Elm: how does Mailbox work? -

say create mailbox using mailbox = signal.mailbox action, type action = blah, later can send actions mailbox using signal.send mailbox.address blah, allows me listen on mailbox.signal, how so? mean, after all, type mailbox alias of {address : signal.address action, signal : signal.signal action}, because that, in elm, there 1 signal type, in above scenario, don't have tell elm bind mailbox.signal mailbox.address, elm figure out because of one-to-one correspondence between address , signal of type?

the ability mailbox put events send signal entirely based on "magic", is, it's implemented natively (in javascript) , not implement yourself. that's why it's built-in in standard libraries.

creating mailbox imperative, effectful action (shhh, don't tell anyone). if use:

mailbox1 = signal.mailbox blah mailbox2 = signal.mailbox blah 

those 2 mailboxes distinct. send message mailbox2.address result in message on mailbox2.signal not mailbox1.signal. breaks referential transparency, bad, it's not breaking everything. (this may go far tangent, because need signal output , can't have signal (signal something), in practice unmanaged effect of mailbox creation not messing things up). "leak" fixed @ point in future. there proposal big changes staged in different versions of language.