4 ms·

Depends on the value of the template parameter. If it's 0 the return type is an int, if it's not 0 the return type is a string. There's a wanting implementatio

Depends on the value of the template parameter. If it's 0 the return type is an int, if it's not 0 the return type is a string. There's a wanting implementation of a sumtype in the standard library (https://dlang.org/phobos/std_variant.html#.Algebraic https://dlang.org/phobos/std_variant.html#.Algebraic), and a much better one as a package: https://code.dlang.org/packages/sumtype https://code.dlang.org/packages/sumtype

RandallBrown 6y ago
Interesting, so the return type isn't known until runtime.
- lumberjackstian 6y ago
  When it comes to templates it's not until instantiation time - when the compiler see the code being used. So this is just an issue during compilation. The docs on static if may shed some more light: https://dlang.org/spec/version.html#staticif https://dlang.org/spec/version.html#staticif
  - RandallBrown 6y ago
    So the previous code wouldn't compile unless the compiler knew what values were going to be passed into whatDoesItReturn?
    - DougBTX 6y ago
      I missed this the first time, but there are two sets of parentheses in the example. Apparently the first set are like template parameters, and the second are the actual arguments to the function.
    - lumberjackstian 6y ago
      Aye correct! I should've been more clear about that, sorry. Template arguments need to be known at compile time, and the extra set of parens is how templates parameters are declared in D.
- gallier2 6y ago
  No. It is known at compile time. Template declarations in D take 2 parameter lists. The first is the template parameters the second the runtime parameter: in auto whatDoesItReturn(int i)() { static if (i == 0) { return int.init; } else { return string.init; } } we have (int i) as template parameter and () as an empty runtime parameter. In C++ syntax whatDoesItReturn<int i>() at instanciation the syntax is different: whatDoesItReturn!0() will instantiate a function returning an int whatDoesItReturn!42() will instantiate a function returning a string.

RandallBrown 6y ago

Interesting, so the return type isn't known until runtime.

lumberjackstian 6y ago
When it comes to templates it's not until instantiation time - when the compiler see the code being used. So this is just an issue during compilation. The docs on static if may shed some more light: https://dlang.org/spec/version.html#staticif https://dlang.org/spec/version.html#staticif
- RandallBrown 6y ago
  So the previous code wouldn't compile unless the compiler knew what values were going to be passed into whatDoesItReturn?
  - DougBTX 6y ago
    I missed this the first time, but there are two sets of parentheses in the example. Apparently the first set are like template parameters, and the second are the actual arguments to the function.
  - lumberjackstian 6y ago
    Aye correct! I should've been more clear about that, sorry. Template arguments need to be known at compile time, and the extra set of parens is how templates parameters are declared in D.
gallier2 6y ago
No. It is known at compile time. Template declarations in D take 2 parameter lists. The first is the template parameters the second the runtime parameter: in auto whatDoesItReturn(int i)() { static if (i == 0) { return int.init; } else { return string.init; } } we have (int i) as template parameter and () as an empty runtime parameter. In C++ syntax whatDoesItReturn<int i>() at instanciation the syntax is different: whatDoesItReturn!0() will instantiate a function returning an int whatDoesItReturn!42() will instantiate a function returning a string.