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<h1>DSL reference: a note on the complexity of Miller’s expression language<a class="headerlink" href="#dsl-reference-a-note-on-the-complexity-of-miller-s-expression-language" title="Permalink to this headline">¶</a></h1>
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<p>One of Miller’s strengths is its brevity: it’s much quicker – and less error-prone – to type <code class="docutils literal notranslate"><span class="pre">mlr</span> <span class="pre">stats1</span> <span class="pre">-a</span> <span class="pre">sum</span> <span class="pre">-f</span> <span class="pre">x,y</span> <span class="pre">-g</span> <span class="pre">a,b</span></code> than having to track summation variables as in <code class="docutils literal notranslate"><span class="pre">awk</span></code>, or using Miller’s out-of-stream variables. And the more language features Miller’s put-DSL has (for-loops, if-statements, nested control structures, user-defined functions, etc.) then the <em>less</em> powerful it begins to seem: because of the other programming-language features it <em>doesn’t</em> have (classes, execptions, and so on).</p>
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<p>When I was originally prototyping Miller in 2015, the decision I had was whether to hand-code in a low-level language like C or Rust, with my own hand-rolled DSL, or whether to use a higher-level language (like Python or Lua or Nim) and let the <code class="docutils literal notranslate"><span class="pre">put</span></code> statements be handled by the implementation language’s own <code class="docutils literal notranslate"><span class="pre">eval</span></code>: the implementation language would take the place of a DSL. Multiple performance experiments showed me I could get better throughput using the former, by a wide margin. So Miller is Go under the hood with a hand-rolled DSL.</p>
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<p>I do want to keep focusing on what Miller is good at – concise notation, low latency, and high throughput – and not add too much in terms of high-level-language features to the DSL. That said, some sort of customizability is a basic thing to want. As of 4.1.0 we have recursive for/while/if structures on about the same complexity level as <code class="docutils literal notranslate"><span class="pre">awk</span></code>; as of 5.0.0 we have user-defined functions and map-valued variables, again on about the same complexity level as <code class="docutils literal notranslate"><span class="pre">awk</span></code> along with optional type-declaration syntax. While I’m excited by these powerful language features, I hope to keep new features beyond 5.0.0 focused on Miller’s sweet spot which is speed plus simplicity.</p>
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