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what *typed* languages have the best support for defining and working with multiple variations of an AST type, transforming between them, writing pattern-matching code for them in a DRY way, etc e.g. "take this function Expr -> Int and adapt it to Expr `Without` ExprLam inputs"

Brendan Zabarauskas

@brendanzab

May 9, 2020

Not typed, but it might be interesting to look at nanopass compilation. You can add and remove bits of AST when you go between fine-grained compilation passes: nanopass.org - but yes, this is something I very much wish I had in Rust.

Replying to

and

There's this common issue in inductive types where you want to add and remove stuff, and add extra data… and be able to reuse existing function implementations on these new types. Feel like this was partly what levitation and ornaments were trying to look at but I need to check.

5:34 AM · May 9, 2020Twitter Web App

Replying to

and

AFAIK, ornaments seem about adding fields or indexes to existing datatypes — letting you relate, say, Peano naturals, lists (Peano naturals ornamented with elements), and vectors (lists ornamented with lengths). Levitation was a self-describing universe construction... OTOH:

Paolo G. Giarrusso

@Blaisorblade

May 9, 2020

The “expression problem” is about adding constructors and extending programs. Meanwhile in the wild, having types for each constructors, and combining them using OO (union types/market traits), seems somewhat popular in the wild (in Scala, and possibly in TypeScript).