11.1 Typesets

Now that you know about polymorphism you might be tempted to write this:

    proc printline[T] : T = "::std::cout << $1 << ::std::endl;" 
      requires header '#include <iostream>'
    ;
  
    printline 1;
    printline "Hello";
    printline 4.2;

1
Hello
4.2

and indeed that would work! But what if C++ doesn't know how to print your type? What if you had a C struct you imported into Felix that didn't have an {operator <<} defined?

Again, your Felix code would compile because you told it the C++ code would work for all type T, but you lied, and the C++ compiler will find you out!

Before I show you the solution, I want to give another motivating example:

  type myshort= "short";
  type myint = "int";
  type mylong = "long";
  
  type myushort= "unsigned short";
  type myuint = "unsigned int";
  type myulong = "unsigned long";
  
  type myfloat = "float";
  type mydouble = "double";
  
  fun + : myshort * myshort -> myshort = "$1+$2";
  fun + : myint * myint -> myint = "$1+$2";
  fun + : mylong * mylong -> myint = "$1+$2";
  fun + : myushort * myushort -> myushort = "$1+$2";
  fun + : myuint * myuint -> myuint = "$1+$2";
  fun + : myulong * myulong -> myulong = "$1+$2";
  fun + : myfloat * myfloat -> myfloat = "$1+$2";
  fun + : mydouble* mydouble -> mydouble = "$1+$2";

And that's just addition! Where's the macro processor??

Well of course you're not just tempted to write:

  fun +[T] : T * T -> T = "$1+$2";

you're more or less forced to: in practice you'd never maintain a set of such bindings effectively. There could be hundreds of lines of this code. The polymorphic function is so much easier.

But it's wrong. How do we fix this?

Here's the answer:

  typeset myints = {myshort, myint, mylong};
  typeset myuints = {myushort, myuint, myulong};
  typeset myfloats = {myfloat, mydouble};
  typeset mynumbers = myints \(\cup\) myuints \(\cup\) myfloats;
  
  fun mymake [T in mynumbers] : int -> T = "int($1)";
  fun *[T in mynumbers] : T * T -> T = "$1*$2";
  proc printit[T in mynumbers] : T = "::std::cout<<$1<<::std::endl;";
  
  var xs = mymake[myshort] (42);
  var xi = mymake[myint] (42);
  var xl = mymake[mylong] (42);
  var xd = mymake[mydouble] (42);
  
  printit$ xs;
  printit$ xi;
  printit$ xl;
  printit$ xd;

42
42
42
42

This is known as constrained polymorphism. The entities myints, myuints, myfloats, and mynumbers are finite sets of concrete types. The setunion operator here is the TeX symbol spelled {\cup}.