• sighoya (13/13) Mar 24 2019 Why
• =?UTF-8?B?UsOpbXkgTW91w6t6YQ==?= (4/18) Mar 24 2019 Because D uses if statements, no if expressions.
• sighoya (3/26) Mar 24 2019 Hmm..., sounds like bad news. Is there any mixin technology for
• Alex (6/8) Mar 24 2019 There was a recent thread on this.
• Adam D. Ruppe (21/23) Mar 24 2019 mixin() works for statements too.
• sighoya (9/12) Mar 24 2019 And because AST Nodes aren't expressions means you can't pass
• Paul Backus (12/26) Mar 24 2019 You can't return the result of a string mixin from a function.
• sighoya (2/13) Mar 24 2019 Thanks for mentioning this, this was where I looking for.

sighoya <sighoya gmail.com> writes:

Why

auto GenIf()()
{
     return mixin("if(true) { return true;} else {return false;}");
}

public bool testFunction2()
{
     GenIf!();
}


gives me:

onlineapp.d-mixin-3(3): Error: expression expected, not if
onlineapp.d(8): Error: template instance `onlineapp.GenIf!()` 
error instantiating

=?UTF-8?B?UsOpbXkgTW91w6t6YQ==?= <remy.moueza gmail.com> writes:

On Sunday, 24 March 2019 at 16:18:49 UTC, sighoya wrote:
 Why

 auto GenIf()()
 {
     return mixin("if(true) { return true;} else {return 
 false;}");
 }

 public bool testFunction2()
 {
     GenIf!();
 }


 gives me:

 onlineapp.d-mixin-3(3): Error: expression expected, not if
 onlineapp.d(8): Error: template instance `onlineapp.GenIf!()` 
 error instantiating

Because D uses if statements, no if expressions.
The equivalent of an if expression is the ternary operator:
     bool-condition ? if-true : if-false;

sighoya <sighoya gmail.com> writes:

On Sunday, 24 March 2019 at 16:57:25 UTC, Rémy Mouëza wrote:
 On Sunday, 24 March 2019 at 16:18:49 UTC, sighoya wrote:
 Why

 auto GenIf()()
 {
     return mixin("if(true) { return true;} else {return 
 false;}");
 }

 public bool testFunction2()
 {
     GenIf!();
 }


 gives me:

 onlineapp.d-mixin-3(3): Error: expression expected, not if
 onlineapp.d(8): Error: template instance `onlineapp.GenIf!()` 
 error instantiating

 Because D uses if statements, no if expressions.
 The equivalent of an if expression is the ternary operator:
     bool-condition ? if-true : if-false;

Hmm..., sounds like bad news. Is there any mixin technology for 
statements?

Alex <sascha.orlov gmail.com> writes:

On Sunday, 24 March 2019 at 18:18:39 UTC, sighoya wrote:
 Hmm..., sounds like bad news. Is there any mixin technology for 
 statements?

There was a recent thread on this.

https://forum.dlang.org/post/mailman.6499.1547823247.29801.digitalmars-d puremagic.com

However, do you have a clear use case, where statement mixin 
would be considerably more profitable than the replacements 
suggested by Remy and Paul?

Adam D. Ruppe <destructionator gmail.com> writes:

On Sunday, 24 March 2019 at 18:18:39 UTC, sighoya wrote:
 Hmm..., sounds like bad news. Is there any mixin technology for 
 statements?

mixin() works for statements too.

It is the *context* though. If mixin() appears where the compiler 
expects an expression, it must give an expression. Ditto for 
statements (and for declarations btw). Consider

int main() {
    // compiler expecting a statement here, so
    mixin("if(true) {}"); // allowed

    // but here it is expecting an expression
    return mixin("some_expression");
}

This is also the reason why mixin sometimes requires ; and 
sometimes requires a LACK of ;

int main() {
    int a;
    mixin("a = 10;"); // the ; inside the mixin is required there
    return mixin("a"); // but mixin("a;"); there would be an error
}


Think of mixin() not as pasting code per se, but pasting an AST 
node inside the compiler's data structures. It must be a complete 
node of that tree that can be substituted in.

sighoya <sighoya gmail.com> writes:

On Sunday, 24 March 2019 at 18:59:45 UTC, Adam D. Ruppe wrote:

 Think of mixin() not as pasting code per se, but pasting an AST 
 node inside the compiler's data structures. It must be a 
 complete node of that tree that can be substituted in.

And because AST Nodes aren't expressions means you can't pass 
them. This makes sense.

I would really like to use normal mixin templates for these 
things, but why I can't insert only declarations with mixin 
templates?

Sure, I can use q{} for better syntax highlighting in string 
mixins but I can't expand args inside them which I can however 
with mixin templates.

Paul Backus <snarwin gmail.com> writes:

On Sunday, 24 March 2019 at 16:18:49 UTC, sighoya wrote:
 Why

 auto GenIf()()
 {
     return mixin("if(true) { return true;} else {return 
 false;}");
 }

 public bool testFunction2()
 {
     GenIf!();
 }


 gives me:

 onlineapp.d-mixin-3(3): Error: expression expected, not if
 onlineapp.d(8): Error: template instance `onlineapp.GenIf!()` 
 error instantiating

You can't return the result of a string mixin from a function. 
Instead, return a string from your `GenIf` function, and mix it 
in at the call site:

string GenIf()
{
     return "if (true) { return true; } else { return false; }";
}

bool testFunction()
{
     mixin(GenIf());
}

sighoya <sighoya gmail.com> writes:

On Sunday, 24 March 2019 at 18:43:51 UTC, Paul Backus wrote:
 You can't return the result of a string mixin from a function. 
 Instead, return a string from your `GenIf` function, and mix it 
 in at the call site:

 string GenIf()
 {
     return "if (true) { return true; } else { return false; }";
 }

 bool testFunction()
 {
     mixin(GenIf());
 }

Thanks for mentioning this, this was where I looking for.