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Macro-Projects.html
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Macro-Projects.html
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<div class="section" id="macro-projects">
<h1>Macro Projects<a class="headerlink" href="#macro-projects" title="Permalink to this headline">¶</a></h1>
<div class="section" id="introduction">
<h2>Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<p>Some common problems arise when working with macros.</p>
<ol class="arabic simple">
<li>The current macro implementation in the compiler requires that macro implementations be compiled before they are used. The solution is typically to put the macros in a subproject or in their own configuration.</li>
<li>Sometimes the macro implementation should be distributed with the main code that uses them and sometimes the implementation should not be distributed at all.</li>
</ol>
<p>The rest of the page shows example solutions to these problems.</p>
</div>
<div class="section" id="defining-the-project-relationships">
<h2>Defining the Project Relationships<a class="headerlink" href="#defining-the-project-relationships" title="Permalink to this headline">¶</a></h2>
<p>The macro implementation will go in a subproject in the <tt class="docutils literal"><span class="pre">macro/</span></tt> directory.
The main project in the project's base directory will depend on this subproject and use the macro.
This configuration is shown in the following build definition:</p>
<p><tt class="docutils literal"><span class="pre">project/Build.scala</span></tt></p>
<div class="highlight-scala"><div class="highlight"><pre><span class="k">import</span> <span class="nn">sbt._</span>
<span class="k">import</span> <span class="nn">Keys._</span>
<span class="k">object</span> <span class="nc">MacroBuild</span> <span class="k">extends</span> <span class="nc">Build</span> <span class="o">{</span>
<span class="k">lazy</span> <span class="k">val</span> <span class="n">main</span> <span class="k">=</span> <span class="nc">Project</span><span class="o">(</span><span class="s">"main"</span><span class="o">,</span> <span class="n">file</span><span class="o">(</span><span class="s">"."</span><span class="o">))</span> <span class="n">dependsOn</span><span class="o">(</span><span class="n">macroSub</span><span class="o">)</span>
<span class="k">lazy</span> <span class="k">val</span> <span class="n">macroSub</span> <span class="k">=</span> <span class="nc">Project</span><span class="o">(</span><span class="s">"macro"</span><span class="o">,</span> <span class="n">file</span><span class="o">(</span><span class="s">"macro"</span><span class="o">))</span> <span class="n">settings</span><span class="o">(</span>
<span class="n">libraryDependencies</span> <span class="o"><+=</span> <span class="n">scalaVersion</span><span class="o">(</span><span class="s">"org.scala-lang"</span> <span class="o">%</span> <span class="s">"scala-compiler"</span> <span class="o">%</span> <span class="k">_</span><span class="o">)</span>
<span class="o">)</span>
<span class="o">}</span>
</pre></div>
</div>
<p>This specifies that the macro implementation goes in <tt class="docutils literal"><span class="pre">macro/src/main/scala/</span></tt> and tests go in <tt class="docutils literal"><span class="pre">macro/src/test/scala/</span></tt>.
It also shows that we need a dependency on the compiler for the macro implementation.
As an example macro, we'll use <tt class="docutils literal"><span class="pre">desugar</span></tt> from <a class="reference external" href="https://github.com/retronym/macrocosm">macrocosm</a>.</p>
<p><tt class="docutils literal"><span class="pre">macro/src/main/scala/demo/Demo.scala</span></tt></p>
<div class="highlight-scala"><div class="highlight"><pre><span class="k">package</span> <span class="nn">demo</span>
<span class="k">import</span> <span class="nn">language.experimental.macros</span>
<span class="k">import</span> <span class="nn">scala.reflect.macros.Context</span>
<span class="k">object</span> <span class="nc">Demo</span> <span class="o">{</span>
<span class="c1">// Returns the tree of `a` after the typer, printed as source code.</span>
<span class="k">def</span> <span class="n">desugar</span><span class="o">(</span><span class="n">a</span><span class="k">:</span> <span class="kt">Any</span><span class="o">)</span><span class="k">:</span> <span class="kt">String</span> <span class="o">=</span> <span class="n">macro</span> <span class="n">desugarImpl</span>
<span class="k">def</span> <span class="n">desugarImpl</span><span class="o">(</span><span class="n">c</span><span class="k">:</span> <span class="kt">Context</span><span class="o">)(</span><span class="n">a</span><span class="k">:</span> <span class="kt">c.Expr</span><span class="o">[</span><span class="kt">Any</span><span class="o">])</span> <span class="k">=</span> <span class="o">{</span>
<span class="k">import</span> <span class="nn">c.universe._</span>
<span class="k">val</span> <span class="n">s</span> <span class="k">=</span> <span class="n">show</span><span class="o">(</span><span class="n">a</span><span class="o">.</span><span class="n">tree</span><span class="o">)</span>
<span class="n">c</span><span class="o">.</span><span class="nc">Expr</span><span class="o">(</span>
<span class="nc">Literal</span><span class="o">(</span><span class="nc">Constant</span><span class="o">(</span><span class="n">s</span><span class="o">))</span>
<span class="o">)</span>
<span class="o">}</span>
<span class="o">}</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">macro/src/test/scala/demo/Usage.scala</span></tt></p>
<div class="highlight-scala"><div class="highlight"><pre><span class="k">package</span> <span class="nn">demo</span>
<span class="k">object</span> <span class="nc">Usage</span> <span class="o">{</span>
<span class="k">def</span> <span class="n">main</span><span class="o">(</span><span class="n">args</span><span class="k">:</span> <span class="kt">Array</span><span class="o">[</span><span class="kt">String</span><span class="o">])</span> <span class="o">{</span>
<span class="k">val</span> <span class="n">s</span> <span class="k">=</span> <span class="nc">Demo</span><span class="o">.</span><span class="n">desugar</span><span class="o">(</span><span class="nc">List</span><span class="o">(</span><span class="mi">1</span><span class="o">,</span> <span class="mi">2</span><span class="o">,</span> <span class="mi">3</span><span class="o">).</span><span class="n">reverse</span><span class="o">)</span>
<span class="n">println</span><span class="o">(</span><span class="n">s</span><span class="o">)</span>
<span class="o">}</span>
<span class="o">}</span>
</pre></div>
</div>
<p>This can be then be run at the console:</p>
<p>Actual tests can be defined and run as usual with <tt class="docutils literal"><span class="pre">macro/test</span></tt>.</p>
<p>The main project can use the macro in the same way that the tests do.
For example,</p>
<p><tt class="docutils literal"><span class="pre">src/main/scala/MainUsage.scala</span></tt></p>
<div class="highlight-scala"><div class="highlight"><pre><span class="k">package</span> <span class="nn">demo</span>
<span class="k">object</span> <span class="nc">Usage</span> <span class="o">{</span>
<span class="k">def</span> <span class="n">main</span><span class="o">(</span><span class="n">args</span><span class="k">:</span> <span class="kt">Array</span><span class="o">[</span><span class="kt">String</span><span class="o">])</span> <span class="o">{</span>
<span class="k">val</span> <span class="n">s</span> <span class="k">=</span> <span class="nc">Demo</span><span class="o">.</span><span class="n">desugar</span><span class="o">(</span><span class="nc">List</span><span class="o">(</span><span class="mi">6</span><span class="o">,</span> <span class="mi">4</span><span class="o">,</span> <span class="mi">5</span><span class="o">).</span><span class="n">sorted</span><span class="o">)</span>
<span class="n">println</span><span class="o">(</span><span class="n">s</span><span class="o">)</span>
<span class="o">}</span>
<span class="o">}</span>
</pre></div>
</div>
</div>
<div class="section" id="common-interface">
<h2>Common Interface<a class="headerlink" href="#common-interface" title="Permalink to this headline">¶</a></h2>
<p>Sometimes, the macro implementation and the macro usage should share some common code.
In this case, declare another subproject for the common code and have the main project and the macro subproject depend on the new subproject.
For example, the project definitions from above would look like:</p>
<div class="highlight-scala"><div class="highlight"><pre><span class="k">lazy</span> <span class="k">val</span> <span class="n">main</span> <span class="k">=</span> <span class="nc">Project</span><span class="o">(</span><span class="s">"main"</span><span class="o">,</span> <span class="n">file</span><span class="o">(</span><span class="s">"."</span><span class="o">))</span> <span class="n">dependsOn</span><span class="o">(</span><span class="n">macroSub</span><span class="o">,</span> <span class="n">commonSub</span><span class="o">)</span>
<span class="k">lazy</span> <span class="k">val</span> <span class="n">macroSub</span> <span class="k">=</span> <span class="nc">Project</span><span class="o">(</span><span class="s">"macro"</span><span class="o">,</span> <span class="n">file</span><span class="o">(</span><span class="s">"macro"</span><span class="o">))</span> <span class="n">dependsOn</span><span class="o">(</span><span class="n">commonSub</span><span class="o">)</span> <span class="n">settings</span><span class="o">(</span>
<span class="n">libraryDependencies</span> <span class="o"><+=</span> <span class="n">scalaVersion</span><span class="o">(</span><span class="s">"org.scala-lang"</span> <span class="o">%</span> <span class="s">"scala-compiler"</span> <span class="o">%</span> <span class="k">_</span><span class="o">)</span>
<span class="o">)</span>
<span class="k">lazy</span> <span class="k">val</span> <span class="n">commonSub</span> <span class="k">=</span> <span class="nc">Project</span><span class="o">(</span><span class="s">"common"</span><span class="o">,</span> <span class="n">file</span><span class="o">(</span><span class="s">"common"</span><span class="o">))</span>
</pre></div>
</div>
<p>Code in <tt class="docutils literal"><span class="pre">common/src/main/scala/</span></tt> is available for both the <tt class="docutils literal"><span class="pre">macro</span></tt> and <tt class="docutils literal"><span class="pre">main</span></tt> projects to use.</p>
</div>
<div class="section" id="distribution">
<h2>Distribution<a class="headerlink" href="#distribution" title="Permalink to this headline">¶</a></h2>
<p>To include the macro code with the main code, add the binary and source mappings from the macro subproject to the main project.
For example, the <tt class="docutils literal"><span class="pre">main</span></tt> Project definition above would now look like:</p>
<div class="highlight-scala"><div class="highlight"><pre><span class="k">lazy</span> <span class="k">val</span> <span class="n">main</span> <span class="k">=</span> <span class="nc">Project</span><span class="o">(</span><span class="s">"main"</span><span class="o">,</span> <span class="n">file</span><span class="o">(</span><span class="s">"."</span><span class="o">))</span> <span class="n">dependsOn</span><span class="o">(</span><span class="n">macroSub</span><span class="o">)</span> <span class="n">settings</span><span class="o">(</span>
<span class="c1">// include the macro classes and resources in the main jar</span>
<span class="n">mappings</span> <span class="n">in</span> <span class="o">(</span><span class="nc">Compile</span><span class="o">,</span> <span class="n">packageBin</span><span class="o">)</span> <span class="o"><++=</span> <span class="n">mappings</span> <span class="n">in</span> <span class="o">(</span><span class="n">macroSub</span><span class="o">,</span> <span class="nc">Compile</span><span class="o">,</span> <span class="n">packageBin</span><span class="o">),</span>
<span class="c1">// include the macro sources in the main source jar</span>
<span class="n">mappings</span> <span class="n">in</span> <span class="o">(</span><span class="nc">Compile</span><span class="o">,</span> <span class="n">packageSrc</span><span class="o">)</span> <span class="o"><++=</span> <span class="n">mappings</span> <span class="n">in</span> <span class="o">(</span><span class="n">macroSub</span><span class="o">,</span> <span class="nc">Compile</span><span class="o">,</span> <span class="n">packageSrc</span><span class="o">)</span>
<span class="o">)</span>
</pre></div>
</div>
<p>You may wish to disable publishing the macro implementation.
This is done by overriding <tt class="docutils literal"><span class="pre">publish</span></tt> and <tt class="docutils literal"><span class="pre">deliver</span></tt> to do nothing:</p>
<div class="highlight-scala"><div class="highlight"><pre><span class="k">lazy</span> <span class="k">val</span> <span class="n">macroSub</span> <span class="k">=</span> <span class="nc">Project</span><span class="o">(</span><span class="s">"macro"</span><span class="o">,</span> <span class="n">file</span><span class="o">(</span><span class="s">"macro"</span><span class="o">))</span> <span class="n">settings</span><span class="o">(</span>
<span class="n">publish</span> <span class="o">:=</span> <span class="o">{},</span>
<span class="n">deliver</span> <span class="o">:=</span> <span class="o">{}</span>
<span class="o">)</span>
</pre></div>
</div>
<p>The techniques described here may also be used for the common interface described in the previous section.</p>
</div>
</div>
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<div class="span3"><p class="contents-title">Contents</p>
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<li><a class="reference internal" href="#">Macro Projects</a><ul>
<li><a class="reference internal" href="#introduction">Introduction</a></li>
<li><a class="reference internal" href="#defining-the-project-relationships">Defining the Project Relationships</a></li>
<li><a class="reference internal" href="#common-interface">Common Interface</a></li>
<li><a class="reference internal" href="#distribution">Distribution</a></li>
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