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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