If I have a data type say:
data Color = Red | Yellow | Green
Is there a way I can turn this into a list of type [Color] getting all possible values? [Red, Yellow, Green]
Perhaps this is a complete anti pattern?
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I believe [toEnum 0 ..] works no matter how you change the data definition.SwiftsNamesake– SwiftsNamesake2017-07-16 00:34:39 +00:00Commented Jul 16, 2017 at 0:34
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4 Answers
Not sure if it is an anti-pattern (nor can I think of a good use right now), but it's possible. Use the Enum (allows to generate a list like [someCtor .. someOtherCtor]) and Bounded (for minBound and maxBound) type classes. Luckily, you can derive both:
data Color = Red
| Yellow
| Green
deriving (Enum, Bounded)
allColors = [(minBound :: Color) ..]
If you ever add another color, allColors get updated automatically. One restriction though: Enum requires all contructors to be nullary, i.e. adding Foo Int breaks the whole thing. Luckily, because a list of all possible values for this would be way too large.
Edit: The other answer works as well, maybe better since it doesn't require deriving Bounded and is therefore a bit shorter. I'll still leave mine because I love over-engineered but extremely generic code ;)
Surely delnan's answer is better. Since I do not know how to include a piece of code in a comment, I'll give a generalisation as a separate answer here.
allValues :: (Bounded a, Enum a) => [a]
allValues = [minBound..]
Now, this works for any type with a Bounded and Enum instance! And allColors is just a special case:
allColors :: [Color]
allColors = allValues
In many cases, you won't even need to define allColors separately.
2 Comments
Guy
I agree with you, since Delnan's solutions doesn't require any reference to a real value from the type. On a SO note, perhaps I accepted too quickly, but the answer by adamse did work exactly as I asked.
data Color = Red
| Yellow
| Green
deriving Enum
allColors = [Red ..]
2 Comments
SwiftsNamesake
I believe
[toEnum 0 ..] works no matter how you change the data definition.Here is an example of using this technique to parse enums with Parsec
data FavoriteColor = Maroon | Black | Green | Red |
Blue | Pink | Yellow | Orange
deriving (Show, Read, Enum, Bounded)
And the parsec parser
parseColor :: Parser FavoriteColor
parseColor = fmap read . foldr1 (<|>) $ map (try . string . show)
[ minBound :: FavoriteColor ..]
of course the try could could be applied by better by pattern matching and a few other things could make it nicer but this is just an example of some usage of the technique.
