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Regarding the type field, look for "enum EventType type;" in the article. (In practice I would not use "enum EventType", but simply "int" - but anyway). What I

Regarding the type field, look for "enum EventType type;" in the article. (In practice I would not use "enum EventType", but simply "int" - but anyway). What I would do with it? Like the result of (x+y), or any type of data, I can use it for whatever I need. I might simply want to count the number of occurences of each "type". Often I'm simply not interested in the data besides the "type", and in these cases I find that case-matching is a lot of boilerplate. And it doesn't reify the "type" to authoritative runtime values. Which, as you say, might be done with reification support from the language, but I don't want to bother with that -- and it would not allow me to choose the possible values of the type field, which might be a requirement if the options are defined somewhere else.

tom_mellior 9y ago
> Regarding the type field, look for "enum EventType type;" in the article. OK. If I understand correctly, you want to use that field in other contexts than doing a switch on it, whereas in Haskell the usual way of consuming the type tag is to do such a switch, as in the article's example: case event of ClickEvent x y -> <do click stuff> PaintEvent color -> <do paint stuff> // there are no other possible cases, so no default case needed > For example, I could separate a large array of sum-type data into separate arrays, and also process them separately. Sure, but you can do that with a case expression as above. Here is a complete example: data Event = ClickEvent Int Int | PaintEvent Int -- separate a list of Events into two lists, the first holding the payloads -- of the click events, the other the payloads of the paint events separate :: [Event] -> ([(Int, Int)], [Int]) separate [] = ([], []) separate (event : events) = case event of ClickEvent x y -> ((x, y) : clicks, colors) PaintEvent color -> (clicks, color : colors) where (clicks, colors) = separate events example = [ClickEvent 1 2, PaintEvent 42, ClickEvent 3 4, PaintEvent 23] Running it: > separate example ([(1,2),(3,4)],[42,23]) You can now go ahead and process the list [(1,2),(3,4)] separately from [42,23]. (Depending on the exact specification and performance requirements, the code might be written a bit differently in the real world.) EDIT: Ah, you edited your post while I was writing. Oh well. Regarding reifying constructors as integers or whatever, yes, you might have to write a two-line function to do that.
- jstimpfle 9y ago
  Yep, sorry for the edit. I did the arrays example but then noticed it was not really an answer to your question, since it can be done with sum types in almost the same way.

tom_mellior 9y ago

> Regarding the type field, look for "enum EventType type;" in the article. OK. If I understand correctly, you want to use that field in other contexts than doing a switch on it, whereas in Haskell the usual way of consuming the type tag is to do such a switch, as in the article's example: case event of ClickEvent x y -> <do click stuff> PaintEvent color -> <do paint stuff> // there are no other possible cases, so no default case needed > For example, I could separate a large array of sum-type data into separate arrays, and also process them separately. Sure, but you can do that with a case expression as above. Here is a complete example: data Event = ClickEvent Int Int | PaintEvent Int -- separate a list of Events into two lists, the first holding the payloads -- of the click events, the other the payloads of the paint events separate :: [Event] -> ([(Int, Int)], [Int]) separate [] = ([], []) separate (event : events) = case event of ClickEvent x y -> ((x, y) : clicks, colors) PaintEvent color -> (clicks, color : colors) where (clicks, colors) = separate events example = [ClickEvent 1 2, PaintEvent 42, ClickEvent 3 4, PaintEvent 23] Running it: > separate example ([(1,2),(3,4)],[42,23]) You can now go ahead and process the list [(1,2),(3,4)] separately from [42,23]. (Depending on the exact specification and performance requirements, the code might be written a bit differently in the real world.) EDIT: Ah, you edited your post while I was writing. Oh well. Regarding reifying constructors as integers or whatever, yes, you might have to write a two-line function to do that.

jstimpfle 9y ago
Yep, sorry for the edit. I did the arrays example but then noticed it was not really an answer to your question, since it can be done with sum types in almost the same way.