panproto-expr

A small functional expression language for writing data transforms in migrations.

What it does

When a schema changes, field values sometimes need to change too: a single name field gets split into firstName and lastName, a numeric status code gets converted to a string enum, a price stored in cents needs to be divided by 100. This crate provides the language for writing those transforms: lambdas, pattern matching, let bindings, and about 50 built-in functions covering arithmetic, string manipulation, list operations, and record access.

Expressions are pure (no IO, no mutable state) and deterministic (same input always produces the same output). They are also bounded: a configurable step counter stops runaway loops before they can hang a migration pipeline. The AST serializes to JSON via serde, so expressions stored in the schema VCS travel alongside the schema versions they belong to.

The expression language is also the computational layer for everything beyond simple field remapping: coercion functions, merge logic for combining conflicting values, default value computation, and conflict resolution policies all use Expr values under the hood.

Quick example

use panproto_expr::{Expr, Literal, BuiltinOp, Env, EvalConfig, eval};

// \s -> concat(s, "_v2")
let add_suffix = Expr::lam(
    "s",
    Expr::builtin(BuiltinOp::Concat, vec![
        Expr::var("s"),
        Expr::Lit(Literal::Str("_v2".into())),
    ]),
);

let applied = Expr::app(add_suffix, Expr::Lit(Literal::Str("widget".into())));
let result = eval(&applied, &Env::new(), &EvalConfig::default()).unwrap();
// result == Literal::Str("widget_v2")

API overview

License

MIT