[PRL] Joel on Software - Making Wrong Code Look Wrong

[Paul A. Steckler]

> I wonder if you could also introduce a "label" mechanism into MrFlow, 
> where you could tag values as having that label, and tag the basic 
> operations with contracts describing what they require/provide with 
> those labels.

Here's a cheap hack to add attributes to types in Haskell using 
phantom types:

 data MyString a = MyString { s :: String }
   deriving (Eq,Show)

 type Unsafe = MyString Int
 type Safe = MyString [Int]

 x :: Unsafe
 x = MyString { s = "foo" }

 y :: Safe
 y = MyString { s = "foo" }

Then, x == x or y == y will type-check,
but not x == y.

The actual type parameters to MyType 
are unimportant, as long as it differs
for Safe and Unsafe.

-- Paul