[Haskell-cafe] pattern matching on data families constructors
f at mazzo.li
Thu Apr 25 20:08:17 CEST 2013
At Thu, 25 Apr 2013 20:29:16 +0400,
Alexey Egorov wrote:
> I'm curious - why data families constructors (such as DInt and DBool) doesn't
> imply such constraints while typechecking pattern matching?
I think you are misunderstanding what data families do. ‘DInt :: DInt -> D Int’
and ‘DBool :: DBool -> D Bool’ are two data constructors for *different* data
types (namely, ‘D Int’ and ‘D Bool’). The type family ‘D :: * -> *’ relates
types to said distinct data types.
On the other hand the type constructor ‘D :: * -> *’ parametrises a *single*
data type over another type—the fact that the parameter can be constrained
depending on the data constructor doesn’t really matter here.
Would you expect this to work?
> newtype DInt a = DInt a
> newtype DBool a = DBool a
> type family D a
> type instance D Int = DInt Int
> type instance D Bool = DBool Bool
> a :: D a -> a
> a (DInt x) = x
> a (DBool x) = x
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