The Matchable Protocol
April 30, 2021

For the last few years, the default setting for all of the Swift code I write has been open source.

As a result, I’ve accumulated a vast number of Github repositories and Swift Package Manager packages.

However, I’ve been really bad at telling people that they exist!

This post is an attempt to start to fix that, by talking about one small package I’ve recently created: Matchable.

First though, some disclaimers:

Caveat Emptor

Part of the barrier to telling people about things I’ve done is sheer time it takes to write even quite a simple post like this one.

So my first disclaimer is just to say that this post is mostly a re-hash of the README file from the Github Repository. Nothing wrong with that I think, but just to be clear…

My second disclaimer is that this is work-in-progress code from the real world.

I’ve encountered a few people who subscribe to a fundamentalist view of open source code: that it’s useless unless it is fully polished, fully tested, 100% supported and actively maintained.

I understand this point of view; we’ve all encountered code that makes great claims and turns out to be broken or mostly unfinished.

Respectfully though, those people are wrong.

Imperfect open-source code can be frustrating. However, it can also be a helpful foundation for someone else to build on, a good example of the pros and cons of particular technique, or a useful supplier of that one crucial line you have been searching the internet for.

Aiming for perfection is setting the barrier way too high. I am as insecure as the next person when it comes to showing my workings in public. I’ve been a professional programmer for more than three decades, but I still suffer from impostor syndrome.

It’s tempting to hide away, but I’m trying to fight the urge, and I’d like to contribute in some small way to an environment where we aren’t scared to risk being wrong.

I offer up all of my open-source code in this spirit. It’s not perfect, because I am busy, and because I am still writing it. I find this code useful, and I hope someone else might. If you do find that it is fundamentally broken, please tell me why. That way I learn something.

That said…

Matchable

The Matchable protocol defines a way to compare two objects, structures or values for equality.

Unlike the Equatable protocol, Matchable works by throwing an error when it encounters a mismatch.

You can view this as an assertion of equality. For this reason, the primary method is named assertMatches.

This makes for compact code since you don’t need to write explicit return statements for every failed comparison.

It also allows the protocol to handle compound structures intelligently.

If a matching check of a structure fails on one of its members, the matchable code will wrap up the error thrown by the member, and throw another error from the structure.

Any catching code can dig down into these compound errors to cleanly report exactly where the mismatch occurred.

Usage

You can check that two values match with:

try x.assertMatches(y)

A sequence of checks can easily be performed – the first failure will throw, causing the remaining checks to be skipped:

try int1.assertMatches(int2)
try double1.assertMatches(double2)
try string1.assertMatches(string2)

A type can implement matching by conforming to the Matchable protocol, and defining the assertMatches method. Inside this method it can perform the necessary checks.

If it finds a failure, it can throw a MatchFailedError to report the mismatch.

Implementations of assertMatches are provided for most of the primitive types, and a few Foundation types (I’ve just done the ones I needed for now - pull requests gratefully received…).

Compound Types

Although you can match primitive value types, the protocol comes into its own when performing memberwise matching of compound types (structs, objects, etc).

In this case a type can conform to the MatchableCompound protocol, and defining the assertContentMatches method.

This works the same way as the basic assertMatches method, except that if a check throws an error inside this method, the error will be wrapped in an outer error reporting that the whole structure failed to match.

Keypaths

As a convenience, we also define a form of assertMatches which takes a key path or list of key paths, and calls assertMatches on each path of two objects in turn.

This helps to keep down the amount of boiler-plate code to a minimum.

Here’s an example combining keypaths and the MatchableCompound. This tests the matchability of a structure that has 13 properties, and manages to do it with a minimum of boilerplate.

extension Task: MatchableCompound {
    public func assertContentMatches(_ other: Task, in context: MatchableContext) throws {
        try assertMatches([\.state], of: other)
        try assertMatches([\.name, \.icon, \.details], of: other)
        try assertMatches([\.started], of: other)
        try assertMatches([\.hasDescription, \.hasDuration, \.isScheduled], of: other)
        try assertMatches([\.duration], of: other)
        try assertMatches([\.scheduledHour, \.scheduledMinute], of: other)
        try assertMatches([\.streaks], of: other)
        try assertMatches([\.restDays], of: other)
    }
}

Note that currently if you pass a list of keys, they all have to resolve to members of the same type. Unfortunately this somewhat reduces the helpfulness of this method.

Unit Testing

The original motivating use-case for this protocol was unit testing, where it’s often necessary to compare two instances of something, and useful to be able to identify the exact point of divergence.

Whilst I still see this as the primary use for the protocol, I have split it out into a standalone package as it may be helpful in other places.

The fact that Matchable is different from Equatable is an advantage for unit testing, as it allows both to co-exist.

In your code, you might define Equatable to only check part of a structure (a unique identifier, for example).

This is good for efficiency in production code, but no use for test code where you really do want to know if all members are equal.

In this situation you can define a thorough check with Matchable, and use that for unit testing, without interfering with the efficient implemention of Equatable.

Initially this protocol was defined as part of my XCTestExtensions package.

That package includes some additions to XCTAssert which use Matchable to let you perform matching checks:

XCTAssert(savedModel, matches: reloadedModel)

This assert method catches any errors and presents them in a nice way by calling XCTFail, identifying the exact point of failure.

Because of the way the match-failure errors are wrapped for compound structures, the method can call XCTFail at all levels of the failure, which results in Xcode showing an error marker at all levels.

This can be helpful when tracking down a mismatch in a deeply nested structure.

Future

This is an early implementation, based on code pulled from elsewhere.

The API probably needs tweaking, and the methods definitely need documenting.

I also intend to explore the idea of using Swift’s introspection to automatically generate assertMatches for structures/classes.

In theory this should work well, but it’s possible that it will hit wrinkles.

All feedback, suggestions, pull requests and bug reports gratefully received!

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