Re: [Haskell-cafe] Why does Enum succ and pred functions throw exception

On 6/21/12 1:15 PM, Brandon Allbery wrote:

On Thu, Jun 21, 2012 at 5:11 AM, Rouan van Dalenwrote:

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Can anyone shed some light on why the succ and pred functions of the Enum typeclass throw exceptions if we go over the upper or lower boundary, and not return Maybe a?

Enum and Bounded have a complicated and arguably incorrect[1] relationship. The whole thing should be overhauled,

Indeed. In light of the exception-throwing specification, it's not trivial to define a safe enumFromDownTo function satisfying the specification: enumFromDownTo x y == reverse (enumFromTo y x) with all the properties we'd like to have (e.g., list fusion). Essentially, we need access to whatever predicate decides to throw an error in the pred function, in order to use it to return the empty list when appropriate. The predicate can be reconstructed from Ord or from Bounded and Eq but both of those constructions make non-trivial assumptions about how the classes interact. Hence, the Enum class in incomplete, in addition to any correctness problems interacting with other classes. -- Live well, ~wren