2013/01/24 07:38:11
This part is not exactly a theory of algebraic theories, but anyway.

Imagine we have a parametric class (e.g. `List[X]`) for which we would like to define methods that would only work if `X` satisfies certain conditions. E.g. define `sum` if it is a numeric type, or define `flatten` if it is some kind of sequence (`Traversable[Y]` would be enough). How can we accomplish it in Scala?!

Suppose we want to declare `flatten`
```trait List[X] {
// bla-bla-bla

def flatten[X <% Iterable[Y]] { ... } // no way, we already mentioned X; this one would shadow the previous one
// OOPS!
}
```

The trick would be to have an implicit transformation handy that transforms `X` into `Iterable[Y]` for some `Y`; and define it so that if `X` is actually some kind of `Iterable`, the implicit would be available, and otherwise not available.

Where can we find such a general transformation? The only thing that comes up to mind is an identity:

`implicit def itsme[Y, X <% Iterable[Y]](x: X): Iterable[Y] = x`

This would be enough in this specific case... but then we would have to define another one for numeric types, and so on.

We can go generic, and write something like this:
```abstract class SafeToCast[-S, +T] extends Function1[S, T]
implicit def canCast[X]: SafeToCast[X, X] = new SafeToCast[A,A] { def apply(a: A) = a }
```

Now we can define `flatten`, like this:
```class List[X] { ...
def flatten[Y](implicit transformer: SafeToCast[X, Iterable[Y]]) { ... }
}
```

All the conditions are satisfied now. Contravariance in the first argument and covariance in the second argument ensure that a subtype can be cast to its supertype... to be more precise, an instance of supertype can be substituted by an instance of subtype.

The only thing is that we can try to make the code more readable, by refactoring.

Step 1. Use the trick in Scala that binary operations can be written in an infix way, even for types. E.g. you can declare `val map: (String Map Int)` - this is the same as `Map[String, Int]`.

```sealed abstract class SafeToCast[-S, +T] extends Function1[S, T]
implicit def canCast[X]: (X SafeToCast X) = new (A SafeToCast A) { def apply(a: A) = a }
...

class List[X] { ...
def flatten[Y](implicit transformer: X SafeToCast Iterable[Y]) { ... }
}
```

Step 2. Rename the method, giving it a more "type-relationship" name: `SafeToCast` -> `<:<`.

```sealed abstract class <:<[-S, +T] extends Function1[S, T]
implicit def canCast[X]: (X <:< X) = new (A <:< A) { def apply(a: A) = a }
...

class List[X] { ...
def flatten[Y](implicit transformer: X <:< Iterable[Y]) { ... }
}
```

That's the trick.

(if you have questions, ask; if you have corrections, please tell me)
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