On Tuesday, 10 July 2018 at 21:08:32 UTC, Cym13 wrote:
First of all I must point that I would very much like to have
seen a code actually producing an error in that article.
Contrary to what is hinted just taking the struct and putting
using it with Nullable or format() caused no error for me and
worked as expected.
To reproduce the format issue, try to print the struct with
writefln!"%s"(MyDomainType()).
To reproduce the Nullable issue, you need to slightly modify the
struct. In Phobos, Nullable will (due to an abundance of caution)
refuse to initialize the struct if the default constructor is
disabled; also you need a destructor. However, for this it is
enough to use any type that has a destructor, so that an implicit
struct destructor is generated. For verbosity, I'll write it out:
struct MyDomainData {
string username;
this(string username) @safe
in(!username.empty) // only non-empty usernames please!
do { this.username = username; }
// let's formalise the restriction.
invariant { assert(!username.empty); }
string toString() { return null; }
~this() @safe { }
}
Then just stick it in a Nullable. No explicit .init needed.
That said, I may be missing something obvious but what prevents
you from overloading the init field?
struct MyDomainData {
string username;
@disable this(); // don't make a MyDomainData() by
accident!
this(string username)
in(!username.empty) // only non-empty usernames please!
do { this.username = username; }
// let's formalise the restriction.
invariant { assert(!username.empty); }
string toString() { ... }
static @property MyDomainData init() {
return MyDomainData("uninitialized");
}
...
}
auto test = MyDomainData.init; // There, no error
Of course that value means nothing but .init isn't meant to
actually mean something anyway, it's just a valid value and
that's what that init is proposing, so it shouldn't cause any
more bugs than empty .init in a normal case.
That would work, it's just a really horrible hack and I hate it.
We're constructing a fictitious domain value that passes our
invariants while having zero correspondence to the real world,
*just to pass our invariants*. It's an obvious sign of a language
issue.
