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id="page-container"><section id="content" role="main" class="main-content content--fancy-strings-in-scala-3"><section class="content__section section-post"><div class="viewport"><div class="post__grid grid--content"><article class="post__content text--rich" role="main" data-blog-post="" data-submenu="" data-submenu_h1="data-submenu_h2="><div class="title-area"><h1 class="banner__title">Fancy strings in Scala 3</h1><div class="taxonomy__tags"><ul class="article__tags tags__list"><li class="list__item item--tag"><a href="/blog/?tag=scala" class="no-border">Scala</a></li><li class="list__item item--tag"><a href="/blog/?tag=functional programming" class="no-border">Functional Programming</a></li><li class="list__item item--tag"><a href="/blog/?tag=type safety" class="no-border">Type Safety</a></li><li class="list__item item--tag"><a href="/blog/?tag=scala 3" class="no-border">Scala 3</a></li><li class="list__item item--tag"><a href="/blog/?tag=functional programming" class="no-border">Functional programming</a></li></ul></div><div class="block-author author--big"><div class="grid--two-columns"><div class="author__meta"><a href="/blog/?author=Adam%20Warski"><figure alt="webp image" class="employee__image" style="background-image:url('/user/webp/user/themes/softwaremill/assets/team/5UM2bvwWkeS4XlH.webp')"/></a><div class="author__desc"><p class="name"><a href="/blog/?author=Adam%20Warski">Adam Warski</a></p><p><span class="date">30 Mar 2022.</span><span class="time-to-read">6 minutes read</span></p></div></div><div class="author__social-media"><div class="author__socialmedia block-socialmedia"><ul class="socialmedia__list"><li><a target="_blank" href="https://linkedin.com/in/adamwarski"><figure class="socialmedia__ico ui-ico ico--black ico--linkedin"><svg version="1.1" class="ico-linkedin" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" viewbox="-949 471.7 20.9 19.3" enable-background="new -949 471.7 20.9 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c-3.6,0.8-4.3-1.7-4.3-1.7c-0.2-0.8-0.7-1.5-1.4-1.9c-1.2-0.8,0.1-0.7,0.1-0.7c0.8,0.1,1.6,0.6,2,1.3c0.7,1.3,2.3,1.8,3.6,1.1 c0,0,0.1,0,0.1,0c0-0.7,0.3-1.3,0.8-1.7c-2.8-0.3-5.8-1.4-5.8-6.3c0-1.3,0.4-2.5,1.3-3.4c-0.4-1.1-0.4-2.3,0.2-3.4 c0,0,1.1-0.3,3.5,1.3c1.1-0.3,2.1-0.4,3.2-0.4c1.1,0,2.2,0.2,3.2,0.4c2.4-1.7,3.5-1.3,3.5-1.3c0.4,1.1,0.5,2.3,0.2,3.4 c0.9,0.9,1.3,2.1,1.3,3.4c0,4.9-3,6-5.8,6.3c0.6,0.6,0.9,1.5,0.8,2.4v3.5c0,0.3,0.2,0.8,0.9,0.6c5.2-1.8,8.7-6.6,8.7-12.2 C-920.3,474.7-926.1,469.1-933.2,469z"/></svg></figure></a></li></ul></div></div></div></div></div><p><img src="/user/webp/user/themes/softwaremill/assets/img/post-image/g87IdJ1jiUl4FNc.webp" alt="Fancy strings in Scala 3 webp image"/></p><div><p>Let’s put some of the new <a href="https://scala-lang.org">Scala 3</a> features to work! While we try to escape using <code>String</code>s as much as possible, we still end up manipulating them in our codebases daily. However, quite often, these are not arbitrary strings but ones with some special properties. In these cases, Scala’s compiler might be able to offer some help.</p><p>Our goal will be to <strong>create dedicated types for non-empty and lowercase strings</strong>. We’ll use <a href="https://docs.scala-lang.org/scala3/book/types-opaque-types.html">opaque types</a> and <a href="https://docs.scala-lang.org/scala3/guides/macros/inline.html">inlines</a>, which are new features of the Scala 3 compiler.</p><h2>Non-empty strings</h2><p>First, let’s create a zero-cost abstraction that will represent non-empty strings. For that, we’ll define an opaque type <code>NonEmptyString</code>:</p><pre><code class="language-scala">opaque type NonEmptyString = String</code></pre><p>At runtime, values of type <code>NonEmptyString</code> will be simply <code>String</code>s (hence, there’s no additional cost to the abstraction that we are introducing). At compile-time, however, these two types are treated by the compiler as completely unrelated, <strong>except</strong> for the scope in which this type alias is defined. Here, if this is a top-level definition, the scope will be the entire file, but if we created the type alias in an object, the scope in which it’s known that a <code>NonEmptyString</code> is in fact a <code>String</code> would be that object.</p><p>A type alias is a good start, but we’ll also need some way to “lift” values from a <code>String</code> into our new type. First, given an arbitrary value at runtime, we can write a function that returns an optional <code>NonEmptyString</code>. Note that this definition needs to be placed next to the opaque type alias as the compiler needs to know that these types are indeed equal:</p><pre><code class="language-scala">opaque type NonEmptyString = String object NonEmptyString: def apply(s: String): Option[NonEmptyString] = if s.isEmpty then None else Some(s)</code></pre><p>It’s worth noting that here we do have some additional runtime cost - allocating the option. However, we can do better with constants. We can check at compile time if they are indeed empty or not! For this, we’ll use an inline method that is guaranteed to be evaluated by the compiler as the compilation happens. In the inline’s method definition, we’ll use an <code>inline if</code> which can be used to verify whether a <strong>constant expression</strong> is true at compile time:</p><pre><code class="language-scala">inline def from(inline s: String): NonEmptyString = requireConst(s) inline if s == "" then error("got an empty string") else s</code></pre><p>We’re also using <code>scala.compiletime.requireConst</code> to get a nice error message if the value passed as a parameter is not a constant (but e.g. a value reference). If the string is empty, we’re using <code>scala.compiletime.error</code> to report a custom error message.</p><p>Finally, we need a way to upcast a <code>NonEmptyString</code> into a <code>String</code>. This can be done using an <a href="https://docs.scala-lang.org/scala3/reference/contextual/conversions.html">implicit conversion</a>. In order to avoid an additional runtime method call, we define the conversion as an <code>inline</code> method as well (evaluated at compile-time). This conversion will be added automatically by the compiler, given that we import it into scope.</p><blockquote><p>As <a href="https://softwaremill.com/fancy-strings-in-scala-3/#comment-5813679026">Julien noted</a>, a better way to achieve the above is to add a type bound to the definition of the opaque type: <code>opaque type NonEmptyString &lt;: String = String</code>. That way, we don't need the implicit "upcasting" conversion.</p></blockquote><p>Here’s the entire <code>NonEmptyString</code> abstraction that we have created:</p><pre><code>import scala.compiletime.{error, requireConst} opaque type NonEmptyString = String object NonEmptyString: def apply(s: String): Option[NonEmptyString] = if s.isEmpty then None else Some(s) inline def from(inline s: String): NonEmptyString = requireConst(s) inline if s == "" then error("got an empty string") else s given Conversion[NonEmptyString, String] with inline def apply(nes: NonEmptyString): String = nes </code></pre><p>Time for some tests! We need to put them in a different file or a different scope so that the opaque type is really opaque:</p><pre><code class="language-scala">object Usage: // compilation fails: // Found: ("abc" : String) // Required: com.softwaremill.test.NonEmptyString val x0: NonEmptyString = "abc" // compilation succeeds val x1: NonEmptyString = NonEmptyString.from("abc") // compilation fails: // got an empty string val x2: NonEmptyString = NonEmptyString.from("") // compilation fails: // expected a constant value but found: Usage.z val z = "x" * 10 val x3: NonEmptyString = NonEmptyString.from(z) // implicit conversion at work def test(param: String) = s"Got: $param" println(test(x1))</code></pre><p>What’s important in the above design is that we’re creating no runtime overhead - all <code>NonEmptyString</code> values at runtime are the same <code>String</code> objects from which they are created; it’s just at compile-time that these types are distinct.</p><h2>Lowercase strings</h2><p>Let’s look at a slightly more complex example - creating a type that represents lowercase strings. We start the same, with an opaque type, implicit conversion to upcast back to <code>String</code>, and a method to lift a <code>String</code> into our <code>LowerCased</code> type. This time, the lifting works slightly differently, as we simply lowercase the parameter:</p><pre><code class="language-scala">opaque type LowerCased = String object LowerCased: def apply(s: String): LowerCased = s.toLowerCase(Locale.US) given Conversion[LowerCased, String] with inline def apply(lc: LowerCased): String = lc</code></pre><p>What about typing known constants as <code>LowerCased</code>? Here, <code>inline</code>s are not powerful enough: in the <code>inline if</code>, we can only check constant conditions. To verify that a constant string is already lower case, we’d have to check that <code>LowerCased(s) == s</code> at compile time. Luckily, unlike inlines, using <a href="https://docs.scala-lang.org/scala3/guides/macros/macros.html">macros</a> we can run arbitrary code. We won’t go in-depth into quoting &amp; splicing (see links to articles dedicated to macros in the next section), instead, we’ll focus on the macro logic that is required here.</p><p>We’ll need two methods. A user-facing one that “lifts” the parameter it receives into the macro-land (so that we can inspect &amp; manipulate it at compile-time) and “splices” the result of our manipulations back so that it is compiled normally. This method needs to be inline to be evaluated at compile-time and its body will call the macro implementation. The quoting and splicing is done using <code>’{}</code> and <code>${}</code>, respectively.</p><p>Additionally, we’ll need to define the macro implementation method, which operates on abstract syntax trees (ASTs) - represented in code as values of type <code>Expr</code> - corresponding to the chunks of code that are passed as macro parameters:</p><pre><code class="language-scala">object LowerCased: inline def from(inline s: String): LowerCased = ${ fromImpl('s) } import scala.quoted.* private def fromImpl(s: Expr[String])(using Quotes): Expr[LowerCased] = ???</code></pre><p>In the implementation, we’ll inspect the abstract syntax tree corresponding to the parameter <code>s</code>. If it is a constant string, we check if the string is already lower case. Otherwise, we report an error:</p><pre><code class="language-scala">object LowerCased: private def fromImpl(s: Expr[String])(using Quotes): Expr[LowerCased] = import quotes.reflect.* s.asTerm match case Inlined(_, _, Literal(StringConstant(str))) =&gt; if LowerCased(str) == str then s else report.errorAndAbort( s"got a string which is not all lower case: $str") case _ =&gt; report.errorAndAbort( s"got a value which is not a constant string: ${s.show}")</code></pre><p>Note that in case of success, the result of the method is <code>s</code> - the expression that is passed as a parameter. Since the macro is defined next to the opaque type alias, the compiler knows that <code>LowerCased == String</code>.</p><p>Let’s test our solution:</p><pre><code class="language-scala">// compilation succeeds val p1: LowerCased = LowerCased("Abc") // compilation succeeds LowerCased.from("abc") // compilation fails: // got a string which is not all lower case: Abc