On Sun, Sep 12, 2010 at 9:24 AM, Dominique Devriese
However, it would make more sense to have it be a type family, without the overhead of data (both in space and in typing).
You can make Tensor a data family and use "newtype instances". As I understand these, there should not be a space overhead. The only overhead I would expect this to introduce is the extra newtype constructor.
Which is only at programming time; newtype constructors do not exist at runtime. They get erased after typechecking. This also means that pattern matching against newtype constructors cannot fail. For example:
data family F a data instance F Bool = B () newtype instance F Int = I ()
fBool :: F Bool -> Int f1 (B _) = 3
fInt :: F Int -> Int f2 (I _) = 4
main = do print (fInt undefined) print (fBool undefined)
This program should print 4 and then exit with an error. -- ryan