On Fri, 2007-10-12 at 11:00 +1300, ok wrote: > On 11 Oct 2007, at 4:06 pm, Tom Davies basically asked for > something equivalent to Ada's > type T is new Old_T; > which introduces a *distinct* type T that has all the operations and > literals of Old_T. In functional terms, suppose there is a function > f :: ... Old_T ... Old_T ... Old_T ... > then you get a function > f :: ... T ... T ... T ... > where Old_T has been uniformly and consistently replaced by T. > More precisely, this happens to the *built in* operations of the > type; it doesn't automatically apply to user-defined operations. > > It's extraordinarily useful in Ada. My standard example is that you > can have a 2D array where row subscripts and column subscripts act in > almost all ways like integers BUT they are incompatible types so you > cannot possibly mix them up. > > Haskell's newtype comes close, but not quite close enough. > If I write > newtype Foo = Foo String > I cannot then write > x :: Foo > x = "boojumed" > Similarly, if I write > newtype Row = Row Int > newtype Col = Col Int > I cannot use 1 as a Row or Col value, but must instead write > Row 1 or Col 1. > > This *is* a limitation of the Haskell type system, and it *does* > on occasion lead to more long-winded code. But I suspect that it > is not a major problem. > > For the specific case of numeric types, it is possible to get > essentially > the Ada result, with the same convenience of use, but not the same > convenience of setup. > > newtype Row = Row Int deriving (Eq,Ord,Show) > > instance Num Row where > (Row x) + (Row y) = Row (x + y) > (Row x) - (Row y) = Row (x - y) > (Row x) * (Row y) = Row (x * y) > fromInteger i = Row (fromInteger i) > ... > instance Integral Row where > ...
*Warning: GHC extension* newtype Row = Row Int deriving (Eq,Ord,Show,Read,Num,Integral,...) jcc _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
