TypeError: result is undefined when using tasks

[hoelzro]

I'm not sure if this belongs here or in evancs/elm-http, but from what I understand, I think this may be a general bug with tasks (if it's a bug at all).

I've been experimenting with Elm, and I wanted to create an example that sends requests to an HTTP server every click, but ignore clicks that happen when a request is in flight, so as to not overload the server or deal with requests that would involve stale data. I very well may be doing in wrong, but my example program results in the following JS error:

TypeError: result is undefined Test.elm:9216:11

The line in question looks like this:

if (tag === 'AndThen' || tag === 'Catch')
    {
        var result = mark('runnable', task.task);
        while (result.status === 'runnable') // line 9216
        {
            result = stepTask(onComplete, root, result.task);
        }

I should also mention that Elm keeps sending requests to the server even when I'm not clicking. The code I wrote to come across this bug is below; please also feel free to give me any feedback on how to write better Elm, as I am a newbie. =)

Test.elm:

import Http

import Graphics.Element exposing (Element, show)
import Mouse
import Task exposing (Task, andThen)

import Debug

type alias ClickState = {
    clicks          : Int,
    requestInFlight : Bool,
    lastResponse    : String
}

initialState : ClickState
initialState = {
    clicks          = 0,
    requestInFlight = False,
    lastResponse    = "" }

type Action =
      Click
    | Response String

squareMailbox : Signal.Mailbox String
squareMailbox = Signal.mailbox "0"

squareTask : Int -> Task Http.Error ()
squareTask value =
  let url    = "http://localhost:5000/square/" ++ (toString value)
      httpTask = Http.getString url
      sendTask = Signal.send squareMailbox.address
  in
      httpTask `andThen` sendTask

applicationEvents : Signal Action
applicationEvents =
  Signal.merge
    (Signal.map (\_ -> Click) Mouse.clicks)
    (Signal.map Response squareMailbox.signal)

update : Action -> ClickState -> ClickState
update action {clicks, requestInFlight, lastResponse} =
  case action of
    Click        -> { clicks = clicks + 1, requestInFlight = True, lastResponse = lastResponse }
    Response res -> { clicks = clicks, requestInFlight = False, lastResponse = res }

mainSignal : Signal ClickState
mainSignal =
  Signal.foldp update initialState applicationEvents

extractRequest : ClickState -> Maybe Int
extractRequest {clicks, requestInFlight} =
    if requestInFlight
      then Nothing
      else Just clicks

port squareTaskPort : Signal (Task Http.Error ())
port squareTaskPort = Signal.map squareTask <| Signal.filterMap extractRequest 0 mainSignal

view : ClickState -> Element
view {clicks, lastResponse} =
  show (clicks, lastResponse)

main : Signal Element
main = Signal.map view mainSignal

app.psgi (a simple stupid Perl webserver that forwards everything but /square/ to elm-reactor; /square/ itself takes a number in the URL, squares that after sleeping a second, and outputs the result)

#!/usr/bin/env perl

use strict;
use warnings;

use Plack::App::Proxy;
use Plack::Builder;

my $square_app = sub {
    my ( $env ) = @_;

    my $value = 0;

    if($env->{'PATH_INFO'} =~ m{/(\d+)$}) {
        sleep 1;
        $value = $1;
    }

    my $answer = $value * $value;

    [
        200,
        ['Content-Type' => 'text/plain'],
        [$answer],
    ]
};

my $proxy = Plack::App::Proxy->new(remote => 'http://localhost:8000');

builder {
    mount '/square' => $square_app;
    mount '/'       => $proxy->to_app;
};

[evancz]

I'm not sure if this problem still exists, but we should migrate it to an http://sscce.org that uses Elm 0.17.1 and the latest libraries.

[evancz]

I'm not sure if this problem still exists, but we should migrate it to an http://sscce.org that uses Elm 0.17.1 and the latest libraries.