Artur Swiderski (Nov 08 2023 at 13:20): Artur Swiderski (Nov 08 2023 at 13:20):Why this code does not work and how to implement equivalent
modifyActive = \ head ->
updatedLst =
if List.isEmpty head.children == Bool.true then
[head]
else
head.children
updatedLst
a = { children : []}
kk =modifyActive a
tree like are rather quite basic structures how to accomplish creating them in Roc
Artur Swiderski (Nov 08 2023 at 16:39):I think I can workaround this problem using Tags a = { children : Childern []} <-- something like that
Artur Swiderski (Nov 08 2023 at 16:40):modifyActive = \ head ->
updatedLst =
when head.children is
Ok children ->
if List.isEmpty head.children == Bool.true then
[head]
else
head.children
Err -> []
updatedLst
This would definitely be something good to add to the examples.
I think with tags you'll still encounter self-referential issues later on.
You could go with something like this:
NodeID : U64
Node : { value: Str, children : List NodeID}
tree : Dict NodeID Node
tree = Dict.empty {}
|> Dict.insert 0 { value: "hey", children: [1]}
|> Dict.insert 1 { value: "hey!", children: []}
This particular code assumes you want to store a Str in each Node.
Anton (Nov 08 2023 at 17:07):If you don't want to store anything in the Node, you could go with this:
NodeID : U64
Children : List NodeID
tree : Dict NodeID Children
tree = Dict.empty {}
|> Dict.insert 0 [1]
|> Dict.insert 1 []
thx I just encounter problems apparently what I am trying is impossible, but your solution will work for sure
Artur Swiderski (Nov 08 2023 at 17:42):btw, this crashes compiler
modifyActivetest = \ head ->
updatedLst =
when head.children is
Children children->
if List.isEmpty children == Bool.true then
[{ locked : Bool.false, children : Children [], value : [] }]
else
when List.last children is
Ok elem ->
if elem.locked == Bool.true then
[{ locked : Bool.false, children : Children [], value : [] }]
else
[modifyActivetest elem]
Err _ -> []
_ -> []
{ locked : head.locked, children : Children updatedLst , value : head.value }
www = { locked : Bool.false, children : Children [], value : [] }
uuu = modifyActivetest www
I think we have a few bugs to file here and track.
Artur Swiderski (Nov 08 2023 at 18:40):Brendan Hansknecht said:
I think we have a few bugs to file here and track.
- Recursion though lists/dicts/etc instead of tags should just work.
- We already have a super old bug for this, but the recursive tag definition issue if they aren't in a single definition.
- The crash is probably a separate issue and should be filed as well.
It turned out that through tags I was not able to accomplish anything as well, it fall apart when I tried do something useful in real life
Artur Swiderski (Nov 08 2023 at 18:50):In terms of language adaptation, as new user intuitively I would rather expect that language (any language) will figure out that what I am trying to do is perfectly fine. But still, if feature is missing (for technical or other reasons) I can live with this Dict approach presented by Anton
Brendan Hansknecht (Nov 08 2023 at 19:52):Yeah, I am not sure about the technical problems here, but I think it should just work. Gonna chat with others.
Brendan Hansknecht (Nov 08 2023 at 20:02):@Artur Swiderski This should work and be a lot nicer than dictionaries:
Node : [Branch (List Node)]
modifyActive : Node -> List Node
modifyActive = \head ->
Branch children = head
if List.isEmpty children then
[head]
else
children
the problem is that my code is sightly bit more complex then example I provided
my target function is
modifyActive : Node -> Node
not
modifyActive : Node -> List Node (this worked in some variations in other it crashed but not self reference problem)
the type I used was: Node : [ { branch : Branch (List Node), ... } ]
I have to check if this works with
Node : [ Element { branch : List Node, ... } ] ?? (I will check my code with this type used)
So to repeat :
Earlier
I fall into problems when I tried (this is equivalent code):
With type like this :
Node : [ { branch : Branch (List Node), ... } ]
I tried something like this :
modifyActive : Node -> Node
modifyActive = \head ->
Branch children = head
updatedList =
if List.isEmpty children then
[ ----> create new node element but no problem here <---]
else
when List.last children is
Ok child ->
# before I checked if child is eligible for modification but here it is irrelevant
List.dropLast children
|> List.append ( modifyActive child ) <--------in this line it failed with reference to itself problem
....
{ head & branch : updatedList } <-- function has this as return value
I think we are just giving you a misleading type error here. This code checks for me and should be similar to your goal above:
modifyActive : Node -> Node
modifyActive = \head ->
Branch children = head
when List.last children is
Ok child ->
List.dropLast children 1
|> List.append (modifyActive child)
|> Branch # This converts back from (List Node) to a Node
Err _ ->
# This is the list empty case, make new node or etc.
head
But I explicitly recover object in line
{ head & branch : Branch updatedList } <--- that was last line of my function
I will check tomorrow but keep in mind that I need different type. then you are using
something like:
Node : [ { branch : (List Node), ... } ] <--- this checked already and it failed on reference to itself
Node : [ { branch : Branch (List Node), ... } ] <--- this checked already and it failed on reference to itself
Node : [ Element { branch : (List Node), ... } ] <--- maybe this will work
Node : [Element { branch : Branch (List Node), ... } ] <--- or this
I need Node to be record (and maybe record usage is the problem ??)
I will try two last options in case of problems , Dict approach will work for sure
Artur Swiderski (Nov 08 2023 at 21:10):I am using like 2 weeks old Roc so maybe that's the problem I will update environment
Brendan Hansknecht (Nov 08 2023 at 21:43):I would expect Node: [Element { branch: List Node, ...}] to work with current roc.
Brendan Hansknecht (Nov 08 2023 at 21:43):Also, I'll try to fix the base roc issue.
Artur Swiderski (Nov 09 2023 at 15:22):this may be the same problem as I mentioned before but this piece of code compiles but crashes
modifyActive = \ op, currentStructHead ->
Node head = currentStructHead
updatedLst =
if List.isEmpty head.children == Bool.true then
op head.children
else
when List.last head.children is
Ok node ->
Node child = node
if child.locked == Bool.true then
op head.children
else
modifyLastInList head.children (modifyActive op node )
Err _ -> []
Node {head & children : updatedLst }
main =
www = Node { locked : Bool.false, children : [], value : [] }
fun1 = (\ lst -> List.append lst (Node { locked : Bool.false, children : [], value : [] } ))
wataC = modifyActive fun1 www
.
.
.
I would rather consider it valid, or at least it would be nice to get some meaningful message out of execution if it is not
Artur Swiderski (Nov 09 2023 at 15:34):I placed it here https://github.com/roc-lang/roc/issues/5970
Last updated: Jul 26 2025 at 12:14 UTC