equal functions in Enum and Seq

[mars0i]

LazyList lacks an equal predicate. Both Enum and Seq include equal, but the syntax is different: Enum has a required equality-predicate parameter for comparison of individual elements, while Seq instead has an optional argument that defaults to =.

I want to submit a PR for equal for LazyList, but I'm not sure which model to follow. I think the optional argument version of Seq makes more sense, since any of the usual standard OCaml types, except functions, will automatically work with =. In fact I was considering submitting code more or less identical to what's in Seq, following kantian's suggestions in this thread in discuss.ocaml.org.

It would be nice if all three equal functions had the same syntax, but I gather that Enum is used a lot in Batteries, so changing Enum's equal might break a lot of things. I don't know how heavily Seq is used, and whether changing its syntax might break things.

Given the difference between Enum's and Seq's equal functions, I'm not sure how best to proceed.

[gasche]

If you grep for val equal in the repository, you will see that having a mandatory argument is much more common than having an optional argument, so I would use that for consistency.

[mars0i]

OK. Should I "fix" Seq's equal, too? Or is that risky?

[UnixJunkie]

@c-cube

[gasche]

You shouldn't change the interface of an existing function unless we do a major release. You could send this change, separately, to the v3 branch -- which we need to rebase on top of the current master, but that is another story.

[mars0i]

If equal has a required eq argument, it's the same function as for_all2, except that it returns false rather than raising an exception if one list is longer and the test succeeds up to that point.

Seq and Enum don't have functions named for_all2, but Enum's equal can also be used like for_all2 if the enumerations are the same length. You could do the same thing with Seq.equal; the function parameter defaults to =, but you could pass any function with the right signature.

I'm just a little befuddled, confused at the moment about what functions are needed. An equal function with a required predicate argument is essentially for_all2, with false if the lists are different lengths. Are both worth having? I certainly like the idea of an equal function, and would write my own if it wasn't provided in LazyList. But it's a somewhat strange idea that an arbitrary function could be passed to equal, and that it's almost the same function as for_all2.

In the abstract I feel that Seq's default test predicate syntax is better--that makes the name equal more appropriate--but I see the logic of following what's mostly done in the rest of the code.

[gasche]

Yes, I personally think that it is fine having two functions doing the same thing with different names that carry different connotations. (Furthermore, in this case, they are not exactly doing the same thing. Note that I wouldn't encourage people to use equal whenever they need "for_all2 on unequal lengths" because that would hurt code readability.)

Two points on the optional arguments:

• polymorphic comparison is arguably a dangerous feature of the OCaml language; it is convenient and used for convenience, but there is a reasonable argument that we should make it easy to not use it, and in particular not bake it implicitly in our APIs
• modular implicits (type-classes for OCaml), if/when they eventually come to the language, will force us to revisit all this design space in any case

[mars0i]

OK. That sounds fine. Maybe I'll note the similairity in the doc comments.

I.e. you're saying that having = as a default would have an undesirable aspect, too, I gather.

[UnixJunkie]

I updated the issue title, because the LazyList part was done.
I guess others are still welcome.