Re: [Haskell-cafe] Data & newtype differences. Today: strictness

An interesting use case for this is that while data Void = Void Void has infinitely many values (undefined, Void undefined, Void (Void undefined) etc), the newtype version newtype Void = Void Void has only one, bottom. This is a way to define the empty datatype without extensions. Erik On Tue, Jan 24, 2012 at 11:25, Ketil Malde wrote:

Yves Parès writes:

...

I had for long thought that data and newtype were equivalent, but then I spotted some differences when it comes to strictness.

data Test = Test Int newtype TestN = TestN Int

Interesting.  I'd thought that

 data Test = Test !Int

and

 newtype Test = Test Int

would be equivalent, but here you (well, I had to add the ! myself) show a situation where they're not.

I guess

 pm (Test _) = 12

is the same as

 pm = \x -> case x of Test _ -> 12

which perhaps makes it clearer why it breaks on undefined...

-k -- If I haven't seen further, it is by standing in the footprints of giants

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