Monica (Jul 01 2024 at 22:03): Monica (Jul 01 2024 at 22:03):Can anyone explain where I'm going wrong with the following:
Students : List Student
Res : Result Students [Err Str]
readStudents : JsonData -> Res
readStudents= \json ->
when json is
Object obj ->
students = Dict.get obj "students"
when students is
Ok a -> readStudentsList a
_ -> Err "students not found in the object"
_ -> Err "Expected Json Object"
readStudentsList : JsonData -> Result Students [Err Str]
...
on my second branch of my second pattern match, I'm getting an error of
Something is off with the 2nd branch of this
when expression:
// code above
This Err tag application has the type:
[
Err Str,
Ok Students,
]
But the type annotation on readStudents says it should be:
Result Students [Err Str]
Is this not how I should be chaining together functions which return Results of the same type?
Luke Boswell (Jul 01 2024 at 23:21):Maybe you want this?
students =
Dict.get obj "students"
|> \studentsResult ->
when studentsResult is
Ok a -> readStudentsList a
_ -> Err "students not found in the object"
Something like this might be easier to follow
students =
Dict.get obj "students"
|> Result.map readStudentsList
|> Result.mapErr \_ -> Err "students not found in the object"
If you can share the type of JsonData and Student I can make a more complete example
Luke Boswell (Jul 01 2024 at 23:28):Ok so this code works, but I've made some assumptions
emptyStudents : Students
emptyStudents = []
readStudents : JsonData -> Result Students [Err Str]
readStudents = \json ->
when json is
Object obj ->
Dict.get obj "students"
|> Result.map \_ -> emptyStudents
|> Result.mapErr \_ -> Err "students not found in the object"
_ ->
Err (Err "Expected Json Object")
I think one possible point of confusion is that you have named the Tag in your error union as Err which is also the name of the tag in a Result, so to return a valid Result _ [Err Str] you need to use Err (Err "my msg")
Luke Boswell (Jul 01 2024 at 23:31):If you change the error to something more descriptive like
readStudents : JsonData -> Result Students [StudentsNotFoundInObj, ExepectedJsonOjbect]
readStudents = \json ->
when json is
Object obj ->
Dict.get obj "students"
|> Result.map \_ -> emptyStudents
|> Result.mapErr \_ -> StudentsNotFoundInObj
_ ->
Err ExepectedJsonOjbect
Is it more idiomatic to to use tag unions like this for errors rather than storing them in Strings?
Luke Boswell (Jul 02 2024 at 08:08):Yes, I would say so. It's nice to be able to merge them and also exhaustively handle them too elsewhere in the program.
Monica (Jul 02 2024 at 08:10):This is where I've gotten to with your help:
Student : {
name : Str,
modules : List (Module)
}
Module : {
name : Str,
credits : I64
}
JsonData : [
String Str,
Number U64,
Null,
Object (Dict Str JsonData),
Arr (List JsonData),
]
readStudents : JsonData -> Result (List Student) [StudentsNotFoundInObj, ExepectedJsonOjbect, EmptyListOfStudents, ExpectedJsonArray]
readStudents= \json ->
when json is
Object obj ->
Dict.get obj "students"
|> Result.map \_ -> readStudentsList
|> Result.mapErr \_ -> StudentsNotFoundInObj
_ -> Err ExepectedJsonOjbect
readStudentsList : JsonData -> Result (List Student) [EmptyListOfStudents, ExpectedJsonArray]
But I am confused about the error message I am getting
This when expression produces:
Result (JsonData -> Result (List Student) [
EmptyListOfStudents,
ExpectedJsonArray,
]) […]
But the type annotation on readStudents says it should be:
Result (List Student) […]
Is this because my call to readStudentsList also returns a result? And I need to flatten them in order to either get my Ok or my Err?
Luke Boswell (Jul 02 2024 at 08:13):Dict.get obj "students" will return a result, you are mapping the Ok value into another function readStudentsList which also returns a result. That is why you have nested results. Also you you have no actually provided a value to call readStudentsList, so that is why it shows as a Fn.
Try using |> Result.try readStudentsList instead
Luke Boswell (Jul 02 2024 at 08:14):Result.try returns a new result, if the previous is an Err than it doesn't do anything.
Monica (Jul 02 2024 at 08:15):Ahhhh
Monica (Jul 02 2024 at 08:15):that compiles yep
Monica (Jul 02 2024 at 08:16):what if I have an error in readStudentsList? Is it possible to bubble up the errors from readStudentsList
Luke Boswell (Jul 02 2024 at 08:21):That is what Result.try will do. :smile:
Monica (Jul 02 2024 at 08:31):Ahhhhh that makes a lot of sense thanks!!
Last updated: Jul 26 2025 at 12:14 UTC