In scala, why string has ordering but Seq[Char] doesn't?

Question

They are pretty much the same thing. Yes, only the first of the following codes can be compiled:

sorting String:

val ss = Seq(
  "abc",
  "def"
)

ss.sorted

sorted Seq:

val ss = Seq(
  "abc",
  "def"
)
.map(_.toSeq)

ss.sorted

What's the point of such design?

If you provide Ordering for Seq[Char], why not for Seq[Int], Seq[Double], Seq[Seq[Seq[Int]]]...? — n. m. could be an AI, Aug 14 '19 at 18:30
@AlexeyRomanov nothing wrong with it, it's just a question about ordering of Seq[Char] is odly specific. — n. m. could be an AI, Aug 15 '19 at 03:58

Alexey Romanov · Answer 1 · 2019-08-15T02:31:48.393

2

You need to add

import scala.math.Ordering.Implicits._

(or scala.math.Ordering.Implicits.seqOrdering).

Why you need this extra import is in the documentation:

Not in the standard scope due to the potential for divergence: For instance implicitly[Ordering[Any]] diverges in its presence.

edited Aug 15 '19 at 02:31

answered Aug 15 '19 at 02:18

Alexey Romanov

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score 1 · Answer 2 · answered Aug 14 '19 at 18:30

1

I think that String and Seq[Char] are isomorphic but not same. They are 'same' in the same way as Int and (Byte, Byte, Byte, Byte). String has additional semantics.

If you add Ordering[Seq[Char]] then it would be consistent to have Ordering[A : Ordered]. And it looks quite opinionated to be in stdlib.

Anyways it's not a 'design' problem since nothing stops you from writhing an instance of Ordering[Seq[Char]] yourself.

answered Aug 14 '19 at 18:30

simpadjo

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Except it _is_ in stdlib, just behind an import for technical reasons. – Alexey Romanov Aug 15 '19 at 02:19
See also https://stackoverflow.com/a/4494095/9204: "I think it's an oversight. Lexicographic ordering does make sense on Seqs. We should add it to the standard library." (in 2010) – Alexey Romanov Aug 15 '19 at 02:30

In scala, why string has ordering but Seq[Char] doesn't?

2 Answers2