I have a function
convertMsg : Msg1 -> List Msg2
where Msg1 and Msg2 are certain message types. And I would like to turn this into a function:
convertCmd : Cmd Msg1 -> Cmd Msg2
Which would for every message in the batch replace it with some messages possibly none or more than 1.
As a Haskell programmer at heart I immediately reach for a monadic bind ((>>=) in Haskell and andThen in the Elm parlance), a function with the type:
bind : (a -> Cmd b) -> Cmd a -> Cmd b
I can easily change my convertMsg to be the following:
convertMsg : msg1 -> Cmd Msg2
At which point it would be just perfect for the bind.
But looking in Platform.Cmd, there isn't such a function I can find. There's a map which is similar, but convertMsg can't really be convertMsg : Msg1 -> Msg2 since it doesn't always give back exactly one message.
Is there a way to achieve this? Is there some limitation to the Cmd type that would prevent this sort of thing?
What you're trying to do to messages, how you might, and whether it's a good plan
I promise I'll try to answer what I think you're trying to do, but first I think there's a more important thing...
You're perhaps assuming that Cmd is analogous to IO from Haskell, but Cmd is asynchronous and isn't designed to chain actions. Your update is what glues consequences to outputs:
update : Msg -> Model -> (Model,Cmd Msg)
At the end of your update, you can issue a Cmd Msg to ask elm to do something externally, usually passing it a constructor with which it can wrap its output. This output comes back to your update function.
What you don't do is chain Cmds together as you would in a monad. There's no bind for Cmd, or to put it another way, the only bind for Cmd is your update function!
Now I suppose that if you wanted to catch a MyComplexMsg : Msg and turn it into [SimpleMsg1,SimpleMsg2], you could pattern match for it in your update function, leave the model unchanged and issue a new Cmd Msg, but what command would you be running the second time?
You could certainly take a pure Msg -> List Msg function and use Cmd.map to apply it, or apply it manually in the pattern match at the beginning of update, like
update msg model = case msg of
MyComplexMsg -> myHandler [SimpleMsg1,SimpleMsg2]
...
or even go full state monad style with
update msg model0 = case msg of
MyComplexMsg ->
let
(model1,cmd1) = update SimpleMsg1 model0
(model2,cmd2) = update SimpleMsg2 model1
in
(model2,Cmd.batch [cmd1,cmd2])
to try to emulate monadic bind, but I don't know why you might ever want this, and a lot of advice in the elm literature and community is that if you're calling update from update you're probably doing it wrong. Make a separate single-purpose helper function for that stuff instead of re-running your entire program logic twice!
Let go of your need to have a monad
I suspect that what's going wrong is that you're not letting go of a monadic control flow mentality. update is where it's at. update is where you make things happen. User input and asynchronous messages are your drivers, not sequencing. Cmd is just for communicating externally. You don't plumb the results back in, the elm architecture does that for you. Just handle the result of your Cmd (which will arrive as a message) as a branch in your update and it'll all progress nicely, and if the user presses some button of their own choice without you making it happen, so be it. You can handle that too.
I worry that you're trying to write a monad transformer stack in elm, which is a bit like trying to write an object oriented programming library in haskell. Haskell doesn't do object oriented programming, and the sooner folks drop the OO thinking and let go of their need to bundle data and functions together, the sooner they're writing good haskell code. Elm doesn't do typeclasses, it does model/view/update, and does it extraordinarily well. Let go of your need to find and use a monad to control the flow of your program, and instead respond to what messages you're given. Make a Msg for something you want to happen, provide a way to trigger it appropriately in your view and then handle it in your update.
When should one message become three messages?
If one of your messages is really three messages, why isn't it three messages already? If it's just that in response to that particular message, you just need to do three things to your model and issue five commands, why not just have one message and get update to do those three things to your model using pure code and issue the five commands in a batch?
If you need to log the successful login, then get the user's photo, then query the database for their recent activity, then display it all, then I disagree about the immediateness, and they're all asynchronous. You can issue commands to do each of those things in a batch, and when the responses come back you will need to separately deal with each - update your model with the image when it arrives, with the list of recent activity when it arrives. Once your model is in the state that the picture and the recent activity are both there you can change the view, but why not show each as soon as they're there?
Using monads sometimes trains us to think sequentially when we're doing effects programming when we needn't, but now, finally, I'll address what to do when there is a compelling need to sequence commands.
Genuinely necessary sequential commands
Perhaps there really is something sequential that you need. Maybe you have to query some data store for something before you send some request elsewhere. You still don't use a bind, you just use your update:
update msg model = case msg of
StartsMultiStageProcess userID ->
({model|multiStageProcessStatus = RequestedData}
, getDataPart1 userID PartOneReceived )
PartOneReceived userData ->
({model|multiStageProcessStatus = Fired}
, fireRockets userData.nemesis.location RocketResult)
RocketResult r -> if model.multiStageProcessStatus == Fire then
case r of
Ok UtterlyDestroyed ->
....
Ok DamagedBeyondUse ->
....
Err disappointment ->
....
...
A handy point is that if your model doesn't have the required data, it automatically won't show the missing data in the view (sum types for the win), and you can store whatever state your multistage process is in in the model.
You might prefer to put all of those messages into a new type so it becomes indented once further in the handler and won't ever be mixed with other ones in the order, like
MSPmsg msg -> case msg of
Started userId ->
...
GotPartOne userData ->
but much more likely use a helper function like MSPmsg msg -> updateMultiStageProcess.
Concluding advice
Maybe there's some great use case for delving into messages and commands and editing them that you haven't made explicit, but Cmd is opaque and all you can do is issue them and handle the resulting messages, so I'm sceptical but definitely interested.
Also in giving you update to write, it's almost like they've given you the app-specific bind to write (but it's not a functor and you absolutely do look at the data), so they've given you the keys to the Tesla. It takes a bit of getting used to but you're really going to like what happens at the traffic lights. Don't attempt to dismantle the door hinges until you've learned to drive it.
Edit: Your specific use case: inter-page communication
It turns out in chat that you're trying to get messages from one page to be usable in other pages or the overall update - sometimes one page needs to tell the app to change page and tell the new page to start an animation. I might have skipped all the advice above if I'd known that initially, but I think it's good advice for anyone coming from Haskell and I'm leaving it in!
Multiple messages
I still think it's important to accept that sometimes a single message needs to cover multiple actions, and you sort that out in your update function rather than try to create multiple messages in response to a single user action.
Lots of elm folks give the advice that your messages, rather than describing something to do like AddProduct they should describe something that happened in the past, partly because that's how messages come to you in your update so your mental model of what the elm runtime is doing is accurate, and partly because you're less likely to want to make two messages and do weird message translations when you ought to make one message.
Do multiple things in your ClickedViewOffers branch of update rather than try to make both a SwitchToOffersPage and a
AnimatePickOfTheDay message.
I'd like to point out that your idea to convert your messages and filter them somehow within the messages type is doing it in the wrong place. Filtering so that your Home page doesn't get all the messages for your Login page is something you have to do in update anyway - don't try to filter them while you're making or passing the messages. update is where it's at for deciding what to do in response to user input. Messages are for describing user input.
OK, but how do you get messages to cross the barriers between pages?!
There are a few ways to achieve this and it might be worth looking into different ways of making a Single Page Application (SPA) in Elm. I found this article by Rogério Chaves on Medium quite enlightening on the topic of various ways of organising messages from child page to parent app. He's done the TodoMVC app all the different ways in this repo A stack overflow post is better if it inlines ideas, so here we go:
Common Msg type across all pages
This can work by having a separate module for your message types which all your modules import. Messages look like ProductsMsg (UserCreatedNewProduct productRecord), as they might well do anyway, but because all the message types are global you can call another page's methods.
Individual pages also return an OutMsg from their update function
Use better names than these (eg Login.Msg rather than LoginMsg), but...
loginPageUpdate : LoginMsg -> LoginModel -> (LoginModel,Cmd LoginMsg,OutMsgFromLogin)
update : GlobalMsg -> GlobalModel -> (GlobalModel,Cmd GlobalMsg)
update msg model = case msg of
LoginMsg loginMsg ->
let (newLoginModel,cmd,outMsgFromLogin) = loginPageUpdate loginMsg model.loginModel
in
...
(You'd need NoOp :: OutMsgFromLogin or use Maybe OutMsgFromLogin there. I'm not a fan of NoOp. It's terribly tempting to use it for unimplemented features, and it's the king of all divorced-from-user-intentions messages that doesn't explain why you ought to do nothing or how you came to write something where you generated a purposeless message. I think it's a code smell that there's a better way of writing something.)
Have a record of messages that you later use to translate your page's Msgs messages into global messages.
(Again, use better domain-specific names, I'm trying to convey usage in my names.)
type LoginMessagesRecord globalMsg =
{ internalLoginMsgTag : LoginMsg -> globalMsg
, loginSucceeded : User -> globalMsg
, loginFailed : globalMsg
, newUserSuccessfullyRegistered : User -> globalMsg
}
and in your main, you would specify these:
loginMessages : LoginMessagesRecord GlobalMsg
loginMessages =
{ internalLoginMsgTag = LocalLoginMsg
, loginSucceeded = LoginSucceeded
, loginFailed = LoginFailed
, newUserSuccessfullyRegistered = NewUserSuccessfullyRegistered
}
You can either parameterise functions in your Login code with those so they all consume a LoginMessagesRecord and produce a msg, or you can use a genuinely local message type and write a translation helper in your Login module:
type HereOrThere here there = Here here | There there
type LocalLoginMessage = EditedUserName String | EditedPassword String | ....
type MessageForElsewhere = LoggedIn User | DidNotLogIn | MadeNewAccount User
type alias LoginMsg = HereOrThere LocalLoginMessage MessageForElsewhere
loginMsgTranslator : LoginMessagesRecord msg -> LoginMsg -> msg
loginMsgTranslator
{ internalLoginMsgTag
, loginSucceeded
, loginFailed
, newUserSuccessfullyRegistered
}
loginMsg = case loginMsg of
Here msg -> internalLoginMsgTag msg
There msg -> case msg of
LoggedIn user -> loginSucceeded user
DidNotLogIn -> loginFailed
MadeNewAccount user -> newUserSuccessfullyRegistered user
and then you can use Html.map loginMsgTranslator loginView in your global view, or Element.map loginMsgTranslator loginView if you're using the utterly brilliant html&css-free way to write elm apps, elm-ui.
Summary / takeaway
Have a single message describing a user intention and use update to handle all the consequences.
Don't edit the messages, respond appropriately in the update
The user is in control. The runtime is in control. You're not in control. Don't generate messages yourself, just respond to them. If you're generating messages rather than the user or the runtime, you're using elm in a weird way that'll be hard.
Your program logic largely resides in update. It doesn't reside in message. Don't try to make things happen in message, just describe what the user did or what the system did in the message.
Use case statements and descriptive tags in message types to help choose which update helper function to run. It can often help to use union types to describe how local a message is. Sometimes you use a local updating function, sometimes a global one.
You might want to also read this reddit thread about scaling elm apps that Rogério Chaves references.
I'm using new WebRTC stats API with RTCStatsCollectorCallback object that is called when stats report is generated. I'm invoke GetStats() and then I can see that OnStatsDelivered is called with a RTCStatsReport that contains just one (empty) item in stats_ member. In GetStats() call I pass my own implementation of RTCStatsCollectorCallback that implements webrtc::RTCStatsCollectorCallback interface. My question is, is neccessary some setup or constraints in PeerConnection in order to get RTCStatsReport with the metrics? I mean, to get, for example kStatsValueNameRtt stat, I need to set something in PeerConnection. Note that I'm using C++ native API in branch 55. Is this new stats API full implemented?
I've used the older GetStats method with success:
bool GetStats(StatsObserver* observer,
webrtc::MediaStreamTrackInterface* track,
StatsOutputLevel level) override;
It could be your problem was due to trying to use an in-progress API (webrtc framework is a bit wild west). Currently, peerconnectiointernface.h says the following:
// Gets stats using the new stats collection API, see webrtc/api/stats/. These
// will replace old stats collection API when the new API has matured enough.
// TODO(hbos): Default implementation that does nothing only exists as to not
// break third party projects. As soon as they have been updated this should
// be changed to "= 0;".
virtual void GetStats(RTCStatsCollectorCallback* callback) {}
As I'm writing this response, your question is 6 months old, so it's possible the this new API is now working (though the comment above suggests otherwise).
I'm new to OTRS (3.2) and also new to PERL but I have been given the task of setting up OTRS so that it will make a call to our remote webservice so a record can be created on our end when a ticket is set as "Closed".
I set up various dynamic fields so the customer service rep can fill in additional data that will be passed into the webservice call along with ticket details.
I couldn't get the webservice call to trigger when the ticket was "Closed" but I did get it to trigger when the "priority" was changed so I'm just using that now to test the webservice.
I'm just using the Test.pm and TestSimple.pm files that were included with OTRS.
When I look at the Debugger for the Webserice, I can see that the calls were being made:
$VAR1 = {
'TicketID' => '6'
};
My webservice currently just has one method "create" which just returns true for testing.
however I get the following from the Test.pm
"Got no TicketNumber (2014-09-02 09:20:42, error)"
and the following from the TestSimple.pm
"Error in SOAP call: 404 Not Found at /TARGET/SHARE/var/otrs/Kernel/GenericInterface/Transport/HTTP/SOAP.pm line 578 (2014-09-02 09:20:43, error)
I've spent countless hours on Google but couldn't find anything on this. All I could find is code for the Test.pm and TestSimple.pm but nothing really helpful to help me create a custom invoker for my needs and configure the webservice in OTRS to get it to work.
Does anyone have any sample invokers that I can look at to see how to set it up?
Basically I need to pass the ticket information along with my custom dynamic fields to my webservice. From there I can create the record on my end and do whatever processing.
I'm not sure how to setup the Invoker to pass the necessary ticket fields and dynamic fields and how to make it call a specific method in my remote webservice.
I guess getting the Test.pm and TestSimple.pm to work is the first step then I can modify those for my needs. I have not used PERL at all so any help is greatly appreciated.
I'm also struggling with similar set of requirements too. I've also never programmed in PERL, but I can tell you at least that the "Got no TicketNumber" in the Test.pm is right from the PrepareRequest method, there you can see this block of code:
# we need a TicketNumber
if ( !IsStringWithData( $Param{Data}->{TicketNumber} ) ) {
return $Self->{DebuggerObject}->Error( Summary => 'Got no TicketNumber' );
}
You should change all references to TicketNumber to TicketID, or remove the validation whatsoever (also there is mapping to ReturnedData variable).
Invoking specific methods on your WS interface is quite simple (but poorly documented). The Invoker name that you specify in the "OTRS as requester" section of web service configuration corresponds to the WS method that will be called. So if you have WS interface with a method called "create" just name the Invoker "create" too.
As far as the gathering of dynamic field goes, can't help you on that one yet, sorry.
Cheers
I have a application with a Django backend and an AngularJS front-end.
I use the angular-gettext plugin along with Grunt to handle translations.
The thing is, I sometimes received dynamic strings from my backend through the API. For instance a MySQL error about a foreign key constraint or duplicate key entry.
How can I add this strings to the .pot file or non harcoded string in general ?
I've tried to following but of course it cannot work :
angular.module('app').factory('HttpInterceptor', ['$q', '$injector', '$rootScope', '$cookieStore', 'gettext', function ($q, $injector, $rootScope, $cookieStore, gettext) {
responseError: function (rejection) {
gettext('static string'); //it works
gettext(rejection.data.error); //does not work
$rootScope.$emit('errorModal', rejection.data);
}
// Return the promise rejection.
return $q.reject(rejection);
}
};
}]);
})();
One solution I could think of would be to write every dynamic strings into a JSON object. Send this json to server and from there, write a static file containing these strings so gettext can extract them.
What do you suggest ?
I also use angular-gettext and have strings returned from the server that need to be translated. We did not like the idea of having a separate translation system for those messages so we send them over in the default language like normal.
To allow this to work we did two things. We created a function in our backend which we can call to retrieve all the possible strings to translate. In our case it's mainly static data that only changes once in a while. Ideally this would be automated but it's fine for now.
That list is formatted properly through code into html with the translate tag. This file is not deployed, it is just there to allow the extraction task to find the strings.
Secondly we created a filter to do the translation on the interpolated value, so instead of translating {{foo}} it will translate the word bar if that's was the value of foo. We called this postTranslate and it's a simple:
angular
.module('app')
.filter('postTranslate', ['gettextCatalog', function (gettextCatalog) {
return function (s) {
return gettextCatalog.getString(s);
};
}]);
As for things that are not in the database we have another file for those where we manually put them in. So your error messages may go here.
If errors are all you are worried about though, you may rather consider not showing all the error messages directly and instead determine what user friendly error message to show. That user friendly error message is in the front end and therefore circumvents all of this other headache :)
This is probably a very simple question but I just can't seem to figure it out.
I am writing a Javascript app to retrieve layer information from a WFS server using a GetCapabilities request using GeoExt. GetCapabilities returns information about the WFS server -- the server's name, who runs it, etc., in addition to information on the data layers it has on offer.
My basic code looks like this:
var store = new GeoExt.data.WFSCapabilitiesStore({ url: serverURL });
store.on('load', successFunction);
store.on('exception', failureFunction);
store.load();
This works as expected, and when the loading completes, successFunction is called.
successFunction looks like this:
successFunction = function(dataProxy, records, options) {
doSomeStuff();
}
dataProxy is a Ext.data.DataProxy object, records is a list of records, one for each layer on the WFS server, and options is empty.
And here is where I'm stuck: In this function, I can get access to all the layer information regarding data offered by the server. But I also want to extract the server information that is contained in the XML fetched during the store.load() (see below). But I can't figure out how to get it out of the dataProxy object, where I'm sure it must be squirreled away.
Any ideas?
The fields I want are contained in this snippet:
<ows:ServiceIdentification>
<ows:Title>G_WIS_testIvago</ows:Title>
<ows:Abstract/>
<ows:Keywords>
<ows:Keyword/>
</ows:Keywords>
<ows:ServiceType>WFS</ows:ServiceType>
<ows:ServiceTypeVersion>1.1.0</ows:ServiceTypeVersion>
<ows:Fees/>
<ows:AccessConstraints/>
Apparently,GeoExt currently discards the server information, undermining the entire premise of my question.
Here is a code snippet that can be used to tell GeoExt to grab it. I did not write this code, but have tested it, and found it works well for me:
https://github.com/opengeo/gxp/blob/master/src/script/plugins/WMSSource.js#L37