I've seen this expressed with GADTs (which I guess many things can),
though I'm not sure if it's the best way, and I'm no type system
wizard. If I recall correctly, this use is normally called phantom
types.
{-# LANGUAGE EmptyDataDecls, GADTs #-}
data Even
data Odd
data Foo a where
One :: Foo Odd
Two :: Foo Even
Three :: Foo Odd
Four :: Foo Even
Then you can write functions that accept only evens:
f :: Foo Even -> ...
or only odds:
f :: Foo Odd -> ...
or either:
f :: Foo a -> ...
Hope that helps,
-Ross
On Jul 27, 2009, at 12:01 AM, Brian Troutwine wrote:
Hello all.
I would like to define a data type that is the super-set of several
types and then each of the proper subset types. For example:
data Foo = One | Two | Three | Four
data Odd = One | Three
data Even = Two | Four
This, of course, does not work. It seems that such a thing should
possible to express entirely in the type system, but I cannot think of
how. Would someone be so kind as to explain how this sort of thing can
be accomplished?
Thanks,
Brian
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