whether two types are the same, you end up having to prove two expressions are equivalent (will always yield the same value at runtime). As a result, if you compare wikipedia's list of dependently typed languages with its list of theorem provers you may notice a suspicious similarity. ;-)

Consider this: in all decent programming languages you can write functions, e.g.

def f(arg) = result

Here, f takes a value arg and computes a value result. It is a function from values to values.

Now, some languages allow you to define polymorphic (aka generic) values:

def empty<T> = new List<T>()

Here, empty takes a type T and computes a value. It is a function from types to values.

Usually, you can also have generic type definitions:

type Matrix<T> = List<List<T>>

This definition takes a type and it returns a type. It can be viewed as a function from types to types.

So much for what ordinary languages offer. A language is called dependently typed if it also offers the 4th possibility, namely defining functions from values to types. Or in other words, parameterizing a type definition over a value:

type BoundedInt(n) = {i:Int | i<=n}

Some mainstream languages have some fake form of this that is not to be confused. E.g. in C++, templates can take values as parameters, but they have to be compile-time constants when applied. Not so in a truly dependently-typed language. For example, I could use the type above like this:

def min(i : Int, j : Int) : BoundedInt(j) =
  if i < j then i else j

Here, the function's result type depends on the actual argument value j, thus the terminology.