• May 17, 2026
• in ponyc, Finite Recursive Type Aliases
• 6 min read

Eleven years to a finite recursive type alias

There’s a pull request open against the Pony compiler. It’s in review right now. When it lands, and it will land soon, you’ll be able to write this:

use "collections"

type JsonValue is
  ( String
  | F64
  | Bool
  | None
  | Array[JsonValue]
  | Map[String, JsonValue])

Today the compiler rejects it. JsonValue mentions Array[JsonValue], which mentions JsonValue, and ponyc throws up its hands: type aliases can't be recursive. That has been true for the entire history of the language. It’s about to stop being true, and the pull request that changes it closes out the oldest open issue in the ponyc repository.