# What's a usage pattern for existential type variables?

**URL:** https://discuss.ocaml.org/t/whats-a-usage-pattern-for-existential-type-variables/8538
**Category:** Learning
**Tags:** type-system, existential-types
**Created:** [September 27, 2021, 5:16pm UTC](https://discuss.ocaml.org/t/whats-a-usage-pattern-for-existential-type-variables/8538 "2021-09-27T17:16:41Z")
**Posts on this page:** 1
**Showing post:** 1

<div class="post-metadata">

### Author: ![hyphenrf](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/hyphenrf/32/2733_2.png) [@hyphenrf](https://discuss.ocaml.org/u/hyphenrf)
#### Post date: [September 27, 2021, 5:16pm UTC](https://discuss.ocaml.org/t/whats-a-usage-pattern-for-existential-type-variables/8538/1 "2021-09-27T17:16:41Z")

</div>

Online, (Scala/Java) and Haskell-specific tutorials on existential types seem to be dominant. Because I wouldn’t get stuck too much on notation, I went with Haskell tutorials, and they all seem to imply that _existentials that are unconstrained are useless_.

The usual example for existentials is information (but not trait) hiding, in which for example you can have a definition like so (with some syntax parallels for those unfamiliar with haskell):

```nohighlight
{-
ocaml: `type gadt = Constr : typexpr`
haskell: `data Gadt where Constr :: typexpr`
-}

data Hidden where
  Hide :: Show a => a -> Hidden

{-
ocaml: iter (fun (Hide x) -> ...) [Hide 1; Hide ...]
-}

main :: IO ()
main =
  mapM_ (\(Hide x) -> print x) [Hide 1, Hide "a", Hide 4.2]

```

The problem with such implication is that I can’t find a usage for an existential introduced by an OCaml GADT because I don’t know how I’d express the `Show a => a` constraint.

```nohighlight
type hidden = Hide : 'a -> hidden

```

Would simply completely cloak `'a`, and any attempt at interacting with it would produce a type scope escaping error.

That considered, since `Show a => a` is best emulated now with passing a first-class module, that was the vector I was walking in with my existentials experiments, but I have a feeling there’s better, more valid, usage of those type variables that isn’t reliant on constraining.

So my question is, have you ever come across a pattern where an existential type variable is the solution?

---

_[View the full topic](https://discuss.ocaml.org/t/whats-a-usage-pattern-for-existential-type-variables/8538)._