On Wednesday, 20 August 2014 at 20:18:15 UTC, Dicebot wrote:
On Wednesday, 20 August 2014 at 20:01:05 UTC, Colin wrote:
I see 3 distinct parts playing a role in my confusion:
A) The 'is' keyword. What does it do when you have
is(expression);
http://dlang.org/expression.html#IsExpression
It is a tool for type checking. It has many options but plain
`is(T)` checks if `T` is a valid type and returns `true` if it
is. `void` is considered a valid type.
B) typeof( expression ); whats this doing? Particularly when
the expression its acting on is a closure that returns
nothing? (at least as far as I can see)
typeof(() {}) evaluates to void:
static assert(is(typeof(() {}) == void));
However, if any compilation errors happen inside the delegate,
it evaluates to special invalid type which yield `false` when
supplied to `is` expression.
Thus `is(typeof(expr))` is a way to express a concept "if
`expr` compiles". Delegate is necessary to test statements
because `typeof` only accepts expressions.
C) The closure expression:
(inout int = 0) {
// Check to see if I can do InputRangy stuff...
}
Why is there a need for that inout int = 0 clause at the start
of it?
Now this is a really weird one. This is necessary for input
ranges with `inout` functions to fit the trait because `inout`
variables can be declared only inside `inout` functions. See
this bug report for details :
https://issues.dlang.org/show_bug.cgi?id=7824
It is one of surprising feature inter-operation cases you don't
expect when designing features :)
Thanks all, that explains it.
It is weird looking code alright, but I guess it has its uses.
I think I prefer the __traits(compiles,...) construct, as it is a
bit more obvious as to what I'm checking. So will use that I
suspect.
Cheers!
