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Date: Sat, 16 Apr 2016 01:16:30 +0800
Message-ID: <CAKSD0w4rzfFOjL5mMNa46jXH-hAwJcThVDnOQNxPaV77+QBgng@mail.gmail.com>
To: Andrea Faulds <ajf@ajf.me>
Cc: "internals@lists.php.net" <internals@lists.php.net>
Content-Type: multipart/alternative; boundary=001a113de8e4fe5e450530892b15
Subject: Re: [PHP-DEV] Improving PHP's type system
From: cornelius.howl@gmail.com (Lin Yo-An)

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Andrea Faulds <ajf@ajf.me> =E6=96=BC 2016=E5=B9=B44=E6=9C=8815=E6=97=A5 =E6=
=98=9F=E6=9C=9F=E4=BA=94=E5=AF=AB=E9=81=93=EF=BC=9A
>
>
> This is something that particularly concerns me about union types, in tha=
t
> they reduce type safety. If you have a union type of Foo|Bar for some
> variable, then the set of methods you can call on that variable is actual=
ly
> the intersection, not the union, of the set of methods you can call on Fo=
o
> and Bar. Which, unless those two classes share some interface, is probabl=
y
> an empty set. So there's nothing you can actually do safely with it witho=
ut
> doing checks within the body of the function, and if you're doing that,
> then why do we have a type declaration? It's only barely more useful than
> omitting a type declaration at all; type declarations are supposed to
> prevent you needing to check. On the other hand, if the two classes share
> some methods, then either there's an interface you can already use here, =
or
> you can create one. Either way, you don't need a union type.

+1 I agree. Language like Golang use interface to intersect the method
calls, I think that's better.



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