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Date: Fri, 15 Apr 2016 13:04:21 -0500
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Subject: Re: [PHP-DEV] Improving PHP's type system
From: larry@garfieldtech.com (Larry Garfield)

On 4/15/16 12:16 PM, Lin Yo-An wrote:
> Andrea Faulds <ajf@ajf.me> 於 2016年4月15日 星期五寫道：
>>
>> This is something that particularly concerns me about union types, in that
>> 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 actually
>> the intersection, not the union, of the set of methods you can call on Foo
>> and Bar. Which, unless those two classes share some interface, is probably
>> an empty set. So there's nothing you can actually do safely with it without
>> 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.

Go's interfaces are implicit rather than explicit. That's what makes 
micro-interfaces like that (which are often single-method) feasible.  
Anthony Ferrara proposed "weak interfaces" a few years ago but it was 
rejected (or didn't make it to a vote; I don't recall which).

Either way, Foo|Bar is not the main use case for union types.  See my 
earlier email.

-- 
--Larry Garfield