Javaで型安全なprintf

gistfile1.java

interface Func<A, B> {
  B apply(final A arg);
}

abstract class Dir<A, B> {
  public Dir() {}
  abstract public Func<Func<String, A>, Func<String, B>> format();
}

class DLit<A> extends Dir<A, A> {
  private String s;
  public DLit(String s) {
    this.s = s;
  }
  public Func<Func<String, A>, Func<String, A>> format() {
    return new Func<Func<String, A>, Func<String, A>>() {
      public Func<String, A> apply(final Func<String, A> cont) {
        return new Func<String, A>() {
          public A apply(final String out) {
            return cont.apply(out + s);
          }
        };
      }
    };
  }
}

class DInt<A> extends Dir<A, Func<Integer, A>> {
  public DInt() {}
  public Func<Func<String, A>, Func<String, Func<Integer, A>>> format() {
    return new Func<Func<String, A>, Func<String, Func<Integer, A>>>() {
      public Func<String, Func<Integer, A>> apply(final Func<String, A> cont) {
        return new Func<String, Func<Integer, A>>() {
          public Func<Integer, A> apply(final String out) {
            return new Func<Integer, A>() {
              public A apply(Integer i) {
                return cont.apply(out + i.toString());
              }
            };
          }
        };
      }
    };
  }
}

class DStr<A> extends Dir<A, Func<String, A>> {
  public DStr() {}
  public Func<Func<String, A>, Func<String, Func<String, A>>> format() {
    return new Func<Func<String, A>, Func<String, Func<String, A>>>() {
      public Func<String, Func<String, A>> apply(final Func<String, A> cont) {
        return new Func<String, Func<String, A>>() {
          public Func<String, A> apply(final String out) {
            return new Func<String, A>() {
              public A apply(String s) {
                return cont.apply(out + s);
              }
            };
          }
        };
      }
    };
  }
}

class DCon<A, B, C, D> extends Dir<C, B> {
  private Dir<A, B> d1;
  private Dir<C, A> d2;
  public DCon(Dir<A, B> d1, Dir<C, A> d2) {
    this.d1 = d1;
    this.d2 = d2;
  }
  public Func<Func<String, C>, Func<String, B>> format() {
    return new Func<Func<String, C>, Func<String, B>>() {
      public Func<String, B> apply(final Func<String, C> cont) {
        return new Func<String, B>() {
          public B apply(final String out) {
            return d1.format().apply(d2.format().apply(cont)).apply(out);
          }
        };
      }
    };
  }
}

class SafeFormatter {
  public static <A> A format(Dir<String, A> d) {
    return d.format().apply(new Func<String, String>() {
      public String apply(final String a) {
        return a;
      }
    }).apply("");
  }
  
  public static <A> Dir<A, A> newDLit(String s) {
    return new DLit<A>(s);
  }
  
  public static <A> Dir<A, Func<Integer, A>> newDInt() {
    return new DInt<A>();
  }

  public static <A> Dir<A, Func<String, A>> newDStr() {
    return new DStr<A>();
  }
  
  public static <A, B, C, D> Dir<C, B> newDCon(Dir<A, B> d1, Dir<C, A> d2) {
    return new DCon<A, B, C, D>(d1, d2);
  }

  public static void main(String[] args) {
    
    //型安全なSystem.out.printf("hello, %s!", "world")を表現している。
    Dir<Func<String, String>, Func<String, String>> d1 = newDLit("hello, ");
    Dir<String, Func<String, String>> d2 = newDStr();
    Dir<String, String> d3 = newDLit("!");
    
    System.out.println(format(newDCon(newDCon(d1, d2), d3)).apply("world"));
    //System.out.println(format(newDCon(newDCon(d1, d2), d3)).apply(123)); //String以外の型はコンパイルエラー

    
    //型安全なSystem.out.printf("a: %d b: %d c: %d", 1, 2, 3)を表現している。
    Dir<Func<Integer, Func<Integer, Func<Integer, String>>>,
      Func<Integer, Func<Integer, Func<Integer, String>>>> d4 = newDLit("a: ");
    Dir<Func<Integer, Func<Integer, String>>,
      Func<Integer, Func<Integer, Func<Integer, String>>>> d5 = newDInt();
    Dir<Func<Integer, Func<Integer, String>>,
      Func<Integer, Func<Integer, String>>> d6 = newDLit(" b: ");
    Dir<Func<Integer, String>,
      Func<Integer, Func<Integer, String>>> d7 = newDInt();
    Dir<Func<Integer, String>, Func<Integer, String>> d8 = newDLit(" c: ");
    Dir<String, Func<Integer, String>> d9 = newDInt();
    
    System.out.println(
      format(newDCon(newDCon(newDCon(newDCon(newDCon(d4, d5), d6), d7), d8), d9))
      .apply(1).apply(2).apply(3)
    );
    //System.out.println(
    //  format(newDCon(newDCon(newDCon(newDCon(newDCon(d4, d5), d6), d7), d8), d9))
    //  .apply('a').apply('b').apply('c')
    //); //Integer以外の型はコンパイルエラー
    
  }
}