[Haskell-cafe] Clearly, Haskell is ill-founded

Stefan O'Rear stefanor at cox.net
Tue Jul 10 15:26:29 EDT 2007

On Tue, Jul 10, 2007 at 08:08:52PM +0100, Andrew Coppin wrote:
> Erm... Wait a sec... coroutines, comonads, coprograms, codata... what in 
> the name of goodness does "co" actually *mean* anyway??


When mathematicians find a new thing completely unlike an OldThing, but
related by some symmetry, they often call the new thing a CoOldThing.

Data can only be constructed using constructors, but can be
deconstructed using recursive folds;
Codata can only be deconstructed using case analysis, but can be
constructed using recursive unfolds.

Monads keep things inside.
Comonads keep things outside.

Homology theory studies the boundaries of shapes.
Cohomology theory studies the insides of curves.


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