Simon Peyton-Jones wrote: > 3. Dictionaries are packaged in data constructors > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > A very useful new feature is this. When a data type is declared in in > GADT syntax, the context of the constructor is > *required* when constructing > *available* when matching > > For example > data T a where > T1 :: Num a => a -> T a > T2 :: Bounded a => T a > > f :: T a -> a > f (T1 x) = x+1 > f T2 = maxBound > > Notice that f is not overloaded. The Num needed in the first branch > is satisfied by the T1 pattern match; and similarly for the T2 pattern > match. > > This feature has been often requested, becuase it allows you to > package a dictionary into an ordinary (non-existential) data type, and > be able to use it. > > NOTE: the Haskell 98 syntax for data type declarations > data Num a => T a = T1 a > behaves exactly as specified in H98, and *not* in the new way. > The Num dictionary is > *required* when constructing, and > *required* when matching > I think this is stupid, but it's what H98 says. To get the new > behaviour, use GADT-style syntax, even though the data type being > defined is does not use the GADT-ness.
We may want to propose to change that for Haskell'. What do you think? Manuel _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
