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Enums and get_next ('succ)
f#

Hi,

I know that you can create an enum in F# by for example

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type colour =
    | red = 1
    | green = 2
    | blue = 3

I would like two library functions of the form

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// true if c has a next colour
let has_next c = not ( c = blue )

// get next colour in enum
let get_next = function
    | red   -> green
    | green -> blue
    | blue -> failwith "Blue has no next colour"

My problem is that they have knowledge of the "structure" of the enum, colour, and are therefore hard to maintain. Try adding yellow (say) after blue.

In Ada I would use something like 

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colour'succ(c)

for get_next. Yes, I would have to write has_next in a more generic manner but that's no problem.

Thanks
Joe

.

It's certainly possible to create a function that does something like what you want. Unfortunately, enumerated types in .NET are a bit strange, so you'll want to think about how to handle corner cases. For instance, it's possible to have instances of an enumerated type which don't match any of the named instances (e.g. 

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let (c:color) = enum 7

. It's also possible to create enums where the same underlying value is given more than one name:

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type weird = | x = 0 | y = 0 | z = 1

Depending on how you want your function to behave, here are a few different approaches you can take. This will give you a new enum with an underlying value which is one higher than that of the enum passed in (regardless of whether that's a valid option):

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let inline succ (e:'en when 'en : enum<'a>) : 'en =
  e 
  |> LanguagePrimitives.EnumToValue
  |> (LanguagePrimitives.AdditionDynamic (LanguagePrimitives.GenericOne:'a))
  |> LanguagePrimitives.EnumOfValue

This will pick the first explicitly defined value higher than the value passed in (or throw if there is none):

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let succ (e:'en when 'en:enum<'a>) : 'en =
  System.Enum.GetValues(typeof<'en>) :?> 'en[]
  |> Array.find (fun e' -> e' > e)
  |> unbox
By on 2/18/2010 8:06 AM ()

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