Brendan Hansknecht (Apr 20 2024 at 15:42): Brendan Hansknecht (Apr 20 2024 at 15:42):Spinoff from #beginners > combining open unions, is the following a bug?
interface Test
exposes []
imports []
generateOpenOkStr : Str -> [Ok Str]*
generateOpenOkStr = \s ->
Ok s
restrictsToClosed : [Ok Str] -> Bool
restrictsToClosed = \Ok s->
!(Str.isEmpty s)
expect
# `in` is an open tag union, returned as `[Ok Str]*`.
# Yet, if I type it as such, it breaks due to type mismatch
in : [Ok Str]*
in = generateOpenOkStr ""
out = restrictsToClosed in
out == Bool.false
This 1st argument to restrictsToClosed has an unexpected type:
18│ out = restrictsToClosed in
^^
This in value is a:
[…]*
But restrictsToClosed needs its 1st argument to be:
[…]
Works if in : [Ok Str]* is commented out.
Anton (Apr 20 2024 at 15:47):It does work with in : [Ok Str]_ I think it's that bug, I'll check if we have an issue for that.
Anton (Apr 20 2024 at 15:54):No, #5660 is different, perhaps this is intended behavior, given that the error message states:
(The * means something different when the tag union is an argument to a
function, though!)
Yeah, it may be a proper error. That said, it definitely feels wrong as a proper error given I am returning a [Ok Str]* from the function.
Brendan Hansknecht (Apr 20 2024 at 15:58):Am I storing the return type [Ok Str]* or the argument type [Ok Str]*? Very unclear.
Brendan Hansknecht (Apr 20 2024 at 15:58):Whatever the case, I definitely think we need a clearer explanation here at a minimum.
timotree (Apr 20 2024 at 17:51):I think this is a reasonable way for * and _ to work. I can reproduce this behavior with List * and List _ and I think it makes sense
constructList : List _
constructList =
empty : List _
empty = []
list1 : List _
list1 = List.append empty 1 # no error
list1
constructList2 : List *
constructList2 =
empty : List *
empty = []
list1 : List *
list1 = List.append empty 1 # ERROR (mismatch between Num * and *)
list1
The mental model here is that within a top-level declaration, * is a placeholder instantiated by each caller and _ is a placeholder which we get to instantiate
timotree (Apr 20 2024 at 17:54):Any time you write myLocal = myTopLevel where myTopLevel : MyType *, you get myLocal : MyType _. This is exactly what it means that * is chosen by each caller; we are a caller of myTopLevel, so now we get to choose how to instantiate the *
Brendan Hansknecht (Apr 20 2024 at 17:56):I agree that without function calls, that all makes sense. I think it feels the worst when returning something from a function:
getEmpty: {} -> List *
getEmpty = \{} -> []
# I know why this doesn't work, but it feels like it should work.
# `empty` is simply specifying the exact same type as what was returned by `getEmpty`
constructList : List _
constructList =
empty : List *
empty = getEmpty {}
list1 = List.append empty 1 # ERROR (mismatch between Num * and *)
list1
I wonder if it would help to have some sort of explicit input type vs return type *....not sure, but this definitely is confusing even if it can be logically explained.
timotree (Apr 20 2024 at 17:57):But constructList can't possibly have type List * there, right? The list it returns always include a number
Brendan Hansknecht (Apr 20 2024 at 17:57):Sorry, meant List _ or List (Num *) there.
timotree (Apr 20 2024 at 18:03):What if we changed the meaning of * inside of type annotations for locals to mean the same thing as _?
timotree (Apr 20 2024 at 18:05):I don't think that would result in any new type errors, because we can always assign _ to * if needed. e.g. the following typechecks
takesAnyList : List * -> {}
takesAnyList = \anyList ->
underscoreList : List _
underscoreList = anyList # no error
{}
But it has the downside of losing some precision in what type annotation are expressible. You could no longer assert that a local has a type which is indeed chosen by the caller.
timotree (Apr 20 2024 at 18:11):Ooh there is some interesting non-uniformity with how local function definitions behave here
myNested : List U32
myNested =
myEmpty : {} -> List *
myEmpty = \{} -> []
List.append (myEmpty {}) 1 # no error, but if we change to `myEmpty : List *` we get an error like before
oh actually this non-uniformity is observable even without type annotations:
nestedPolymorphism : (List U32, List Str)
nestedPolymorphism =
# myEmpty : {} -> List *
myEmpty = \{} -> []
(List.append (myEmpty {}) 1, List.append (myEmpty {}) "1")
failedPolymorhpism : (List U32, List Str)
failedPolymorhpism =
# myEmpty : List _
myEmpty = []
(List.append myEmpty 1, List.append myEmpty "1") # ERROR (mismatch between Str and Num *)
I guess local function definitions can be polymorphic but local values cannot
Brendan Hansknecht (Apr 20 2024 at 18:19):Yeah, requiring functions for polymorphism was something @Ayaz Hafiz did for complexity and compile time reasons.
Top levels are special to allow polymorphism. They are always turned into functions.
timotree (Apr 20 2024 at 18:27):Oh wait, it seems like * in local annotations is currently completely useless, because it doesn't even specify which caller-chosen variable it's referring to
twoEmptyLists : (List *, List *)
twoEmptyLists =
empty1 : List * # The two `*` in the signature are distinct variables. Which one are we referring to?
empty1 = []
empty2 : List *
empty2 = []
(empty1, empty2) # ERROR (mismatch between (List *, List *) and (List *, List *))
In fact, you get the same error (albeit with a better message) even if there's only one * in the signature
myEmpty : List *
myEmpty =
empty : List *
empty = []
empty # ERROR
I think the only way to use * in a type annotation for a local successfully is if the enclosing function has _s in its type
myEmpty2 : List _
myEmpty2 =
empty : List *
empty = []
empty # no error
Yeah, so maybe the error should say "Putting * in a type definition is not really useful. Consider using _ instead." The only cases where * is really useful is for function inputs.
Brendan Hansknecht (Apr 20 2024 at 18:31):That may not be a totally fair statement, but I do think it is mostly accurate
timotree (Apr 20 2024 at 21:31):Brendan Hansknecht said:
Top levels are special to allow polymorphism. They are always turned into functions.
Is this accurate? I just read Let-generalization: Let’s not? and it seems to specify that only lambdas and numerals can be polymorphic, regardless of whether they're top-level or local. Isn't the fact that top-levels constants can't always be polymorphic exactly why we have Dict.empty {} instead of just Dict.empty?
timotree (Apr 20 2024 at 21:33):I think the bug tracked at https://github.com/roc-lang/roc/issues/5536 is the same as the bug in this thread. In that example it's actually a top-level definition being annotated!
Brendan Hansknecht (Apr 20 2024 at 21:37):I thought we made all top levels into thunks to help with this
Brendan Hansknecht (Apr 20 2024 at 21:37):But I don't know all the details
Ayaz Hafiz (Apr 20 2024 at 22:13):only values that are syntactic functions or numbers can be polymorphic, regardless of whether they're on the toplevel or not
Ayaz Hafiz (Apr 20 2024 at 22:13):And in nested contexts, only syntactic functions can be polymorphic
Ayaz Hafiz (Apr 20 2024 at 22:16):To the original post, I think the program should fail to compile, but the bug is that you should not be able to type in : [Ok Str]*. That type doesn't make any sense in Roc, since it claims that in is polymorphic, but it's not.
Brendan Hansknecht (Apr 20 2024 at 23:04):Thanks for the clarity
Richard Feldman (Apr 20 2024 at 23:32):ok cool, so it sounds like the fix here is to make a new compiler error for annotations of this form, yeah?
Ayaz Hafiz (Apr 20 2024 at 23:39):yeah I think so. I'm kind of surprised such a check isn't already around
Anton (Apr 22 2024 at 09:06):I want to make an issue for this, can someone summarize in which cases an annotation containing ]* needs to error?
timotree (Apr 22 2024 at 15:43):If an annotation contains * or introduces a new type variable, the associated definition must be either a syntactic function or a number. So e.g.
# OK. It's a number
three : Int *
three = 3
# OK. It's a function
returnEmpty : {} -> List *
returnEmpty \{} -> []
# Error. It's a list literal
empty : List *
empty = []
# Error. It's a tag
red : [Red, Green, Blue]*
red = Red
# Error. It's a division operation
pi : Frac *
pi = 22 / 7
Let-generalization: Let’s not? has an error message we could use as a starting point for all these cases:
-- NUMBER IS NOT POLYMORPHIC -- The type annotation on `piApprox` suggests that it can be used polymorphically: 1 | piApprox : Frac * 2 | piApprox = 22 / 7 Unfortunately, I can't use `piApprox` as any fractional type! I can only use it as exactly one of `Dec`, `F32`, or `F64`. If you want me to infer the fractional type that should be used, you can use an inference annotation instead of `*`: piApprox : Frac _ If you explicitly want `piApprox` to be able to be used polymorphically, consider making it a thunk: piApprox : {} -> Frac * piApprox = \{} -> 22 / 7
Anton (Apr 22 2024 at 16:33):Lovely thanks @timotree! I made #6663
Last updated: Jul 26 2025 at 12:14 UTC