Artur Swiderski (Dec 10 2023 at 22:12): Artur Swiderski (Dec 10 2023 at 22:12):hi is there any equivalent of c++ template in Roc
I am not saying it should be but just asking
specifically is there a way to do this :
template <class A>
f(A a )
{
// do something with a in generic way
}
No, there is no template system.
Artur Swiderski (Dec 10 2023 at 22:17):btw. templates are the reason among many others why I consider C++ piece of garbage, so lack of this ability although annoying at times may prove to be correct design choice, but indeed sometimes I am lacking something : )
Brian Carroll (Dec 10 2023 at 22:17):But you can have generic types. For example List a means a list of any type.
Artur Swiderski (Dec 10 2023 at 22:17):how to use that in function definition
Brian Carroll (Dec 10 2023 at 22:18):Can you say more about what you want to do?
Artur Swiderski (Dec 10 2023 at 22:19):let say I want to create generic function to add two numbers how to do that ?
Brian Carroll (Dec 10 2023 at 22:21):add : Num a, Num a -> Num a
add = \a, b -> a+b
You can only add numbers so it can't work on any type
Artur Swiderski (Dec 10 2023 at 22:22):will it work if I pass Float number to it ?
Brian Carroll (Dec 10 2023 at 22:22):Yes
Artur Swiderski (Dec 10 2023 at 22:23):Num a stands for something or it is just tag as any in reality ?
Artur Swiderski (Dec 10 2023 at 22:24):I mean what to do for below function to be equivalent
add : Numm a, Numm a -> Numm a
add = \a, b -> a+b
Brian Carroll (Dec 10 2023 at 22:25):Doesn't work. Numbers are special.
Artur Swiderski (Dec 10 2023 at 22:26):and I can't force Numm to act as Num ?
Brian Carroll (Dec 10 2023 at 22:26):No
Artur Swiderski (Dec 10 2023 at 22:26):ok
Brian Carroll (Dec 10 2023 at 22:26):It is built in
Artur Swiderski (Dec 10 2023 at 22:27):what is the role fo this "a" added after Num isn't Num just enough if it is build in ?
Brian Carroll (Dec 10 2023 at 22:28):We started with the most complicated example unfortunately!
Brian Carroll (Dec 10 2023 at 22:28):The a is a type variable. It's similar to the typename in the C++ template
Artur Swiderski (Dec 10 2023 at 22:29):but Num specifies some restriction on this a ??
Brian Carroll (Dec 10 2023 at 22:31):Yes Roc has builtin number types and all of them are specializations of the generic typeNum a.
Brian Carroll (Dec 10 2023 at 22:31):Numbers are the most complicated, let's talk about lists first for a minute
Brian Carroll (Dec 10 2023 at 22:32):So List a means a list of anything
List User is a list of some type called User
List U8 is a list of unsigned 8 bit integers
Brian Carroll (Dec 10 2023 at 22:33):So List a is a generic type and List U8 is a specialized or concrete type.
Brian Carroll (Dec 10 2023 at 22:33):That is how generic types work
Artur Swiderski (Dec 10 2023 at 22:34):can I put List b or "a" is keyword also ?
Brian Carroll (Dec 10 2023 at 22:34):Yes
Brian Carroll (Dec 10 2023 at 22:34):List b or List whatever
Brian Carroll (Dec 10 2023 at 22:34):But you can't change the List and you can't change the Num
Brian Carroll (Dec 10 2023 at 22:35):For numbers, U8 is a type alias for some Num a where I can't remember what the a is
Artur Swiderski (Dec 10 2023 at 22:35):Btw different topic ,I have this problem something is returning
-> Task {} *
but I somehow was not able to return
Task.err "something"
Brian Carroll (Dec 10 2023 at 22:36):It's time for bed where I live so maybe someone else can answer that one, and I'd suggest making a new topic
Artur Swiderski (Dec 10 2023 at 22:37):ok thx anyway
Anton (Dec 11 2023 at 10:39):If a function is returning Task {} *, the * means the error part of the Task is empty. With -> Task {} * you could only return Task.ok {} or call a function like Stdout.line "done", which also returns Task {} *.
If you want to return Task.err "something" the signature would need to be -> Task <whatever> Str. <whatever> depends on what you want the function to return on success.
Artur Swiderski (Dec 11 2023 at 11:20):thx for explanation but what * means ? I was thinking it means anything just "*" coincides with anything in my mind : )
Anton (Dec 11 2023 at 12:00):It used to confuse me too :)
We have a brief section on it in the tutorial.
The * means compatible with any type.
For List.len we have the signature len : List * -> Nat because we can pass it a list of Str, U64, ... . To calculate the number of elements in the list, it does not actually matter what an element is.
When you return something with a wildcard type, it may not be immediately clear what that means. Say we have wildcardList : List *, what could we fill in for wildcardList = ...? The only value that can represent a list of any type is the empty list. So the only option is wildcardList = [].
With Task it's very similar. If we have the type Task Str *, we have Str for the success value and * for the error value. Similar to List *, the only error type that could satisfy * is the empty error.
Isaac Van Doren (Dec 11 2023 at 13:22):What’s the reason for using Task a * rather than Task a []?
Anton (Dec 11 2023 at 13:30):Excellent question! I don't know, @Richard Feldman?
Fletch Canny (Dec 11 2023 at 15:47):There's still some details about Roc's type system that I'm hazy on, but wouldn't the difference be that you can't use Task a * when chaining Tasks with a tagged union error type?
Task.await has a type of Task a b, (a -> Task c b) -> Task c b, and if I use Task a [] for Task a b and Task c [A, B, C] for Task c b, the type system can unify [] and[A, B, C]. But if the error type were say, Task a {} then it couldn't unify those types. Since Task a * my be Task a {} that means we can't necessarily unify those types and thus we would get a type error.
Brendan Hansknecht (Dec 11 2023 at 15:59):Give return types are open tags by default now, I believe that is correct.
Brendan Hansknecht (Dec 11 2023 at 15:59):In the old days, that would not be the case
Brendan Hansknecht (Dec 11 2023 at 16:03):That said, even with a closed tag they are slightly different. * has no constructable error case. It suggests that someone has handled all errors and made them into successes.
[] has an error case with no info in it. So it basically means something failed but we have dropped all information about the failure
Isaac Van Doren (Dec 11 2023 at 16:16):I don’t see how [] is an error case with no info given that it is impossible to construct an element of []
Richard Feldman (Dec 11 2023 at 16:30):Isaac Van Doren said:
What’s the reason for using Task a * rather than Task a []?
the answer varies depending on how it's being used - where are you seeing the type in question? :big_smile:
Isaac Van Doren (Dec 11 2023 at 16:34):I don’t see how [] is an error case with no info given that it is impossible to construct an element of []
Ah wait I forgot that [] is open
Isaac Van Doren (Dec 11 2023 at 16:34):The first one that comes to mind is Stdout.line
Isaac Van Doren (Dec 11 2023 at 16:34):So Task {} *
Brendan Hansknecht (Dec 11 2023 at 17:14):Yeah sorry, I think Task {} [] and Task {} * are equivalent if [] is a closed union.
Brendan Hansknecht (Dec 11 2023 at 17:15):But given roc makes all returned unions into open unions, they quickly become not equivalent.
Richard Feldman (Dec 11 2023 at 17:16):well also even an open [] is more restrictive than * because [] says "this must be some sort of tag union"
Richard Feldman (Dec 11 2023 at 17:16):and you can have errors that aren't tag unions
Brendan Hansknecht (Dec 11 2023 at 17:16):Definitely problematic to send an opinion union version of Task {} [] back to a platform. It would not have a know layout when generating the platform glue
Richard Feldman (Dec 11 2023 at 17:17):so if Stdout.line returned a Task {} [] then you couldn't use it with a Task {} Str, for example (e.g. if someone used Task.mapErr to turn an error into a string)
Artur Swiderski (Dec 11 2023 at 19:05):ok I can't intellectually follow the discussion : ) but I gather this is the reason why Task {} * is used ?
That said, even with a closed tag they are slightly different. * has no constructable error case. It suggests that someone has handled all errors and made them into successes.
btw what {} stands for here, why specifically this value?
Brendan Hansknecht (Dec 11 2023 at 19:16):{} is just an empty record.
Brendan Hansknecht (Dec 11 2023 at 19:17):It is common used for kinda a unit type. A concrete type that compiles to nothing.
Isaac Van Doren (Dec 12 2023 at 03:28):and you can have errors that aren't tag unions
Aren't there no possible errors for a Task {} *?
Regardless, isn't there not much you can do with errors if you don't know what type they are?
Brendan Hansknecht (Dec 12 2023 at 03:40):Aren't there no possible errors for a Task {} *?
This is were it gets confusing depend on how things are worded. Really easy to say something that is easily misinterpreted from what was meant. You are correct that there are no possible constructable errors that match the type Task {} *.
Richard meant that Task {} * will unify with any error type. That means I can do:
Note: Stdout.line is a Task {} *. I has a * error case but merges to a Str error case here just fine.
main =
errStr <- strErrTask |> Task.onErr
Stdout.line "error was: \(errStr)"
strErrTask =
_ <- Stdout.line "Some text" |> Task.await
Task.err "This is a string err"
If I were to redefine Stdout.line to be Task {} [], I would get this error:
── TYPE MISMATCH ───────────────────────────────────── examples/helloWorld.roc ─
This 2nd argument to |> has an unexpected type:
11│> _ <- Stdout.line "Some text" |> Task.await
12│> Task.err "This is a string err"
The argument is an anonymous function of type:
{} -> Task * Str
But |> needs its 2nd argument to be:
{} -> Task * []
Task {} [] can only unify with other errors that are tags.
Isaac Van Doren (Dec 12 2023 at 04:52):Aha! That makes sense. Thanks for the detailed explanation!
Last updated: Jul 26 2025 at 12:14 UTC