| layout | title | partof | num | next-page | previous-page | redirect_from |
|---|---|---|---|---|---|---|
tour |
Implicit Conversions |
scala-tour |
29 |
polymorphic-methods |
implicit-parameters |
/tutorials/tour/implicit-conversions.html |
Implicit conversions are a powerful Scala feature that enable two common use cases:
- allow users to supply an argument of one type, as if it were another type, to avoid boilerplate.
- in Scala 2, to provide additional members to closed classes (replaced by [extension methods][exts] in Scala 3).
{% tabs implicit-conversion-defn class=tabs-scala-version %}
{% tab 'Scala 2' %}
In Scala 2, an implicit conversion from type S to type T is defined by either an [implicit class]({% link _overviews/core/implicit-classes.md %}) T that has a single parameter of type S, an [implicit value]({% link _tour/implicit-parameters.md %}) which has function type S => T, or by an implicit method convertible to a value of that type.
{% endtab %}
{% tab 'Scala 3' %}
In Scala 3, an implicit conversion from type S to type T is defined by a [given instance]({% link _tour/implicit-parameters.md %}) which has type scala.Conversion[S, T]. For compatibility with Scala 2, they can also be defined by an implicit method (read more in the Scala 2 tab).
{% endtab %}
{% endtabs %}
Implicit conversions are applied in two situations:
- If an expression
eis of typeS, andSdoes not conform to the expression's expected typeT. - In a selection
e.mwitheof typeS, if the selectormdoes not denote a member ofS.
In the first case, a conversion c is searched for, which is applicable to e and whose result type conforms to T.
An example is to pass a scala.Int, e.g. x, to a method that expects scala.Long. In this case, the implicit conversion Int.int2long(x) is inserted.
In the second case, a conversion c is searched for, which is applicable to e and whose result contains a member named m.
An example is to compare two strings "foo" < "bar". In this case, String has no member <, so the implicit conversion Predef.augmentString("foo") < "bar" is inserted. (scala.Predef is automatically imported into all Scala programs.)
Further reading: [Implicit Conversions (in the Scala book)]({% link _overviews/scala3-book/ca-implicit-conversions.md %}).
[exts]: {% link _overviews/scala3-book/ca-extension-methods.md %}