Microlibrary for testing with expectations

matklad · July 9, 2023, 12:12pm

Last week, this great post was making rounds on HN & lobste.rs:

It describes “Testing With Expectations”, also known as “Snapshot Testing” — a powerful “thinking outside of the box” technique for writing unit-tests which suck less.

Coincidentally, I’ve implemented a tiny variation of the idea for TigerBeetle:

github.com

tigerbeetle/tigerbeetle/blob/a0aa383ddbf06a487393d3bb87f3d9edb63a79ff/src/testing/snaptest.zig

//! A tiny pattern/library for testing with expectations ([1], [2]).
//!
//! On a high level, this is a replacement for `std.testing.expectEqual` which:
//!
//!   - is less cumbersome to use for complex types,
//!   - gives somewhat more useful feedback on a test failure without much investment,
//!   - drastically reduces the time to update the tests after refactors,
//!   - encourages creation of reusable visualizations for data structures.
//!
//! Implementation-wise, `snaptest` provides a `Snap` type, which can be thought of as a Zig string
//! literal which also remembers its location in the source file, can be diffed with other strings,
//! and, crucially, can _update its own source code_ to match the expected value.
//!
//! Example usage:
//!
//! ```
//! const Snap = @import("snaptest.zig").Snap;
//! const snap = Snap.snap;
//!
//! fn check_addition(x: u32, y: u32, want: Snap) !void {

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I don’t intend to publish this as a proper library at this point, but feel free to take a look and steal the pattern for your projects, it’s very simple!

kristoff · July 9, 2023, 1:18pm

that’s a very neat idea