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?