[Haskell-cafe] Re: Why is Day and Month bounded?
Malcolm.Wallace at cs.york.ac.uk
Fri Jul 14 05:30:52 EDT 2006
"Johan Holmquist" <holmisen at gmail.com> wrote:
> If Day (and Month) where NOT instances of Bounded, the following would
> be possible:
> [Saturday .. Tuesday]
> => should return [Saturday, Sunday, Monday, Tuesday]
> => but returns 
This does seem like a reasonable argument to me. Some enumerations are
semantically cyclic, rather than linear.
But how do you feel about the way it breaks some algebraic laws?
fromEnum x < fromEnum (succ x)
fromEnum x > fromEnum (pred x)
Of course, such laws are not explicitly stated anywhere in the
definition of the classes, so one would be foolish to rely on them in
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