[Haskell-beginners] private types

[Dominic Mulligan]

Hi Stephen,

Yes, that is correct.  But I don't see why this matters (unless I'm 
missing something obvious?).  In this case, you cannot construct a value 
of type T using TView's constructors, and presumably all the functions 
defined in module View1 will be defined over T, not TView.  If the 
question was to find a perfect analogue to O'Caml's private types, then 
probably one doesn't exist, but GHC's view patterns are the closest 
Haskell feature that does the intended purpose of allowing pattern 
matching without breaking abstraction (that I'm aware of, of course :)).

Stephen Tetley wrote:
> Hi Dominic
>
> They can get a bit closer but (I think) you would still need to define
> a auxiliary view type that exports its constructor:
>
>
> module View1 (
>   T,         -- opaque
>   TView(..), -- not opaque
>   tview,
>   mkT
>   )
>   where
>
> data T = PrivateT Int   deriving (Eq,Show)
>
> -- auxillary type
> data TView = TView Int  deriving (Eq,Show)
>
> tview :: T -> TView
> tview (PrivateT i) = TView i
>
> mkT :: Int -> T
> mkT i = PrivateT i
>
>
> -- client module with views (new file) :
>
> {-# LANGUAGE ViewPatterns #-}
>
> module UseView where
>
> import View1
>
> -- Use the view pattern:
> add1 :: T -> T
> add1 (tview -> TView i) = mkT (i+1)
>
> -- or long hand...
> add1_alt :: T -> T
> add1_alt t = case tview t of
>                TView i -> mkT i
>
>
>
>
>
>
>
> On 8 April 2010 21:35, Dominic Mulligan
> <dominic.p.mulligan at googlemail.com> wrote:
>   
>> The GHC extension "view patterns" does roughly what you want.
>>
>> See here: http://hackage.haskell.org/trac/ghc/wiki/ViewPatterns
>>
>> Ashish Agarwal wrote:
>>     
>>> Is there a Haskell analogue to OCaml's private types? In OCaml, you can do
>>> this (converting to pseudo-Haskell syntax):
>>>
>>> type T = private A Int
>>>
>>> Then, A cannot be used directly to construct values of type T.
>>> A 3
>>> Error: Cannot create values of the private type T
>>>
>>> You must provide another constructor, makeT, which takes an Int and
>>> returns a T. This allows you to do additional checks on the Int argument.
>>> However, you can still deconstruct values of type T.
>>>
>>> f (A x) = x + 2
>>>
>>> This works even when f is defined in a different module. The benefit is
>>> that you can restrict the allowed values of T, but still have the
>>> convenience of using existing operations on these values once constructed.
>>>
>>> The solution that comes to mind is to make T abstract and an instance of
>>> some appropriate class, but there is no class that lets you pattern match on
>>> arbitrary variant types.
>>>
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>>>
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