Bool values cannot be the member of Set

[jinjor]

I'm facing a problem with making a Set that contains Bool values.
And also, lists or tuples that contains Bool cause the same trouble.

Here is the simple example and the error message.

set = Set.fromList [True]

The 1st argument to function `fromList` has an unexpected type.

4| set = Set.fromList [True]
Note: comparable types include Int, Float, Char, String, lists, and tuples.

As I infer the type of values flowing through your program, I see a conflict
between these two types:

    comparable

    Bool

I understand what the compiler is saying, but not sure why Set or Dict keys need to be comparable. Should they always be sorted?

I think the possible solutions are...

• Bool (or other union types) should be comparable
• there should be another type instead of comparable

Any ideas? Or is there any workaround?

Thanks.

[jvoigtlaender]

Set is implemented via Dict, and Dict is implemented via some sort of balanced trees, operations on which do in fact require a total order on entries/keys. So the implementation indeed requires comparable for types that should be put into a Set or used as key in a Dict.

So probably the right feature request here is that Bool should become comparable. Making all union types comparable would be more complicated, but special-casing for Bool could make sense.

[jvoigtlaender]

Set is implemented via Dict, and Dict is implemented via some sort of balanced trees, operations on which do in fact require a total order on entries/keys. So the implementation indeed requires comparable for types that should be put into a Set or used as key in a Dict.

So probably the right feature request here is that Bool should become comparable. Making all union types comparable would be more complicated, but special-casing for Bool could make sense.

[evancz]

Why do you want a set of booleans? Any case I can think of can also be handled just by booleans.

[evancz]

Why do you want a set of booleans? Any case I can think of can also be handled just by booleans.

[jvoigtlaender]

Well, what about a set of tuples, some tuple component of which is a boolean? Certainly that's a valid use case that is not covered by just booleans alone?

[jvoigtlaender]

Well, what about a set of tuples, some tuple component of which is a boolean? Certainly that's a valid use case that is not covered by just booleans alone?

[evancz]

Ah, I was just looking at the example given. Nested stuff makes sense. Let's hear @jinjor's specific case though.

[evancz]

Ah, I was just looking at the example given. Nested stuff makes sense. Let's hear @jinjor's specific case though.

[jinjor]

Yeah, exactly the case @jvoigtlaender said. (Sorry, the example above is not real one. )

Here is the specific case in my WebRTC app.

type alias Peer = Int
type alias MediaType = String
type alias Upstream = Bool
type alias Connection = (Peer, MediaType, Upstream)

type alias Model =
  {  connections : Set Connection
  ,  videoUrls : Dict Connection String

  ...

  }

Oh, now I found MediaType can also be written as an union type, like type MediaType = Audio | Video.

[jinjor]

Yeah, exactly the case @jvoigtlaender said. (Sorry, the example above is not real one. )

Here is the specific case in my WebRTC app.

type alias Peer = Int
type alias MediaType = String
type alias Upstream = Bool
type alias Connection = (Peer, MediaType, Upstream)

type alias Model =
  {  connections : Set Connection
  ,  videoUrls : Dict Connection String

  ...

  }

Oh, now I found MediaType can also be written as an union type, like type MediaType = Audio | Video.

[evancz]

Okay, makes sense. I don't know what the short term solution is, but I made an aggregate issue at elm/compiler#1008 so we can keep track of all the issues along these lines. Hopefully the different perspectives will help us come to a good answer.

[evancz]

Okay, makes sense. I don't know what the short term solution is, but I made an aggregate issue at elm/compiler#1008 so we can keep track of all the issues along these lines. Hopefully the different perspectives will help us come to a good answer.