[Haskell-beginners] private types

Dominic Mulligan dominic.p.mulligan at googlemail.com
Thu Apr 8 16:35:44 EDT 2010

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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