What's the best way to represent type variables inside an AST?

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I have a toy hobby compiler I want to add some kind of generic functionality to, to be able to express things like “output should be same as input”. Case in point, an array_slice function:

/**
 * @template A
 * @param array<A> $input
 * @param int $index
 * @return array<A>
 */
array_slice(array $input, int $index): array

Currently, the types in the AST look like:

type typ =
  | Int
  | Float
  | Dynamic_array of typ

So I guess a natural extension would be something like:

type typ =
  | Type_variable of string
  | Int
  | Float
  | Dynamic_array of typ

And then when inferring the content of the type variable, just check for the string name, “A”? No fancy unification algorithm is needed, I think.

Any comments or ideas?