Scala lambda with multiple =>

Question

I am learning Scala and see the follow code in a tutorial

case class Email (
    subject: String,
    text: String,
    sender: String,
    recipient: String)

type EmailFilter = Email => Boolean
def newMailsForUser(mails: Seq[Email], f: EmailFilter) = mails.filter(f)

val notSentByAnyOf: Set[String] => EmailFilter = 
    senders => email => !senders.contains(email.sender)

I understand that notSentByAnyOf is a function type that has the signature type Set[String] => EmailFilter.

However, I am not sure what senders => email => !senders.contains(email.sender) means.

Can I please get some explanation?

Thank you very much for your help!

Kraylog · Accepted Answer · 2018-01-08 07:04:29Z

1

The answer is in what the type EmailFilter is.

An EmailFilter is defined as:

type EmailFilter = Email => Boolean

Which means it's a function from Email to Boolean. Now let's look at notSentByAnyOf:

senders => ...

is a function that given senders returns something. We know from the signature that something needs to be an EmailFilter. Since we know EmailFilter is really a function Email => Boolean, then that is what should be on the right side of the arrow¹:

senders => (email => !senders.contains(email.sender))
           |________________________________________|
                               |
                            EmailFilter

So, notSentByAnyOf, is a function that given a set of String, returns a function Email => Boolean.

_{[1] The extra parenthesis in the example are just for emphasis. As in the original code, they can be omitted.}

edited Jan 8, 2018 at 7:04

answered Jan 8, 2018 at 6:58

Kraylog

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