Trouble with mapping Tasks

[Daniel Bachler]

Hi,

I am new to Elm and I am having trouble mapping a Task. The code I am working on is based on https://github.com/simonh1000/file-reader which in turn is pretty close to the example 5 in the elm architecture https://github.com/evancz/elm-architecture-tutorial/blob/master/examples/5/RandomGif.elm .

The problem I have is in the method that get's called as part of the update function when the user gives me a file to load. There is a native method that expects a JS File object and returns a Task giving the content as a DataURL (or an error).

It looks like this:
readAsDataUrl : Json.Value -> Task Error Json.Value
readAsDataUrl = Native.FileReader.readAsDataUrl

I try to use it like so, but this doesn't work

loadData : Json.Value -> Effects Action
loadData file =
    readAsDataUrl file
        |> Task.map wrapInResult
        `Task.onError` (\err -> Task.succeed (Result.Err err))
        |> Task.map LoadImageCompleted       
        |> Effects.task

wrapInResult : Json.Value -> Result a Json.Value
wrapInResult val = Result.Ok val

What I expected this to do was
readAsDataUrl returns a Task Error Value
this is mapped to a Task Error (Result a Value)
the error case is mapped to a Task x (Result Error Value)
the task is mapped into a Task x Action // one of the action cases expects a result as specified in the line above)
the task is turned into an Effects Action

But what happens is that I get two type mismatch errors:

λ elm make .\DataUrlExample.elm
-- TYPE MISMATCH ------------------------------------------ .\DataUrlExample.elm

The right argument of (|>) is causing a type mismatch.

122│     readAsDataUrl file
123│>        |> Task.map wrapInResult
124│>        `Task.onError` (\err -> Task.succeed (Result.Err err))

(|>) is expecting the right argument to be a:

    Task.Task Error Value -> a

But the right argument is:

    Task.Task a (Result b c)

Hint: I always figure out the type of the left argument first and if it is
acceptable on its own, I assume it is "correct" in subsequent checks. So the
problem may actually be in how the left and right arguments interact.

-- TYPE MISMATCH ------------------------------------------ .\DataUrlExample.elm

The left argument of `onError` is causing a type mismatch.

123│>           Task.map wrapInResult
124│         `Task.onError` (\err -> Task.succeed (Result.Err err))

`onError` is expecting the left argument to be a:

    Task.Task a b

But the left argument is:

    Task.Task a Value -> Task.Task a (Result Error Value)

Detected errors in 1 module.

I thought that the type signatures should check out, but I am obviously missing something. Unfortunately, I don't really understand where the compiler and I differ. Does it have to do with the concrete bound types for Result?

I had the whole thing working when I used toMaybe instead of mapping and `onError` and gave the action a Maybe Value, but I would like to keep the error so I can display a decent error message, hence the attempt to use the Result type.

Any help would be very much appreciated. Thanks!

Daniel

[Janis Voigtländer]

Have you considered using toResult?

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[Daniel Bachler]

... Doh! You are right of course. For some reason I thought that this would not match the types I need but it is precisely what I need of course. Thanks for the quick solution!

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Daniel Bachler
http://www.danielbachler.de
http://www.iconoclash-photography.com
phone, address: http://www.danielbachler.de/contact

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