Intro

This is Gruntwork’s style guide for the Go programming language. It aims to help us ensure that the code we write is clear, readable, idiomatic Go code. The conventions detailed in this guide are our preferences and should be thought of as guidelines rather than hard rules.

Starting point

We use the excellent Effective Go guide as the starting point for our style guide. Unless explicitly mentioned otherwise, we default to following the conventions outlined in Effective Go.

Another helpful resource is the CodeReviewComments section of the Go GitHub wiki page.

Below you will find the list of conventions that differ from the above mentioned documents, either because they are specific to Gruntwork, or because we consciously chose to go against what Effective Go and CodeReviewComments recommend.

Additional conventions

General

Before committing any Go code, run go fmt. We run this as part of the CI build, so this will prevent your build from failing.

Comments

Every public function Foo should have a // comment of the style Foo <explanation>, where explanation says what Foo does, and more importantly, why it does that, what assumptions it makes, and other aspects not obvious from the function name.

Control structures

When possible, avoid nesting if/else statements. (Of course, a single, non-nested if/else is perfectly fine.) Prefer multiple return statements over nested code. This makes the code more readable and avoids complexity.

E.g:

// Do this
if something {
	doThis()
	return this
}
if orOther {
	return that
}
return theOther
// Don't do this
if something {
	doThis()
} else {
	if orOther {
		return that
	}
}

Error handling

Prefer using the errors standard library package for handling single errors. For operations that can produce multiple errors, leverage the MultiError package by accumulating all the errors into a single MultiError and returning that, rather than returning every error individually as it comes up.

Don’t panic (hat tip, Douglas Adams). Any method that can have an error should return that error as its final value. This should be passed up through each layer, which can decide what to do with it, all the way up to the very entrypoint of the app (or main) if necessary. You should almost NEVER use panic.

Use custom error types. Create your own types that implement the error interface so that error messages are clear and have well-defined types you can check against. For some examples of this, see e.g. the custom erros in the aws package of terratest.

Include stack traces. In most of our code, we have to wrap errors with errors.WithStackTrace(e) to add the stack trace. Go annoyingly doesn’t do this by default, but without it, sorting out an error can be very tricky.

Pointer usage

Prefer using value type over pointers, unless necessary. Generally speaking, there are only a few cases where pointer usage would be justified:

  1. You have very large structs containing lots of data. Instead of copying these around, passing pointers may be more efficient.

  2. You need to mutate a variable you passed to a function. Generally speaking, you should avoid this anyway (see the section on immutability below), but sometimes it is necessary (think "rename" methods, etc).

  3. You need to return nil as the default zero value (the default zero value for pointers is nil, as opposed to other data types that have non-nil default zero values).

Testing

In terms of testing, we don’t necessarily aim for 100% test coverage. Rather, our goal is to have enough testing to give us confidence that our code works and allow us to update it quickly. When adding tests, keep in mind these factors:

  1. The likelihood of bugs: some code is harder to get right than others, and merits correspondingly more tests.

  2. The cost of bugs: some bugs are much more costly than others — e.g., bugs in payment systems or security functionality — so it makes sense to invest more in testing there.

  3. The cost of tests: some types of tests are cheap and easy to write and maintain, whereas others are very costly and brittle.

Unless there’s a reason not to, tests should run in parallel. This is done by including a call to t.Parallel in the test function.

Setup and Teardown pattern

In some cases you will want to write a group of tests that use a common resource, such as a Docker image or VPC. In this case, you will want to setup the common resource once, run a bunch of tests, and then teardown the resource. To achieve this, you can follow the subtest pattern of Go.

Always use table driven tests where possible to make the subtest routines maintainable. Briefly, this means that you group your test cases using a test struct that reflects the unique parameters of the test cases. Then you can conveniently loop over the test cases in parallel, taking advantage of uniformity and speed.

Note that the subtest pattern has gotchas when running tests in parallel:

  • The main test function will not wait for the subtest to run if it uses t.Parallel. To avoid this, you need to wrap the parallel subtests in a synchronous, blocking subtest. In the example below, the group subtest is synchronous (no call to t.Parallel) and thus the main function will wait until that test finishes. The group test does not finish until all the subtests it spawns are finished, even if they are non-blocking and parallel, and thus the tearDownVPC call does not happen until all subtests are done.

  • If you are using table driven tests, the range variable will be updated to the next iteration before it is used within the subtest. That is, in the example below, if we did not have the testCase := testCase line in the range block, the testCase reference used in the subtest after the t.Parallel call will correspond to the last testCase in the testCases list. To avoid this, we create a new variable in the scope of the range block so that it does not get updated during the loop.

Example:

func TestECS(t *testing.T) {
    t.Parallel()

    defer tearDownVPC()
    deployVPC()

    // Wrap the parallel tests in a synchronous test group to
    // ensure that the main test function (the one calling
    // `tearDownVPC` and `deployVPC`) waits until all the
    // subtests are done before running the deferred function.
    t.Run("group", func(t *testing.T) {
        for _, testCase := range testCases {
            // To avoid the range variable from getting updated in the
        	// parallel tests, we bind a new name that is within the
        	// scope of the for block.
            testCase := testCase
            t.Run(testCase.name, func(t *testing.T) {
                t.Parallel()
                testCase.testCode()
            })
        }
    })
}

Naming things

Prefer descriptive names over short ones. In particular, avoid one-letter variable names, other than in case of very well known and widely understood conventions, such as i for index (e.g. in a loop). A more descriptive name helps with code understanding and maintenance, at very little cost, given the auto-complete feature in most IDEs and editors.

If a name contains an acronym, only capitalize the first letter of the acronym. E.g. use someEksCluster rather than someEKSCluster. We go against the recommendation here in order to follow the convention already in use by some third party packages we heavily rely on (e.g. aws-sdk-go).

Constants

Since many languages use ALL_CAPS for constants, it is worth calling out explicitly that Effective Go recommends using MixedCaps for all names, including constants. Therefore, region or testRegion for private constants and Region or TestRegion for public ones is preferred over REGION or TEST_REGION.

Functional programming practices

Immutability

Prefer returning a new value rather than mutating an existing one.

// Don't do this
var result int = 0

func main() {
    add(1, 1, &result)
    fmt.Println(result)
}

func add(a, b int, result *int) {
    *result = a + b
}
// Do this instead
func main() {
    fmt.Println(add(1, 1))
}

func add(a, b int) int {
    return a + b
}

Pure functions

Prefer functions that take all their inputs as function parameters, instead of reading those inputs via side effects (e.g., reading from disk or the network or global vars), and whose entire behavior is to return values (note: errors are values too!), rather than performing side effects (e.g. by writing to disk or the network or global vars). Of course, you can’t avoid side effects forever if you want your code to do something useful, but try to do as much of your logic with pure functions as you can, try to pass everything around as explicit parameters and return everything as explicit values, and centralize the side effecting code to a few isolated places.

Composition

Build your code out of small, reusable, named functions, that you compose together.

// Don't do this
func main() {
    fmt.Println(mulOfSums(1, 1))
}

func mulOfSums(a, b int) int {
    return (a + b) * (a + b)
}
// Do this instead
func main() {
    fmt.Println(mul(add(1, 1), add(1, 1)))
}

func add(a, b int) int {
    return a + b
}

func mul(a, b int) int {
    return a * b
}

Repo-specific conventions

terratest

Note the existence of methods in terratest which are suffixed with the letter E, e.g. GetAccountIdE. Methods that have the suffix E return an error as the last return value; methods without E mark the test as failed (e.g., via calling t.Fail()) instead of returning an error.