Can you explain this?

Dicebot via Digitalmars-d-learn digitalmars-d-learn at puremagic.com
Wed Aug 20 13:18:14 PDT 2014


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 :)


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