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<ol class="chapter"><li class="chapter-item expanded affix "><a href="u00-intro.html">Golocron</a></li><li class="chapter-item expanded affix "><li class="part-title">The Style Guide</li><li class="chapter-item expanded "><a href="u01-00-introduction.html"><strong aria-hidden="true">1.</strong> Project Layout</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="u01-01-good-and-bad-layout.html"><strong aria-hidden="true">1.1.</strong> Good and Bad Layout</a></li><li class="chapter-item expanded "><a href="u01-02-types-of-layouts.html"><strong aria-hidden="true">1.2.</strong> Types of Layouts</a></li><li class="chapter-item expanded "><a href="u01-03-common.html"><strong aria-hidden="true">1.3.</strong> Common</a></li><li class="chapter-item expanded "><a href="u01-04-library.html"><strong aria-hidden="true">1.4.</strong> Library</a></li><li class="chapter-item expanded "><a href="u01-05-single-application.html"><strong aria-hidden="true">1.5.</strong> Single Application</a></li><li class="chapter-item expanded "><a href="u01-06-monorepo.html"><strong aria-hidden="true">1.6.</strong> Monolithic Repository</a></li><li class="chapter-item expanded "><a href="u01-07-monorepo-extra.html"><strong aria-hidden="true">1.7.</strong> Monorepo: Additional Chapters</a></li><li class="chapter-item expanded "><a href="u01-08-versioning-and-go.html"><strong aria-hidden="true">1.8.</strong> Versioning and Go</a></li><li class="chapter-item expanded "><a href="u01-09-notes-on-release-notes.html"><strong aria-hidden="true">1.9.</strong> Notes on Release Notes</a></li><li class="chapter-item expanded "><a href="u01-10-summary.html"><strong aria-hidden="true">1.10.</strong> Summary</a></li></ol></li><li class="chapter-item expanded "><a href="u02-00-introduction.html"><strong aria-hidden="true">2.</strong> Package Layout</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="u02-01-what-is-a-package.html"><strong aria-hidden="true">2.1.</strong> What is a Package</a></li><li class="chapter-item expanded "><a href="u02-02-when-to-create-a-package.html"><strong aria-hidden="true">2.2.</strong> When to Create a Package</a></li><li class="chapter-item expanded "><a href="u02-03-keep-public-api-narrow.html"><strong aria-hidden="true">2.3.</strong> Keep Public API as Narrow as Possible</a></li><li class="chapter-item expanded "><a href="u02-04-the-main-package.html"><strong aria-hidden="true">2.4.</strong> The Main Package</a></li><li class="chapter-item expanded "><a href="u02-05-package-provides-something.html"><strong aria-hidden="true">2.5.</strong> Package Provides Something</a></li><li class="chapter-item expanded "><a href="u02-06-naming-a-package.html"><strong aria-hidden="true">2.6.</strong> Naming a Package</a></li><li class="chapter-item expanded "><a href="u02-07-structure.html"><strong aria-hidden="true">2.7.</strong> Structure</a></li><li class="chapter-item expanded "><a href="u02-08-files-in-a-package.html"><strong aria-hidden="true">2.8.</strong> Files in a Package</a></li><li class="chapter-item expanded "><a href="u02-09-cross-platform-code.html"><strong aria-hidden="true">2.9.</strong> Cross-Platform Code</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="u02-10-basic-principles-cp.html"><strong aria-hidden="true">2.9.1.</strong> Basic Principles of Writing Cross-Platform Code</a></li><li class="chapter-item expanded "><a href="u02-11-cp-options-in-go.html"><strong aria-hidden="true">2.9.2.</strong> Cross-Platform Options in Go</a></li></ol></li><li class="chapter-item expanded "><a href="u02-12-cp-package-and-file-organisation.html"><strong aria-hidden="true">2.10.</strong> Package and File Organisation</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="u02-13-cp-keep-code-at-minimum.html"><strong aria-hidden="true">2.10.1.</strong> Keep Platform-Dependent Code at Minimum</a></li><li class="chapter-item expanded "><a href="u02-14-cp-simple-branching.html"><strong aria-hidden="true">2.10.2.</strong> Use Simple Branching in Trivial Cases</a></li><li class="chapter-item expanded "><a href="u02-15-cp-no-platform-code-in-main.html"><strong aria-hidden="true">2.10.3.</strong> No Platform-Specific Code in Main</a></li><li class="chapter-item expanded "><a href="u02-16-cp-file-suffix-by-default.html"><strong aria-hidden="true">2.10.4.</strong> Use File Suffix by Default</a></li><li class="chapter-item expanded "><a href="u02-17-cp-build-tags-in-mixed-cases.html"><strong aria-hidden="true">2.10.5.</strong> Use Build Tags in Mixed Cases</a></li><li class="chapter-item expanded "><a href="u02-18-cp-advanced-example.html" class="active"><strong aria-hidden="true">2.10.6.</strong> An Advanced Example</a></li><li class="chapter-item expanded "><a href="u02-19-cp-platform-independent-tests.html"><strong aria-hidden="true">2.10.7.</strong> Strive for Platform-Independent Tests</a></li><li class="chapter-item expanded "><a href="u02-20-cp-cross-platform-tests.html"><strong aria-hidden="true">2.10.8.</strong> Provide Cross-Platform Tests Only When You Must</a></li></ol></li><li class="chapter-item expanded "><a href="u02-21-summary.html"><strong aria-hidden="true">2.11.</strong> Summary</a></li></ol></li><li class="chapter-item expanded "><div><strong aria-hidden="true">3.</strong> File Layout</div></li><li class="chapter-item expanded affix "><li class="part-title">Foundation</li><li class="chapter-item expanded affix "><li class="part-title">Application Design</li></ol>
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<h1 class="menu-title">Golocron – Software Development With Go</h1>
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<h3 id="an-advanced-example"><a class="header" href="#an-advanced-example">An Advanced Example</a></h3>
<p>Here is another question: What if we need to support a third feature which is unique to the Mac, and is unavailable for all other platforms? In this case, we should use the full power available at our disposal, and also avoid exposing the difference to <code>main</code>. How can this be accomplished?</p>
<p>We can use our example to illustrate the approach. Let's assume there should be a feature that does something on <code>darwin</code> but does not exist/has no effect on other platforms. There are two approaches available:</p>
<ul>
<li>mock the behaviour on the platforms that don't have the feature, so it can be called in a platform-independent way, i.e. make it no-op</li>
<li>or encapsulate the platform-dependent call such that it's not exposed to the platform-independent code, hence is transparent.</li>
</ul>
<p>The latter is available and really helpful in real life when implementing unique parts nested within a higher-level code that is called by code that is unaware of different platforms. For our example we'll follow the former way, but the second is preferred whenever it's possible.</p>
<p>Also note, that a simple <code>if runtime.GOOS == "darwin" { // unique code }</code> won't do the trick. Why? Because it will be attempted to be included for all platforms during compilation, and conditioned only at runtime.</p>
<p>Let's introduce a third feature which should work only on the Mac, while all existing conditions must remain. In order to make this happen, we:</p>
<ul>
<li>implement the new feature for <code>darwin</code></li>
<li>mock it for <code>windows</code> and <code>linix</code></li>
<li>the second step implies that we have to add a Linux-specific file. We'll use the suffix for that.</li>
</ul>
<p>We also don't want to make a special case for the existing code shared between Linux and macOS. The fact that we're now going to support all three platforms, does not mean we should have a separate platform-specific implementation, just for the sake of separation. As one of the guiding principles says, the more code can be shared and/or abstracted, the better. And if the second feature introduced earlier remains the same for the two platforms, there is absolutely no point in splitting it.</p>
<p>Let's have a look at the project after the necessary have been made.</p>
<ul>
<li>directory listing:</li>
</ul>
<pre><code class="language-bash">.
├── bin
├── cmd
│ └── myapp
│ └── main.go
└── something
├── something.go
├── something_darwin.go
├── something_linux.go
├── something_posix.go
└── something_windows.go
</code></pre>
<p>Notice the two new files, <code>something_darwin.go</code> and <code>something_linux.go</code>.</p>
<ul>
<li>the <code>main.go</code> file:</li>
</ul>
<pre><code class="language-golang">package main
import (
"fmt"
"../../something"
)
func main() {
fmt.Println(something.Feature1)
fmt.Println(something.Feature2)
something.DoFeature3()
}
</code></pre>
<p>Notice how the code does not depend on platform here.</p>
<ul>
<li><code>something.go</code></li>
</ul>
<pre><code class="language-golang">package something
const (
Feature1 = "common feature for all platforms"
)
</code></pre>
<p>This piece of code is common among all supported platforms.</p>
<ul>
<li><code>something_darwin.go</code></li>
</ul>
<pre><code class="language-golang">package something
import (
"fmt"
)
func DoFeature3() {
fmt.Println("this is a unique feature for the mac")
}
</code></pre>
<p>This code is compiled for and executed only when running on the Mac.</p>
<ul>
<li><code>something_linux.go</code></li>
</ul>
<pre><code class="language-golang">package something
func DoFeature3() {}
</code></pre>
<p>Here we mock the missing functionality for Linux.</p>
<ul>
<li><code>something_posix.go</code></li>
</ul>
<pre><code class="language-golang">// +build linux darwin
package something
const (
Feature2 = "this is a version supported by a number of unix-like systems"
)
</code></pre>
<p>This code remains the same, as it works nicely for the two platforms, hence no change is needed. This file is included for both platforms.</p>
<ul>
<li><code>something_windows.go</code></li>
</ul>
<pre><code class="language-golang">package something
const (
Feature2 = "this is a windows specific version"
)
func DoFeature3() {}
</code></pre>
<p>Finally, for Windows, the new feature is mocked in this existing file.</p>
<p>Here are a few results of building and running:</p>
<pre><code class="language-bash"># Build for darwin.
GOOS=darwin go build -o bin/myapp-darwin ./cmd/myapp
# Run on darwin.
$ ./bin/myapp-darwin
common feature for all platforms
this is a version supported by a number of unix-like systems
this is a unique feature for the mac
# Build for linux.
$ GOOS=linux go build -o bin/myapp-linux ./cmd/myapp
# Run on linux.
$ docker run -it --rm -v $(pwd)/bin:/root/bin/ alpine:3.13 ash
% /root/bin/myapp-linux
common feature for all platforms
this is a version supported by a number of unix-like systems
</code></pre>
<p>As you see, the feature works as expected on macOS, and is non-existent on Linux. <em>Quod Erat Demonstrandum.</em></p>
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