• Chris Katko (13/13) Feb 17 2016 Hello. I'm almost brand-new to the D language and still absorbing
• Jonathan M Davis via Digitalmars-d-learn (7/20) Feb 18 2016 D has no such feature, and very little D code is going to use return cod...
• tsbockman (41/54) Feb 18 2016 Off hand, I see two reasonable ways to do this:
• Marc =?UTF-8?B?U2Now7x0eg==?= (48/58) Feb 18 2016 As Jonathan said, there's no such built-in feature, and exception
• Chris Katko (3/10) Feb 21 2016 Thank you all for your insight. I think I have all the

Chris Katko <ckatko gmail.com> writes:

Hello. I'm almost brand-new to the D language and still absorbing 
things.

I'm wondering if it's possible to fire off a compile-time (or 
worst case, a run-time) warning or error if a function is called, 
but the return value is not checked.

I'm not trying to enforce whether someone actually deciphers the 
value's meaning correctly. I just want to enforce that somewhere, 
a variable or expression is receiving the return value of a 
particular function.

Any ideas?

I imagine I could use a compiler flag to warn, but that's a 
global setting. I'm looking more for a specified subset of 
functions.

Jonathan M Davis via Digitalmars-d-learn writes:

On Thursday, February 18, 2016 07:21:05 Chris Katko via Digitalmars-d-learn
wrote:
 Hello. I'm almost brand-new to the D language and still absorbing
 things.

 I'm wondering if it's possible to fire off a compile-time (or
 worst case, a run-time) warning or error if a function is called,
 but the return value is not checked.

 I'm not trying to enforce whether someone actually deciphers the
 value's meaning correctly. I just want to enforce that somewhere,
 a variable or expression is receiving the return value of a
 particular function.

 Any ideas?

 I imagine I could use a compiler flag to warn, but that's a
 global setting. I'm looking more for a specified subset of
 functions.

D has no such feature, and very little D code is going to use return codes.
D has exceptions, and throwing an exception is the normal way to indicate
that there was a problem. You certainly _can_ choose to use error codes if
you'd like, but it's not normal practice, and the language does nothing
extra to support it.

- Jonathan M Davis

tsbockman <thomas.bockman gmail.com> writes:

On Thursday, 18 February 2016 at 07:21:05 UTC, Chris Katko wrote:
 Hello. I'm almost brand-new to the D language and still 
 absorbing things.

 I'm wondering if it's possible to fire off a compile-time (or 
 worst case, a run-time) warning or error if a function is 
 called, but the return value is not checked.

 I'm not trying to enforce whether someone actually deciphers 
 the value's meaning correctly. I just want to enforce that 
 somewhere, a variable or expression is receiving the return 
 value of a particular function.

 Any ideas?

 I imagine I could use a compiler flag to warn, but that's a 
 global setting. I'm looking more for a specified subset of 
 functions.

Off hand, I see two reasonable ways to do this:

1) Instead of actually returning the value, require the caller to 
pass the destination variable as a `ref` or `out` parameter.

Disadvantages: This could be a bit slower than using actual 
return values, and it would change the syntax.

2) Wrap the return value in a struct like this:

struct MustUse(T) {
pure: nothrow:

private:
     T payload;
     bool used;

public:
      property ref T use()  nogc {
         used = true;
         return payload;
     }
     alias use this;

     this(T payload)  nogc {
         this.payload = payload;
         this.used = false;
     }	
     ~this() {
         if(!used)
             throw new Error(T.stringof ~ " return value was not 
used.");
     }

}

// For convenience, use this function to infer T from the type of 
paylaod.
MustUse!T mustUse(T)(T payload) {
     return MustUse!T(payload); }

Example on DPaste: http://dpaste.dzfl.pl/8ba6ebf05f32

With inlining and optimizations, even the above generic 
implementation might be faster than passing by ref/out. A 
specialization for non-null pointer values could almost certainly 
be faster.

Disadvantages: On the other hand, it might be slower. The only 
way to know for sure, is to write some non-trivial examples and 
benchmark. Regardless, the fact that it signals the error at 
runtime instead of compile time could be annoying.

Marc =?UTF-8?B?U2Now7x0eg==?= <schuetzm gmx.net> writes:

On Thursday, 18 February 2016 at 07:21:05 UTC, Chris Katko wrote:
 Hello. I'm almost brand-new to the D language and still 
 absorbing things.

 I'm wondering if it's possible to fire off a compile-time (or 
 worst case, a run-time) warning or error if a function is 
 called, but the return value is not checked.

 I'm not trying to enforce whether someone actually deciphers 
 the value's meaning correctly. I just want to enforce that 
 somewhere, a variable or expression is receiving the return 
 value of a particular function.

 Any ideas?

As Jonathan said, there's no such built-in feature, and exception 
are preferred over return codes. However, you can implement such 
a check at run time:

struct ForceCheck(T) {
     private T payload;
     private bool checked = false;
      disable this();
      disable this(this);
     this()(auto ref T payload) { this.payload = payload; }
     ref T get() {
         this.checked = true;
         return payload;
     }
     alias get this;
     void ignore() {
         this.checked = true;
     }
     ~this() {
         assert(this.checked, "you forgot to check the return 
value");
     }
}

auto forceCheck(T)(auto ref T value) {
     return ForceCheck!T(value);
}

auto foo() {
     return forceCheck(42);
}

void main() {
     {
         auto a = foo();
         if(a != 42) { }         // stored values
     }
     {
         if(foo() != 42) { }     // direct access
     }
     {
         foo().ignore();         // explicitly ignore return value
     }
     {
         auto b = foo();         // this one asserts
         foo();                  // as does this one
     }
}

I guess it's reasonably performant; it could be optimized further 
by only adding the `checked` member if assertions are enabled 
(using `version(assert)`).

Chris Katko <ckatko gmail.com> writes:

On Thursday, 18 February 2016 at 10:46:03 UTC, Marc Schütz wrote:
 On Thursday, 18 February 2016 at 07:21:05 UTC, Chris Katko 
 wrote:
[...]

 As Jonathan said, there's no such built-in feature, and 
 exception are preferred over return codes. However, you can 
 implement such a check at run time:

 [...]

Thank you all for your insight. I think I have all the 
information I need.