For functions in the same scope as the higher-order function, you can use the same ::-prefix syntax.

In this case, both lessThanZero() and hasMatch() are member functions of Thingy, so matchify() can pass ::lessThanZero to hasMatch().

You can learn more about this in:
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any() any() and Short Circuits Chaining Higher-Order Functions first() first() Sans Lambda firstOrNull() firstOrNull() and the Elvis Operator fold() Function Reference Function Reference for Member Functions Function References and Receivers Function Types Function Types and Direct Invocation Function Types and Parameters Functional Interfaces joinToString() Type Aliases
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