In my previous post Learning Prolog to be a better Haskell programmer, I advocated learning Prolog is quite helpful to get more intuitions on Haskell type-level programming.

I think a good next step is to learn a dependent typed programming language such as Agda or Epigram. As learning Haskell is a good way to develop oneself as a better Java programmer, learning a dependent typed programming language is a good way to develop oneself as a better Haskell programmer.

Among many dependent typed programming languages, I recommend Agda simply because its surface syntax is quite similar to that of Haskell. Because dependent typed programming languages in general are not mature enough to perform day-to-day programming task and most of them are more or less equivalent in powers, choosing a syntactically familiar language helps you understand more advanced type system behind the syntax.

As the name “dependent type” implies, the biggest difference lies in the type system. While Haskell’s type system strictly splits values and types, Agda blurs the distinction between types and values. Type level programming in Haskell with type families or functional dependencies is esoteric at best, but type level programming in Agda is a norm.

For example, it is possible to define a type of lists of a certain length. In this setting, it is a type error to pass an empty list to head.

```
data Vec (A : Set) : Nat -> Set where
[] : Vec A zero
_::_ : {n : Nat} -> A -> Vec A n -> Vec A (suc n)
head : {A : Set}{n : Nat} -> Vec A (suc n) -> A
head (x :: xs) = x
```

Please refer to Dependently Typed Programming in Agda and Daniel Peebles’s introduction on Agda for more information on Agda.