Utilise des mélanges de chaîne pour la réutilisation de code un anti-motif?

Question

Pour ceux d'entre vous qui ne connaissent pas les mélanges de string, ils sont essentiellement des dépenses de temps. Vous pouvez prendre n'importe quelle chaîne d'heure de compilation (à savoir une évaluation de la fonction de métaprogramming de modèle ou de la fonction de compilation) et de la compiler en tant que code. Si vous utilisez un simple littéral à chaîne, c'est essentiellement la pâte de copie automatisée de compilateur.

Considérez-vous un anticipé d'utiliser des mélanges de chaîne de littéraux comme un moyen de réutilisation de code simple lorsque d'autres méthodes d'affacturage ne correspondent pas parfaitement? D'une part, c'est essentiellement une copie littérale automatisée automatisée et automatisée, ce qui signifie qu'une fois mélangé dans les cas n'a rien de quoi faire les uns avec les autres. Les mauvaises choses se produiront (bien que lors de la compilation, pas au moment de l'exécution) Si un symbole dans le mixage de chaîne collecte avec un symbole dans la portée mixte. Il est relativement non structuré en ce que l'on peut, par exemple, mélanger une chaîne au milieu d'une fonction qui fonctionnera si et uniquement si les variables dans la portée sont nommées selon une convention donnée. Les mélanges peuvent également déclarer des variables que les champs extérieurs peuvent ensuite utiliser comme ils le voient.

D'autre part, car la copie-and-coller est compilée automatisée, il existe un point de vérité unique pour le code en question au niveau source et s'il doit être modifié, il n'a que modifier à peine Un endroit, et tout reste en synchronisation. Les mélanges de chaîne simplifient considérablement la réutilisation du code très difficile à facteur de toute autre manière et auraient autrement une probabilité très élevée d'être coupée manuellement et collé manuellement.

Answer 1

Toutes les critiques que vous avez soulevées sont vraies.

Peu importe, il est toujours supérieur à la copie manuelle.

En réalité, j'ai quelque chose de similaire dans ma bibliothèque d'outils, une expansion de la table de cordes.Exemple de code, à partir d'une mise en oeuvre de la valeur dynamique de chemin de traqueur:

  T to(T)() {
    static if (!is(T == Scope)) {
      T value;
      if (flatType == FlatType.ScopeValue) value = sr.value().to!(T);
    }
    const string Table = `
                 | bool          | int         | string               | float   | Scope
      -----------+---------------+-------------+----------------------+---------+----------
      Boolean    | b             | b           | b?q{true}p:q{false}p | ø       | ø
      Integer    | i != 0        | i           | Format(i)            | i       | ø
      String     | s == q{true}p | atoi(s)     | s                    | atof(s) | ø
      Float      | ø             | cast(int) f | Format(f)            | f       | ø
      ScopeRef   | !!sr          | ø           | (sr?sr.fqn:q{(null:r)}p) | ø   | sr
      ScopeValue | value         | value       | value                | value   | sr`;
    mixin(ctTableUnrollColMajor(Table,
      `static if (is(T == $COL))
        switch (flatType) {
          $BODY
          default: throw new Exception(Format("Invalid type: ", flatType));
        }
      else `,
      `case FlatType.$ROW:
        static if (q{$CELL}p == "ø")
          throw new Exception(q{Cannot convert $ROW to $COL: }p~to!(string)~q{! }p);
        else return $CELL;
      `
    ).litstring_expand() ~ `static assert(false, "Unsupported type: "~T.stringof); `);
  }

Je suis sûr qu'il est facile de voir quel désordre horrible et redondant d'instructions imbriquées et des déclarations de cas sans mélanges de cordes - de cette façon, toute la laideur est concentrée en bas et le comportement réel de la fonction est facile à lire en un coup d'œil.

Answer 2

While other, more elegant solutions may be better to use if you can, string mixins can be extremely useful. They allow for both code re-use and code generation. They're checked at compile time. The code that results is exactly the same as if you'de written it yourself by hand, so it's not any less safe than if you had written it yourself by hand.

The problem with string mixins is that they're harder to control than hand-written code in the sense that it's not physically laid out in your source in the same manner with line numbers clearly traceable to errors, and it may be harder to debug. For instance, take hello world with a string mixin:

import std.stdio;

void main()
{
    mixin(hello());
}

string hello()
{
    return "
    writeln(\"hello world\");
";
}

If we were to remove the semicolon after writeln(), then the error we got would be

d.d(7): found 'EOF' when expecting ';' following statement

The mixin is done on line 5. Line 7 is a blank line. So, the line number is of limited usefulness here. Now, this mixin is short enough that we could have put it on a single line and gotten it to say that the error was on the same line as the mixin, but with more complicated mixins, that obviously won't work. So, by using a string mixin, your ability to figure out where an error is is impaired. If the code is generated using CTFE, then it would become that much more difficult to figure out exactly what the code even looks like in order to figure out what's wrong with it. It's a lot like figuring out what code a C-style macro turns into, except that it could be worse because they could be generated rather than a direct replacement. However, they don't replace except where you explicitly tell them to, so they're much safer than C-style macros.

String mixins are totally safe, and there's nothing particularly wrong with them, but they do make maintenance harder in some ways. The corresponding hand-written code would be easier to debug. However, string mixins are powerful enough that they can do a lot of code generation for you and save you a lot of maintainence costs in that sense, and they allow you to re-use code, which can be a big maintanence gain as well.

So, whether using a string mixin is a good idea in a particular situation depends on that situation. I don't see anything particularly wrong with them, and I certainly wouldn't call them an anti-pattern, but there are both pros and cons to using them such that whether they're a good idea depends on what you're doing. In many cases, there are more elegant, cleaner solutions which would be better. In others, they're exactly what the doctor ordered.

Personally, I think that they're fantastic if you're looking to generate code, saving yourself the effort of having to write that code by hand, and possibly making it easier to generate correct code for a variety of situations and avoiding risking creating new bugs like you might have had you written it yourself in each of those places where you used the mixin. It also is one of the ways to just outright re-use code without having to worry about the cost of a function call or issues with the limits of single-inheritance or anything else that makes code re-use by calling functions or inheritance harder. You're simply copying and pasting the code into each place in a manner which makes it so that if you change the code, the changes will be properly pasted everywhere without you having to worry about tracking them all down like if you had hand copy and pasted.

So, use string mixins where appropriate, and it's probably best not to use them if they're not needed, but there's nothing really wrong with using them.

Answer 3

String mixin is like goto: it should be avoided where ever possible and should be used wherever required.