Checking function parameters in Phobos

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On Wednesday, 20 November 2013 at 18:30:58 UTC, Dmitry Olshansky 
wrote:
> 20-Nov-2013 22:01, Simen Kjærås пишет:
>> On 20.11.2013 18:45, Simen Kjærås wrote:
> [snip]
>>> May I suggest:
>>>
>>> struct Validated(alias fn, T) {
>>>     private T value;
>>>     @property inout
>>>     T get() {
>>>         return value;
>>>     }
>>
>> Uh-hm. Add this:
>>        alias get this;
>>
>
> And it decays to the naked type in a blink of an eye. And some 
> function down the road will do the validation again...
>
>>> }
>>>
>>> Validated!(fn, T) validate(alias fn, T)(T value) {
>>>     Validated!(fn, T) result;
>>>     fn(value);
>>>     result.value = value;
>>>     return result;
>>> }
>>>
>>> void 
>>> functionThatTakesSanitizedFileNames(Validated!(sanitizeFileName,
>>> string) path) {
>>>    // Do stuff
>>> }

Yes. It is very important not to allow direct access to the 
underlying value. This is important for ensuring that it is not 
put in an invalid state. This is a mistake that was made with 
std.typecons.Nullable, making it useless for anything other than 
giving a non-nullable type a null state (which, in fairness, is 
probably all that it was originally intended for).