What do _._1 and _++_ mean in Scala (two separate operations)?

Question

My interpretation of _._1 is:

_ = wildcard parameter _1 = first parameter in method parameter list But when used together with . what does it signify?

This is how its used :

.toList.sortWith(_._1 < _._1)

For this statement:

_++_

I'm lost. Is it concatenation two wildcard parameters somehow? This is how its used:

.reduce(_++_)

I would be particularly interested if they above code could be made more verbose and remove any implicits, just so I can understand it better?

Answer 1

_._1 calls the method _1 on the wildcard parameter _, which gets the first element of a tuple. Thus, sortWith(_._1 < _._1) sorts the list of tuple by their first element.

_++_ calls the method ++ on the first wildcard parameter with the second parameter as an argument. ++ does concatenation for sequences. Thus .reduce(_++_) concatenates a list of sequences together. Usually you can use flatten for that.

Answer 2

_1 is a method name. Specifically tuples have a method named _1, which returns the first element of the tuple. So _._1 < _._1 means "call the _1 method on both arguments and check whether the first is less than the second".

And yes, _++_ concatenates both arguments (assuming the first argument has a ++ method that performs concatenation).