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Refactor documentation for custom link feature
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--- | ||
layout: singlepage-overview | ||
title: String Interpolation | ||
partof: string-interpolation | ||
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languages: [es, ja, zh-cn] | ||
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permalink: /overviews/core/:title.html | ||
--- | ||
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**Josh Suereth** | ||
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## Introduction | ||
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Starting in Scala 2.10.0, Scala offers a new mechanism to create strings from your data: String Interpolation. | ||
String Interpolation allows users to embed variable references directly in *processed* string literals. Here's an example: | ||
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{% tabs example-1 %} | ||
{% tab 'Scala 2 and 3' for=example-1 %} | ||
val name = "James" | ||
println(s"Hello, $name") // Hello, James | ||
{% endtab %} | ||
{% endtabs %} | ||
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In the above, the literal `s"Hello, $name"` is a *processed* string literal. This means that the compiler does some additional | ||
work to this literal. A processed string literal is denoted by a set of characters preceding the `"`. String interpolation | ||
was introduced by [SIP-11](https://docs.scala-lang.org/sips/pending/string-interpolation.html), which contains all details of the implementation. | ||
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## Usage | ||
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Scala provides three string interpolation methods out of the box: `s`, `f` and `raw`. | ||
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### The `s` String Interpolator | ||
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Prepending `s` to any string literal allows the usage of variables directly in the string. You've already seen an example here: | ||
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{% tabs example-2 %} | ||
{% tab 'Scala 2 and 3' for=example-2 %} | ||
val name = "James" | ||
println(s"Hello, $name") // Hello, James | ||
{% endtab %} | ||
{% endtabs %} | ||
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Here `$name` is nested inside an `s` processed string. The `s` interpolator knows to insert the value of the `name` variable at this location | ||
in the string, resulting in the string `Hello, James`. With the `s` interpolator, any name that is in scope can be used within a string. | ||
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String interpolators can also take arbitrary expressions. For example: | ||
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{% tabs example-3 %} | ||
{% tab 'Scala 2 and 3' for=example-3 %} | ||
println(s"1 + 1 = ${1 + 1}") | ||
{% endtab %} | ||
{% endtabs %} | ||
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will print the string `1 + 1 = 2`. Any arbitrary expression can be embedded in `${}`. | ||
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For some special characters, it is necessary to escape them when embedded within a string. | ||
To represent an actual dollar sign you can double it `$$`, like here: | ||
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{% tabs example-4 %} | ||
{% tab 'Scala 2 and 3' for=example-4 %} | ||
println(s"New offers starting at $$14.99") | ||
{% endtab %} | ||
{% endtabs %} | ||
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which will print the string `New offers starting at $14.99`. | ||
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Double quotes also need to be escaped. This can be done by using triple quotes as shown: | ||
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{% tabs example-5 %} | ||
{% tab 'Scala 2 and 3' for=example-5 %} | ||
val person = """{"name":"James"}""" | ||
{% endtab %} | ||
{% endtabs %} | ||
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which will produce the string `{"name":"James"}` when printed. | ||
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### The `f` Interpolator | ||
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Prepending `f` to any string literal allows the creation of simple formatted strings, similar to `printf` in other languages. When using the `f` | ||
interpolator, all variable references should be followed by a `printf`-style format string, like `%d`. Let's look at an example: | ||
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{% tabs example-6 %} | ||
{% tab 'Scala 2 and 3' for=example-6 %} | ||
val height = 1.9d | ||
val name = "James" | ||
println(f"$name%s is $height%2.2f meters tall") // James is 1.90 meters tall | ||
{% endtab %} | ||
{% endtabs %} | ||
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The `f` interpolator is typesafe. If you try to pass a format string that only works for integers but pass a double, the compiler will issue an | ||
error. For example: | ||
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{% tabs f-interpolator-error class=tabs-scala-version %} | ||
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{% tab 'Scala 2' for=f-interpolator-error %} | ||
```scala | ||
val height: Double = 1.9d | ||
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scala> f"$height%4d" | ||
<console>:9: error: type mismatch; | ||
found : Double | ||
required: Int | ||
f"$height%4d" | ||
^ | ||
``` | ||
{% endtab %} | ||
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{% tab 'Scala 3' for=f-interpolator-error %} | ||
```scala | ||
val height: Double = 1.9d | ||
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scala> f"$height%4d" | ||
-- Error: ---------------------------------------------------------------------- | ||
1 |f"$height%4d" | ||
| ^^^^^^ | ||
| Found: (height : Double), Required: Int, Long, Byte, Short, BigInt | ||
1 error found | ||
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``` | ||
{% endtab %} | ||
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{% endtabs %} | ||
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The `f` interpolator makes use of the string format utilities available from Java. The formats allowed after the `%` character are outlined in the | ||
[Formatter javadoc](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Formatter.html#detail). If there is no `%` character after a variable | ||
definition a formatter of `%s` (`String`) is assumed. | ||
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### The `raw` Interpolator | ||
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The raw interpolator is similar to the `s` interpolator except that it performs no escaping of literals within the string. Here's an example processed string: | ||
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{% tabs example-7 %} | ||
{% tab 'Scala 2 and 3' for=example-7 %} | ||
scala> s"a\nb" | ||
res0: String = | ||
a | ||
b | ||
{% endtab %} | ||
{% endtabs %} | ||
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Here the `s` string interpolator replaced the characters `\n` with a return character. The `raw` interpolator will not do that. | ||
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{% tabs example-8 %} | ||
{% tab 'Scala 2 and 3' for=example-8 %} | ||
scala> raw"a\nb" | ||
res1: String = a\nb | ||
{% endtab %} | ||
{% endtabs %} | ||
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The raw interpolator is useful when you want to avoid having expressions like `\n` turn into a return character. | ||
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In addition to the three default string interpolators, users can define their own. | ||
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## Advanced Usage | ||
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In Scala, all processed string literals are simple code transformations. Anytime the compiler encounters a string literal of the form: | ||
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{% tabs example-9 %} | ||
{% tab 'Scala 2 and 3' for=example-9 %} | ||
id"string content" | ||
{% endtab %} | ||
{% endtabs %} | ||
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it transforms it into a method call (`id`) on an instance of [StringContext](https://www.scala-lang.org/api/current/scala/StringContext.html). | ||
This method can also be available on implicit scope. | ||
To define our own string interpolation, we need to create an implicit class (Scala 2) or an `extension` method (Scala 3) that adds a new method to `StringContext`. | ||
Here's an example: | ||
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{% tabs json-definition-and-usage class=tabs-scala-version %} | ||
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{% tab 'Scala 2' for=json-definition-and-usage %} | ||
```scala | ||
// Note: We extends AnyVal to prevent runtime instantiation. See | ||
// value class guide for more info. | ||
implicit class JsonHelper(val sc: StringContext) extends AnyVal { | ||
def json(args: Any*): JSONObject = sys.error("TODO - IMPLEMENT") | ||
} | ||
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def giveMeSomeJson(x: JSONObject): Unit = ... | ||
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giveMeSomeJson(json"{ name: $name, id: $id }") | ||
``` | ||
{% endtab %} | ||
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{% tab 'Scala 3' for=json-definition-and-usage %} | ||
```scala | ||
extension (sc: StringContext) | ||
def json(args: Any*): JSONObject = sys.error("TODO - IMPLEMENT") | ||
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def giveMeSomeJson(x: JSONObject): Unit = ... | ||
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giveMeSomeJson(json"{ name: $name, id: $id }") | ||
``` | ||
{% endtab %} | ||
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{% endtabs %} | ||
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In this example, we're attempting to create a JSON literal syntax using string interpolation. The `JsonHelper` implicit class must be in scope to use this syntax, and the json method would need a complete implementation. However, the result of such a formatted string literal would not be a string, but a `JSONObject`. | ||
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When the compiler encounters the literal `json"{ name: $name, id: $id }"` it rewrites it to the following expression: | ||
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{% tabs extension-desugaring class=tabs-scala-version %} | ||
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{% tab 'Scala 2' for=extension-desugaring %} | ||
```scala | ||
new StringContext("{ name: ", ", id: ", " }").json(name, id) | ||
``` | ||
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The implicit class is then used to rewrite it to the following: | ||
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```scala | ||
new JsonHelper(new StringContext("{ name: ", ", id: ", " }")).json(name, id) | ||
``` | ||
{% endtab %} | ||
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{% tab 'Scala 3' for=extension-desugaring %} | ||
```scala | ||
StringContext("{ name: ", ", id: ", " }").json(name, id) | ||
``` | ||
{% endtab %} | ||
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{% endtabs %} | ||
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So, the `json` method has access to the raw pieces of strings and each expression as a value. A simplified (buggy) implementation of this method could be: | ||
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{% tabs json-fake-implementation class=tabs-scala-version %} | ||
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{% tab 'Scala 2' for=json-fake-implementation %} | ||
```scala | ||
implicit class JsonHelper(val sc: StringContext) extends AnyVal { | ||
def json(args: Any*): JSONObject = { | ||
val strings = sc.parts.iterator | ||
val expressions = args.iterator | ||
val buf = new StringBuilder(strings.next()) | ||
while (strings.hasNext) { | ||
buf.append(expressions.next()) | ||
buf.append(strings.next()) | ||
} | ||
parseJson(buf) | ||
} | ||
} | ||
``` | ||
{% endtab %} | ||
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{% tab 'Scala 3' for=json-fake-implementation %} | ||
```scala | ||
extension (sc: StringContext) | ||
def json(args: Any*): JSONObject = | ||
val strings = sc.parts.iterator | ||
val expressions = args.iterator | ||
val buf = new StringBuilder(strings.next()) | ||
while strings.hasNext do | ||
buf.append(expressions.next()) | ||
buf.append(strings.next()) | ||
parseJson(buf) | ||
``` | ||
{% endtab %} | ||
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{% endtabs %} | ||
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Each of the string portions of the processed string are exposed in the `StringContext`'s `parts` member. Each of the expression values is passed into the `json` method's `args` parameter. The `json` method takes this and generates a big string which it then parses into JSON. A more sophisticated implementation could avoid having to generate this string and simply construct the JSON directly from the raw strings and expression values. |
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--- | ||
title: Improving Your Workflow | ||
description: This page describes improving efficiency of debugging the Scala 3 compiler. | ||
redirect_to: https://dotty.epfl.ch/docs/contributing/workflow/efficiency.html | ||
--- |
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