On Sat, Oct 26, 2013 at 03:12:36AM +0700, Kim-Ee Yeoh wrote:
On Sat, Oct 26, 2013 at 12:26 AM, Nicholas Vanderweit < nick.vanderweit@gmail.com> wrote:
Or, alternatively, using fix from Data.Function:
fix NotVoid
Yes, and all of them are indistinguishable from bottom or 'undefined :: forall a. a', the ghost that inhabits all types, including the empty one: Void.
No, Nicholas was using NotVoid, which I defined as data NotVoid = NotVoid NotVoid in this case fix NotVoid *is* distinguishable from bottom. But if NotVoid is defined using newtype, it is not distinguishable.
What trips up beginners is the interpretation of indistinguishable-ness. At the level of denotational design, 'undefined' and 'error "(sadface)"' are to be treated as equal, although beginners would point out: "hey look, I get different output!"
Call it denotational design, or call it beautiful FP modelling, this stuff is really where it's at.
-- Kim-Ee
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