purescript data as array of all possible data inhabitants

Question

I got the following data type:

data Icon = IconCircle | IconSquare | IconStar

I need an array of all possible Icons?:

allPossible :: Array Icon
allPossible = [IconCircle, IconSquare, IconStar]

Is there a shortcut to create this array? e.g when there are 20 (or a lot more) possible Icons?

When not:

Is it possible to create a type for allPossible that forces all possible Icons to be included?

My goal is to force that this array is never incomplete.

Fyodor Soikin · Accepted Answer · 2022-11-16 15:12:10Z

2

Yes, there is!

You can derive Generic and then use GenericEnum in combination with GenericBottom to enumerate all constructors:

data Icon = IconCircle | IconSquare | IconStar
derive instance Generic Icon _

allPossible :: Array Icon
allPossible = unfoldr (\a -> Tuple a <$> genericSucc a) genericBottom

And you can even make this completely generic, so it would work for any type, not just for Icon:

allPossible :: forall a rep. 
  Generic a rep =>
  GenericEnum rep => 
  GenericBottom rep => 
  Array a
allPossible = unfoldr (\a -> Tuple a <$> genericSucc a) genericBottom

Of course, this will only work for types that are actually enums - i.e. every constructor is either parameterless or its parameters have instances of Enum. If the type is not an enum in this sense, this will fail to compile, complaining that it's impossible to construct a GenericEnum instance.

answered Nov 16, 2022 at 15:12

Fyodor Soikin

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1 Comment

christian wuensche Over a year ago

Hint: these imports are necessary: ``` import Data.Functor ((<$>)) import Data.Tuple import Data.Unfoldable (unfoldr) import Data.Bounded.Generic (genericBottom) import Data.Enum.Generic (genericSucc) import Data.Generic.Rep (class Generic) ```

Dirk · Accepted Answer · 2022-12-13 19:19:07Z

1

Fyodor's answer is incorrect unfortunately. The last value is omitted because it is mapped on it's non existing successor (i.e. Nothing).

A working alternative might be:

allPossible :: Array Icon
allPossible = unfoldr (\b -> b >>= next) $ Just genericBottom
  where
    next a = Just $ Tuple a $ genericSucc a

answered Dec 13, 2022 at 19:19

Dirk

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