www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - static if - is the 'static' really needed?

reply "comco" <void.unsigned gmail.com> writes:
Imagine a world in which a simple 'if' has the semantics of a 
static if, if the condition is evaluable at CT. Is this a world 
you would rather live in?

template Fac(int i) {
if (i == 0) { // static if; doesn't introduce scope
enum Fac = 1;
} else {
enum Fac = i * Fac!(i-1);
}
}

// If the condition is not evaluable at CT, the ordinary runtime 
if semantics (introducing scope) are used.

Me:
pros: simpler syntax
cons: harder to reason about; I recall Andrei's talk about the 
static if proposal to C++: "we don't need _static else_" -- why 
do we even need 'static' in 'static if' by this reasoning?
Dec 13 2013
next sibling parent reply "Nicolas Sicard" <dransic gmail.com> writes:
On Friday, 13 December 2013 at 12:10:02 UTC, comco wrote:

 Imagine a world in which a simple 'if' has the semantics of a 
 static if, if the condition is evaluable at CT. Is this a world 
 you would rather live in?

 template Fac(int i) {
 if (i == 0) { // static if; doesn't introduce scope
 enum Fac = 1;
 } else {
 enum Fac = i * Fac!(i-1);
 }
 }

 // If the condition is not evaluable at CT, the ordinary 
 runtime if semantics (introducing scope) are used.

 Me:
 pros: simpler syntax
 cons: harder to reason about; I recall Andrei's talk about the 
 static if proposal to C++: "we don't need _static else_" -- why 
 do we even need 'static' in 'static if' by this reasoning?


What would happen when the condition is sometimes evaluable at 
compile time and sometimes not?

void foo(alias a)() {
     /* static */ if (a)
         int x = 1;
     else
         int x = 42;
     doSomethingWith(x);
}
Dec 13 2013
parent "comco" <void.unsigned gmail.com> writes:
On Friday, 13 December 2013 at 12:50:01 UTC, Nicolas Sicard wrote:

 On Friday, 13 December 2013 at 12:10:02 UTC, comco wrote:

 Imagine a world in which a simple 'if' has the semantics of a 
 static if, if the condition is evaluable at CT. Is this a 
 world you would rather live in?

 template Fac(int i) {
 if (i == 0) { // static if; doesn't introduce scope
 enum Fac = 1;
 } else {
 enum Fac = i * Fac!(i-1);
 }
 }

 // If the condition is not evaluable at CT, the ordinary 
 runtime if semantics (introducing scope) are used.

 Me:
 pros: simpler syntax
 cons: harder to reason about; I recall Andrei's talk about the 
 static if proposal to C++: "we don't need _static else_" -- 
 why do we even need 'static' in 'static if' by this reasoning?


 What would happen when the condition is sometimes evaluable at 
 compile time and sometimes not?

 void foo(alias a)() {
     /* static */ if (a)
         int x = 1;
     else
         int x = 42;
     doSomethingWith(x);
 }


Multiple versions of the foo function - depending on the caller - 
as it is with templates? Static ifs already can produce a 
combinatorial explosion of different method implementations :)

At least in the places where declarations are expected and normal 
if doesn't make sense: like template/class/struct body, if can be 
implicitly static. Now it is implicitly static right after 
function / class / template declarations.
class A(T) {
if (is(T : B))
B b;
else
T b;
...
}
Dec 13 2013
prev sibling next sibling parent "Jesse Phillips" <Jesse.K.Phillips+D gmail.com> writes:
On Friday, 13 December 2013 at 12:10:02 UTC, comco wrote:

 Imagine a world in which a simple 'if' has the semantics of a 
 static if, if the condition is evaluable at CT. Is this a world 
 you would rather live in?

 template Fac(int i) {
 if (i == 0) { // static if; doesn't introduce scope
 enum Fac = 1;
 } else {
 enum Fac = i * Fac!(i-1);
 }
 }


I would not rather live in it. I would rather live in:

     static foreach(...) // but that hasn't happened yet.
Dec 13 2013
prev sibling next sibling parent "Chris Cain" <clcain uncg.edu> writes:
On Friday, 13 December 2013 at 12:10:02 UTC, comco wrote:

 Imagine a world in which a simple 'if' has the semantics of a 
 static if, if the condition is evaluable at CT. Is this a world 
 you would rather live in?


They are fundamentally two different things. Eliding the 
difference is not a good idea. I'm with Jesse, I wish we had a 
"static foreach" because it's also a significant difference as 
well.


 "we don't need _static else_" -- why do we even need 'static' 
 in 'static if' by this reasoning?


Not exactly sure what the context of this is, but I suspect he 
was saying "we don't need _static else_" because we don't need it 
to resolve ambiguities in the AST (which is true). OTOH we would 
need 'static' in 'static if' to resolve the ambiguities in the 
semantics.
Dec 13 2013
prev sibling parent Parke <parke.nexus gmail.com> writes:
On Fri, Dec 13, 2013 at 4:10 AM, comco <void.unsigned gmail.com> wrote:

 Imagine a world in which a simple 'if' has the semantics of a static if, if
 the condition is evaluable at CT. Is this a world you would rather live in?


Even so, I would still want static if, so that I would get a compile
time error if I mistaken thought code could be evaluated at CT.
Dec 14 2013