Howdy! On the discord channel this morning, someone brought up the fact that in type foo = | A of int*int | B of (int*int) that A and B are quite different. A is a constructor taking two arguments, even though it looks like it takes a tuple, while B actually takes a tuple. This means that the tw…

[image] octachron: I think it should be possible to a small paragraph at the end of the variants and records section of the manual. That would seem like the right location. I’d propose something like the following language: Note that although the syntax is almost the same, there is a signif…

This is a good start but I think your text is context-heavy for a beginner, the many circumstances would be really unclear for this audience. An alternative path might be Beware that the syntax for a constructor with multiple arguments type t = A of float * float looks a lot like a tuple but th…

I find the beginning of what you wrote there very clear, i.e.: Beware that the syntax for a constructor with multiple arguments type t = A of float * float looks a lot like a tuple but this is not the case: but I think as a beginner I would have found the rest quite confusing. It discusses thi…

(It might be reasonable to then show a difference in pattern matching between A and B.)

Yes, the part about memory was too technical. I agree that an example would complement nicely your text: let a_to_tuple a = match a with | A (x,y) -> (x,y) let b_to_tuple b = match b with | B t -> t Also, I would rename the second type to s, to avoid using type-directed disambiguation (which …

[image] n4323: I wonder what Reason does here? Reason does the same (different syntax for each), but specifically mentioned about it . Be careful not to confuse a constructor carrying 2 arguments with a constructor carrying a single tuple argument: type account = | Facebook(string, int) /*…

Ok, but to construct the values, does it also allow the same syntax in both cases like OCaml syntax does? I.e. is let a = Facebook("", 0) and let a2 = Instagram("",0) valid? I find that aspect the most confusing.

IMO the most confusing part is that let b_to_tuple b = match b with B (x, y) -> (x, y) is also valid.

I see your point, but since B t is a valid pattern, it seems coherent that expanding t to (x,y) works.

[image] n4323: IMO the most confusing part is that let b_to_tuple b = match b with B (x, y) -> (x, y) is also valid. Yah. It might be important to note that but note the difference in what it means. Because the syntax is confusing, it would be best to err on the side of explicitness and cla…

Sum type constructor declaration subtlety (and documentation wanted!)

Ecosystem

Yaron_Minsky May 4, 2018, 10:39am 21

RWO now has some coverage of this:

https://dev.realworldocaml.org/variants.html

It’s in the end of the opening section.

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Sum type constructor declaration subtlety (and documentation wanted!)

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