On Monday, 29 March 2021 at 16:20:59 UTC, Ali Çehreli wrote:
auto myFunc(F)(string name, F func)
{
// This condition could be a template constraint but they don't
  // support error messages.
  static assert (is (Parameters!func == AliasSeq!(string)),
"'func' must be a callable that takes 'string'.");
  return func(name);
}

void main() {
// One trouble with this "solution" is that for the compiler to
  // know the return type of the lambda, the parameter must be
  // declared as 'string' (in this case).
  writeln(myFunc("foo", (string a) => a ~ '.'));
}

Ali

Alternatively:

    auto myFunc(F)(string name, F func)
    {
      static assert (__traits(compiles, (string s) => func(s)),
"'func' must be a callable that takes 'string'.");
      return func(name);
    }

    void main() {
      // No need to write out the argument type
      writeln(myFunc("foo", a => a ~ '.'));
    }

You can generalize this into a helper template:

    enum bool isCallableWith(alias fun, ArgTypes...) =
      __traits(compiles, (ArgTypes args) => fun(args));

Usage:

    static assert(isCallableWith!(func, string));

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