[Haskell-beginners] private types

[Ashish Agarwal]

Thanks for all the suggestions. I also see that Haskell allows the same
record field name in multiple constructors. This also helps for the type
definition I had in mind:

module XYZ
 (
   T(foo,bar),
   makeA,
   makeB

 ) where

data T
    = PrivateA {
        foo :: Int
      }
    | PrivateB {
        foo :: Int,
        bar :: Int
      }
    deriving (Eq,Show)

makeA :: Int -> T
makeA i = PrivateA {foo=i}

makeB :: Int -> Int -> T
makeB i j = PrivateB {foo=i, bar=j}


On Thu, Apr 8, 2010 at 5:27 PM, Dominic Mulligan <
dominic.p.mulligan at googlemail.com> wrote:

> 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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