[Haskell-beginners] private types

[Ashish Agarwal]

This makes T fully abstract, which is what I'm trying to avoid. Private
types provide an intermediate level of abstraction. The real example I have
involves defining a type with numerous constructors (or a record with
numerous fields), and I'd like to avoid writing all the corresponding
destructors. Even if I did do this, it burdens clients of the module by
requiring them to use the alternative destructors.

I think the feature I'm looking for is "guarded constructors", but to my
knowledge there is no such thing.


On Thu, Apr 8, 2010 at 3:42 PM, Stephen Tetley <stephen.tetley at gmail.com>wrote:

> Hi
>
> I don't think there is a direct equivalence. You can make T an opaque
> type using a module where you do not export the constructor. However,
> you can't then pattern match on the type so you have to supply a
> destructor - here called unT. Any module that imports XYZ only sees
> the type T, the 'handmade' constructor makeT and the destructor unT:
>
>
>
> module XYZ
>  (
>    T,         -- this does not export the constructor
>    unT
>    makeT
>
>  ) where
>
> data T = PrivateT Int   deriving (Eq,Show)
>
> unT :: T -> Int
> unT (PrivateT i) = i
>
> -- Add any checking you want here...
> makeT :: Int -> T
> makeT i = PrivateT i
>
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