[Feature Request] User-defined generics (type parameters for classes) #160

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adam · 2025-12-30T01:21:39+01:00

adam commented

2025-12-30 01:21:39 +01:00

Originally created by @moritztim on GitHub (May 15, 2024).

Type parameters are only allowed in standard library

But why?

Originally created by @moritztim on GitHub (May 15, 2024). > Type parameters are only allowed in standard library _But why?_

adam commented

2025-12-30 01:21:39 +01:00

@bioball commented on GitHub (Jun 10, 2024):

Generics add a lot of complexity to the language, and is hard to get right. This is something we would like to add to the language at some point, but I would say that we're not close yet.

For example, one thing to consider is how to implement variance checking. Today, Pkl does not perform subtype checking, and this is not trivial because of type constraints.

We also have some bugs with type parameters in the core language that will need to be fixed before we can start to consider something like this.

@bioball commented on GitHub (Jun 10, 2024): Generics add a lot of complexity to the language, and is hard to get right. This is something we would like to add to the language at some point, but I would say that we're not close yet. For example, one thing to consider is how to implement [variance checking](https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)). Today, Pkl does not perform subtype checking, and this is not trivial because of [type constraints](https://pkl-lang.org/main/current/language-reference/index.html#type-constraints). We also have some [bugs with type parameters in the core language](https://github.com/apple/pkl/issues/405) that will need to be fixed before we can start to consider something like this.

adam commented

2025-12-30 01:21:39 +01:00

@joshqou commented on GitHub (Dec 13, 2025):

Ran into this limitation myself. Definitely would be an appreciated feature

@joshqou commented on GitHub (Dec 13, 2025): Ran into this limitation myself. Definitely would be an appreciated feature

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Reference: starred/pkl#160