I asked ChatGPT: “Explain GADTs in OCaml” and it came up with the following answer:

GADT stands for Generalized Algebraic Data Type. In OCaml, GADTs are a language extension that allows defining more precise types for data structures.

In a traditional algebraic data type (ADT), each constructor has a fixed type signature. For example, consider a simple ADT for expressions:

```
type expr =
| Int of int
| Plus of expr * expr
| Times of expr * expr
```

Here, `Int`

constructor has type `int -> expr`

, `Plus`

constructor has type `expr -> expr -> expr`

, and `Times`

constructor has type `expr -> expr -> expr`

.

In contrast, a GADT allows us to define different types for each constructor. We can use this to express more precise type information about the constructors. For example:

```
type _ expr =
| Int : int -> int expr
| Plus : int expr * int expr -> int expr
| Times : int expr * int expr -> int expr
```

Here, each constructor has a different type signature. `Int`

constructor has type `int -> int expr`

, `Plus`

constructor has type `int expr -> int expr -> int expr`

, and `Times`

constructor has type `int expr -> int expr -> int expr`

.

The `_`

symbol before the `expr`

type parameter is a type variable that represents the “unknown” type. This allows us to define different types for different constructors.

By using GADTs, we can enforce more precise typing constraints on our data structures. For example, in the above example, we can now ensure that `Plus`

and `Times`

constructors only accept integer expressions as their arguments, and also guarantee that the result of applying these constructors will always be an integer expression.

GADTs can also be used to define types that are parameterized by other types, allowing for even more precise type information.

This is quite good IMHO.

My questions to you:

- Do you see any problems with ChatGPT’s text as a small summary?
- The example code is taken from neither “Real World OCaml” nor the OCaml manual. Does anybody know the source of this? Maybe it translated an example for another programming language? ChatGPT couldn’t come up with this on it’s own, could it?