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I am currently trying to extend Kotlins String class with a method in a file StringExt.kt

fun String.removeNonAlphanumeric(s: String) = s.replace([^a-ZA-Z0-9].Regex(), "")

But Kotlin in not allowing me to use this method in a lambda:

s.split("\\s+".Regex())
.map(String::removeNonAlphanumeric)
.toList()

The error is:

Required: (TypeVariable(T)) -> TypeVariable(R)
Found: KFunction2<String,String,String>

What confuses me about this is that Kotlins Strings.kt has very similar methods and I can call them by reference without Intellij raising this kind of issue. Any advice is appreciated.

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2 Answers 2

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This is because you have declared an extension function that accepts an additional parameter and should be used as s.replace("abc").

I think what you meant is the following:

fun String.removeNonAlphanumeric(): String = this.replace("[^a-ZA-Z0-9]".toRegex(), "")

This declaration doesn't have an extra parameter and uses this to refer to the String instance it is called on.

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1 Comment

Thank you! This is absolutely correct :) Calling it as the below works as well: fun String.removeNonAlphanumeric() = replace("[^a-ZA-Z0-9]".toRegex(), "")
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I thing this is because a lambda is an anonymous function and dose not access to the scope of a extension file.

Check this link maybe contains some useful information: https://kotlinlang.org/docs/reference/extensions.html

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