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Date: Thu, 9 Mar 2017 20:09:04 +0000
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Subject: Re: [PHP-DEV] [Discussion] is_string(), string type and objects
 implementing __toString()
From: rowan.collins@gmail.com (Rowan Collins)

On 08/03/2017 23:32, Andrey Andreev wrote:
> For example, a Cookie object may have the cookie attributes (domain,
> path, etc.) as value objects, but they can easily be created from raw
> strings, while other types would be ambiguous.
> A similar effect could be desirable for HTTP headers.

OK, now we have some concrete examples, thanks. I would say that in 
these cases, what you actually want is a much tighter contract: not just 
"can be converted to string", but "intended to be used in this context".

It fits with what I was saying before about "if you can't name it, maybe 
it isn't the right abstraction". In this case, you want to accept some 
particular value objects but not, say, Exceptions - which are as useless 
for your purpose as an integer, even though (string)$foo would work fine 
on both.

So you want objects that have promised to behave appropriately for this 
context; that could be as simple as:

interface HTTPHeaderObject {
     public function __toString();
}

...with appropriate documentation that objects declaring that they 
implement this interface promise to behave in a specific way when cast 
to string. This is much clearer than detecting __toString(), which only 
promises "I can be cast to string" - the same promise that is made by 
all scalar values.


In order to accept this or a string in a type hint, you need Union Types:

function setHeaderValue(string|HTTPHeaderObject $value) { ...


For broader use, we could perhaps have named unions:

type HTTPHeaderType = union(string, HTTPHeaderObject);
if ( $value instanceOf HTTPHeaderType ) { ...


Meanwhile, of course, you can just use a boring old user-defined function:

function is_valid_http_header_value($value) {
     return is_string($value) || $value instanceOf HTTPHeaderObject;
}


Regards,

-- 
Rowan Collins
[IMSoP]