kr.dev/diff

Hello, friend! This is a Go module to print the differences between any two Go values.

diff.Test(t, t.Errorf, got, want)

There are a few ways to use this module, but this here ☝️ is probably the most common. In test code, to check whether the result of your test is what you expected.

Usage

Test for expected value in a test:

func TestFoo(t *testing.T) {
    got := ...
    want := ...
    diff.Test(t, t.Errorf, got, want)
}

Log diffs in production:

diff.Log(a, b)

Print diffs to stdout:

diff.Each(fmt.Printf, a, b)

There are also several options to change how it works if the default behavior isn't what you need. Check out the godoc at kr.dev/diff.

Design Philosophy

Here are some general guidelines I try to follow for this module:

make it easy to read for humans (not computers)
use Go-style notation for familiarity
favor being concise over being explicit
but be explicit where necessary to avoid confusion

These aren't hard rules.

For instance, although it is a goal to make the output readable to someone who's familiar with Go, I make no effort to strictly adhere to Go syntax.

Don't Rely on Code Under Test

We also avoid calling methods on the values being compared. For instance, package time defines a method Time.Equal, that tells whether two Time values are the same instant, regardless of their locations. But we don't use it or any other Equal method. Instead, we have custom comparison logic (TimeEqual) to compare Time values while ignoring their locations.

The reason for this is that you might be trying to test your Equal method! It would be really confusing if there's a bug in the code you're testing, and that causes this package to produce incorrect results. You might end up with a "passing" test because Equal returns true even when the values are different. We want to reliably show you when the values are different.

So our policy is this module doesn't call methods on the values being compared. Instead, if you need to customize how comparisons are done, you can install a transform. See Custom Comparison. Your transform is free to call methods on the values being compared, that is up to you; this module will simply not do so directly. Our hope is that if you're doing it yourself, it'll be less surprising.

Custom Comparison

Sometimes you want or need to customize how values of a given type are compared. Here's how.

Let's say you're testing code that uses temporary files with randomized names, and your result might contain values of fs.PathError. You want to check that the errors are the same, but you need to ignore the file name. By design, the name is different every time.

var ignorePath = diff.Transform(func(pe fs.PathError) any {
    pe.Path = "" // it's ok to modify this copy
    return pe
})

diff.Test(t, t.Errorf, a, b, ignorePath)

In this case, you use the Transform option to change each fs.PathError into a new value, so that the transformed values are equal as long as the other fields, Op and Err, are equal, and unequal otherwise.

There's also a shorthand notation for this pattern. This example can be rewritten using the ZeroFields helper option.

var ignorePath = diff.ZeroFields[fs.PathError]("Path")

There are also a couple of predefined transforms exported by this package. Their definitions are visible in the godoc at kr.dev/diff.

Side note. Why doesn't the option look like this?

diff.CustomCompare(equal func(a, b T) bool) Option

That's because, under the hood, we don't always directly compare two values for equality. Sometimes we also hash the values and compare their hashes. If you wanted to define a custom boolean equality function, you'd also have to provide a custom hashing function. But with a transform, this module can do both the hashing and comparison for you, and you only need to write one relatively easy function.

Custom Formatting

This package tries to provide readable and useful output out of the box. But sometimes you can do a lot better by tailoring the output to a specific type. In that case, you can use the Format option to define a custom format function.

var fmtFoo = diff.Format(func(a, b Foo) string {
    // TODO(kr): I need a good example to put here.
    // For now, look at diff.TimeDelta in the godoc.
})

diff.Test(t, t.Errorf, a, b, fmtFoo)

Your formatter takes two values of the given type and returns a description of the difference between them. It only gets called when the values are already determined to be unequal, so you don't have to compare them. You can assume they are different. (If you need to customize how values are compared, see Custom Comparison.)

There are also a couple of predefined custom formats exported by this package. Their definitions are visible in the godoc at kr.dev/diff.

Compatibility

The output of this package is mainly meant for humans to read, and it's not a goal to be easy for computers to parse. We will occasionally change the format to try to make it better. So please keep this in mind if you want to make a tool that consumes the output of this module; it might break!

On top of that, this is still a v0 module, so we might also change the API in a way that breaks.

Roadmap

No promises here, but this is what I intend to work on:

Feedback

If you find bugs or want more features or have design feedback about the interface, please file issues or pull requests! Thank you!

kr/diff