Max, apparently you didn’t understand what Maxime’s proposal is *at all*.
For example, he didn’t propose anything that would change the existence of this type: type Direction = Up | Down | Left | Right 2017-01-15 23:47 GMT+01:00 Max Goldstein <maxgoldste...@gmail.com>: > First, I appreciate that this proposal is being made in the spirit of Elm: > seeking to simplify the language, with reference to existing usage (red > sentence in the first post), and trying to solve an existing problem > (serialization of union types). It's clear that Maxime wants to learn and > improve the language. Thank you. > > So let's look at how union types are used, what jobs they are hired to do, > if you will. You've noticed in core that union types with one tag are used > to hide things. Sometimes that's as an opaque type: there is one tag that's > not exposed, and is pattern-matched easily, and the data the tag holds can > change even in a patch release (example here > <https://github.com/mgold/elm-animation/blob/master/Animation.elm#L89-L105>). > Core also contains a rare pattern of, say *type Task x a = Task* where > the *Task* value defined is not actually used anywhere because it's > actually a native implementation. Serializing arbitrary union types will > have to account for this case. > > A more realistic use of union types is as a finite set of labels, even if > these labels don't carry any information along with them. For example: > > type Direction = Up | Down | Left | Right > > From this we derive pattern matches with static > (compile-time) exhaustiveness checking, which is a huge improvement in > reliability and refactorability over "stringly typed" conventions (instead > of strongly typed, get it?) that you'll see in JavaScript and the like. > Replacing the Direction type with a record including a string removes this > huge improvement. > > Adding data to these tags allows for the classic use case of data that > only makes sense in certain contexts. I remember working with a C++ > graphics library where certain fields of structs were only defined if an > enum was set to a particular value. But this isn't really a separate use > case from labels without data, since you can freely mix within a type. The > RemoteData > type > <http://package.elm-lang.org/packages/krisajenkins/remotedata/4.2.1/RemoteData#RemoteData>does > this to great effect. > > More theoretically, union types and records are not the same; they are > actually duals. Union types are referred to as "sum types" and records (and > tuples) are "product types". Here's the reasoning: consider two types, *a* > and *b*. Let's denote the number of values of type *a* as |a|, and > similarly |b|. If we construct the sum type *Result a b* then there are > |a| values for the Err case and |b| values for the Ok case. So |Result a b| > = |a| + |b|, hence sum types. For the pair (a, b) we can pick any *a* and > any *b* so |(a,b)| = |a|*|b|, hence produce types. > > If you want to produce (create) a value of sum type, such as *Result a b*, > you can pick either case to work with. But when you consume (inspect) a > value of that type, you must be prepared to handle all cases. For product > types it is reversed: to produce a value of type (a,b) you must have both > an *a* and a *b*, but when you consume such a value, you can pick out the > one you want. > > So, while I appreciate the gesture, I don't think this will work. > > -- > You received this message because you are subscribed to the Google Groups > "Elm Discuss" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to elm-discuss+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Elm Discuss" group. To unsubscribe from this group and stop receiving emails from it, send an email to elm-discuss+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
