CODEWITHSUNDEEP
haskellerror-handlingtypeclassoption-type
gallabytes
gallabytes

Reputation: 267

Generalizing isNothing

So let's say we want to deal with computations that can fail using a container similar to Maybe. If I were to specify a typeclass for what I want to do, it'd look something like this:

class Alternative f => Nullable f where
  isEmpty :: f a -> Bool

Such that isEmpty x == True if and only if x = empty

Is there some existing typeclass that I should use for this functionality or should I just use the class above?

EDIT: Some more context

I'm trying to implement matching for logic programming, and I want to be able to have a "not pattern" i.e. "succeed if and only if this program fails, then do ___".

Upvotes: 2

Views: 179

Answers (2)

duplode
duplode

Reputation: 34398

As had been foretold in the comments to danidiaz's answer, base-4.8 generalised the type of null, making it an easy at reach emptiness test for Foldable containers:

GHCi> :t null
null :: Foldable t => t a -> Bool
GHCi> null Nothing
True
GHCi> null (Left "An error")
True
GHCi> null []
True
GHCi> import qualified Data.Map as Map
GHCi> null Data.Map.empty
True

Upvotes: 1

danidiaz
danidiaz

Reputation: 27766

Perhaps you could use Foldable in this manner. Maybe, [], Either, ErrorT, ExceptT are all Foldable. You can just pattern-match on the result of toList.

And in cases when you want to do something with the results of a successful execution and nothing in case of failure, you can use forM_ directly.

Alternatively, you could use the MonoidNull typeclass from monoid-subclasses, that provides the null predicate.

Upvotes: 5

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