[Haskell-beginners] Options for creating a multiple select form via Yesod

Michael Litchard michael at schmong.org
Thu Jun 16 00:40:32 CEST 2011 

David,
        I checked-out yesod-forms from the repository and see that
you've made the changes. Thank you once again. My follow-up question
is for anyone who cares to answer. How do I swap the new yesod-forms
and the old one from 0.2.0? I don't want to hose
my system. This is new ground for me.

On Tue, Jun 14, 2011 at 10:56 AM, Michael Litchard <michael at schmong.org> wrote:
> Oh sweet! Thanks!
>
> On Tue, Jun 14, 2011 at 9:43 AM, David McBride <dmcbride at neondsl.com> wrote:
>> I think I'll give it a shot.  Fun problem and something I'll
>> definitely need eventually.  I'll try and get you a pull request
>> tonight if all goes well.
>>
>> On Tue, Jun 14, 2011 at 11:40 AM, Michael Snoyman <michael at snoyman.com> wrote:
>>> Hi David,
>>>
>>> Thank you for the analysis, you're absolutely correct that this is a
>>> shortcoming in the API of yesod-form as it stands. I had an idea for a
>>> possibly simple modification to fix the situation: change fieldParse
>>> to
>>>
>>>    [Text] -> Either msg (Maybe a)
>>>
>>> We don't really want to support returning singles or doubles from the
>>> same Field; a multiSelectField will automatically make the "a"
>>> variable a list. Said another way, the types should be:
>>>
>>>    selectField :: [(Text, a)] -> Field xml msg a
>>>    multiSelectField :: [(Text, a)] -> Field xml msg [a]
>>>
>>> Another related change: we don't really need to separate out
>>> fieldRender from fieldView I believe. Instead, we can have fieldView
>>> be:
>>>
>>>    fieldView :: Text -- ^ ID
>>>                 -> Text -- ^ name
>>>                 -> a -- currently, this is another Text
>>>                 -> Bool -- ^ required?
>>>                 -> xml
>>>
>>> I'm willing to test out these changes myself, but wanted to (1) get
>>> input from you and (2) see if you or anyone else wanted to take a
>>> crack at it.
>>>
>>> Michael
>>>
>>> On Tue, Jun 14, 2011 at 4:25 PM, David McBride <dmcbride at neondsl.com> wrote:
>>>> I gave a shot at this last night,and didn't quite pull it off.  But I
>>>> got pretty far and I'd rather you have my work than try it from
>>>> scratch.
>>>>
>>>> The first method won't work because the Field type is used in
>>>> validating the get parameters in the mhelper function in
>>>> yesod.form.functions.  That means that you have to have a unified type
>>>> for field that can do everything.  Maybe there is a more succinct way
>>>> with classes, but I just couldn't think of an elegant way to do it
>>>> that way.
>>>>
>>>> So what I tried to do was make the fieldParser callback into its own type:
>>>>
>>>> newtype FieldParser msg a = FieldParser (Either
>>>>  (Maybe Text -> Either msg (Maybe a))
>>>>  (Maybe [Text] -> Either msg (Maybe [a])))
>>>>
>>>> That means a field parser can either take one text and return one
>>>> item, or it takes a list and returns a list of items.
>>>>
>>>> Then I went through Yesod.Form.Fields and changed about 12 or 15 references of
>>>> { fieldParse = blank $ \s ->
>>>> to
>>>> { fieldParse = FieldParser . Left $ blank $ \s ->
>>>>
>>>> Cool, now when you write your multipleSelectField, you'll set the fieldParser to
>>>> FieldParser . Right $ etc...
>>>>
>>>> The very last thing you have to do to fix this is the mhelper
>>>> function, which is where I lost steam.  Right now it looks up the name
>>>> of the field in the get/post params that were passed in, and then
>>>> hands the value to fieldParse.  What it needs to do is check to see
>>>> whether Field Parser is left or right, and then pass in the params
>>>> slightly differently depending on which it is.  I don't know how the
>>>> parameters will end up getting passed into askParams though.  Right
>>>> now askParams returns a list of names to value pairs, so hopefully you
>>>> will end up with a list of multiple entries for the name of your mutli
>>>> select and a different value for each entry, which you need to filter
>>>> out and collect into a single list and then run the fieldParser on it.
>>>>
>>>> Hopefully that is not too bad.
>>>>
>>>>
>>>> On Tue, Jun 14, 2011 at 4:30 AM, Michael Litchard <michael at schmong.org> wrote:
>>>>> Well I will try the easier way first, and having accomplished that I
>>>>> will look into doing it the better way. If people can call dibs, I'd
>>>>> like to.
>>>>>
>>>>> On Mon, Jun 13, 2011 at 9:47 PM, David McBride <dmcbride at neondsl.com> wrote:
>>>>>> After looking at the source, you should be aware that
>>>>>>
>>>>>> 1) yesod-form has been updated to 2.0,
>>>>>> 2) it is a lot easier to understand than 1.x was.
>>>>>>
>>>>>> The main obstacle I see is that the library uses the Field datatype,
>>>>>> that has a fieldParse method of Maybe Text -> Either msg (Maybe a).
>>>>>> The problem with that is that a multiple select box should require
>>>>>> [Text] or perhaps Maybe [Text] rather than Maybe Text.  It is making
>>>>>> the assumption that there can only be one piece of data per field,
>>>>>> which holds for everything except multiple selects and multiple radio
>>>>>> buttons.
>>>>>>
>>>>>> So looking at this, it looks like you'd have to add another field type
>>>>>> "FieldMulti" to Yesod/Form/Types.hs, which allows for multiple values.
>>>>>>  Then add a new version of selectFieldHelper that accepts fieldMultis
>>>>>> instead of fields, and then it is trivial to change selectField to be
>>>>>> a multi field.
>>>>>>
>>>>>> Alternatively you could change Field to accept either single or
>>>>>> multiple values and change its use everywhere else, which is probably
>>>>>> the better answer, but more involved.
>>>>>>
>>>>>> I don't know if this is the best way to go about it, but it seems like
>>>>>> it should work.
>>>>>>
>>>>>> On Mon, Jun 13, 2011 at 8:11 PM, Michael Litchard <michael at schmong.org> wrote:
>>>>>>> Thank you David. I'm trying to figure out step-by-step, exactly how
>>>>>>> selectFields binds field values. One thing I'm having trouble with is
>>>>>>> visualizing return values.
>>>>>>> Beginning with askParams.
>>>>>>>
>>>>>>> askParams :: Monad m => StateT Ints (ReaderT Env m) Env
>>>>>>> askParams = lift askenv <- askParams
>>>>>>>
>>>>>>>
>>>>>>> Here's the example from selectFields
>>>>>>> env <- askParams
>>>>>>> later on env is used in with the lookup function
>>>>>>>
>>>>>>> let res = case lookup name env of
>>>>>>> seeing as lookup is checking for value of type a in a [(a,b)]
>>>>>>> and given the type of askParams
>>>>>>> I have no idea what is going on here. I don't see a [(a,b)] in
>>>>>>> askParams :: Monad m => StateT Ints (ReaderT Env m) Env.
>>>>>>>
>>>>>>> So if someone could answer how env <- askParams yields a [(a,b)] for
>>>>>>> lookup to use as input, I would appreciate it.
>>>>>>>
>>>>>>>
>>>>>>> On Mon, Jun 13, 2011 at 2:54 PM, David McBride <dmcbride at neondsl.com> wrote:
>>>>>>>> The read function is sort of the opposite of the show function.  Take
>>>>>>>> a string, give me a value.  reads is like read, however it has some
>>>>>>>> traits that read doesn't have.
>>>>>>>>
>>>>>>>> The problem with read is that if you go: read "asdf" :: Int, it will
>>>>>>>> die with an exception, and that is something you don't want in a web
>>>>>>>> app.  Also it doesn't tell you what the rest of the string is, so you
>>>>>>>> have no real way of finding out what was left of the string after the
>>>>>>>> part you wanted to parse.
>>>>>>>>
>>>>>>>> So there is the reads function that returns [(a,String)] which is a
>>>>>>>> list of pairs of the answer a, and the rest of the string String.  As
>>>>>>>> a bonus, it returns a list so if it can't parse the string you pass
>>>>>>>> it, then it just returns an empty list.  Why didn't it use Maybe you
>>>>>>>> ask?  I bet it probably has to do with the function being one of the
>>>>>>>> first functions ever written for haskell, long before Maybe existed.
>>>>>>>>
>>>>>>>> So all it is there is unpack this bytestring into a string, then parse
>>>>>>>> it into a value, and please don't blow up if the input is invalid.
>>>>>>>>
>>>>>>>> On Mon, Jun 13, 2011 at 5:28 PM, Michael Litchard <michael at schmong.org> wrote:
>>>>>>>>> I was a bit hasty. I can render a multi-select field easily enough.
>>>>>>>>> However, I'm having difficulty following how selectField makes a value
>>>>>>>>> from the select field accessible from the handler code calling
>>>>>>>>> selectField. Once I figure that out, I can modify multiSelectField
>>>>>>>>> accordingly.
>>>>>>>>>
>>>>>>>>> The goal here being to modify selectField so that a list of field
>>>>>>>>> values can be bound .
>>>>>>>>>
>>>>>>>>> Here's what I have so far:
>>>>>>>>> multiSelectField is thus far identical in every way to selectField
>>>>>>>>> save for the following change in the Hamlet part.
>>>>>>>>>
>>>>>>>>> <select multiple="#{theId}" id="#{theId}" name="#{name}">
>>>>>>>>>
>>>>>>>>> My thinking was that the value bound to multiple was arbitary, and I'd
>>>>>>>>> use theId until I figured out something that made more sense.
>>>>>>>>>
>>>>>>>>> Here's where I am focusing my efforts next
>>>>>>>>>
>>>>>>>>> http://hpaste.org/47774
>>>>>>>>>
>>>>>>>>> Specifically
>>>>>>>>> (x', _):_ ->
>>>>>>>>>                            case lookup x' pairs' of
>>>>>>>>>                                Nothing -> FormFailure ["Invalid entry"]
>>>>>>>>>                                Just (y, _) -> FormSuccess y
>>>>>>>>> I'm thinking this is where selectField binds a value from the select
>>>>>>>>> field form. I'm confused by the (x',_):_. At first I thought it meant
>>>>>>>>> that just the first pair in a list of pairs is pattern matched
>>>>>>>>> against, and the rest discarded. But then I ask myself where the list
>>>>>>>>> is coming from. In a select field there would only be one pair, not a
>>>>>>>>> list of them. Here's where I get confused. Because if this is not
>>>>>>>>> where the values of the select field get bound, I don't know where
>>>>>>>>> it's happening.
>>>>>>>>>
>>>>>>>>> Is my confusion clear enough such that I could get some clarifying
>>>>>>>>> feedback? If not, what is unclear?
>>>>>>>>>
>>>>>>>>> On Sat, Jun 11, 2011 at 11:03 AM, Michael Snoyman <michael at snoyman.com> wrote:
>>>>>>>>>> The best way for code contributions in general is to submit a pull
>>>>>>>>>> request on Github. If that's a problem, sending a patch via email
>>>>>>>>>> works as well (either directly to me or to web-devel).
>>>>>>>>>>
>>>>>>>>>> Michael
>>>>>>>>>>
>>>>>>>>>> On Sat, Jun 11, 2011 at 1:14 AM, Michael Litchard <michael at schmong.org> wrote:
>>>>>>>>>>> Hey! I just added multiSelectField to the Forms library. I'm only
>>>>>>>>>>> getting the first value selected, but I think that's because of how
>>>>>>>>>>> I'm using multiSelecrField. I'm going to try to change the client code
>>>>>>>>>>> to fix this. I'll let you know how it goes. when I get a
>>>>>>>>>>> maybeMultiSelectField added I'll show you what I have. What would be
>>>>>>>>>>> the best way to submit this?
>>>>>>>>>>>
>>>>>>>>>>> On Thu, Jun 9, 2011 at 10:05 PM, Michael Snoyman <michael at snoyman.com> wrote:
>>>>>>>>>>>> Hi Michael,
>>>>>>>>>>>>
>>>>>>>>>>>> There's nothing jQuery or Javascript specific about a multi-select
>>>>>>>>>>>> field: it's just a normal select field with a "multiple" attribute. I
>>>>>>>>>>>> would recommend taking the selectField code from yesod-form and
>>>>>>>>>>>> modifying it to be multi-select. I'll likely do this myself
>>>>>>>>>>>> eventually, but it could be a good learning experience in Yesod (and a
>>>>>>>>>>>> great introduction to contributing to the framework if you're so
>>>>>>>>>>>> inclined).
>>>>>>>>>>>>
>>>>>>>>>>>> Michael
>>>>>>>>>>>>
>>>>>>>>>>>> On Thu, Jun 9, 2011 at 8:29 PM, Michael Litchard <michael at schmong.org> wrote:
>>>>>>>>>>>>> I'm trying to create a multiple select form, as illustrated on the following:
>>>>>>>>>>>>> http://api.jquery.com/selected-selector/
>>>>>>>>>>>>>
>>>>>>>>>>>>> Here's the options I see possible:
>>>>>>>>>>>>>
>>>>>>>>>>>>> (1) Write a jQuery widget.
>>>>>>>>>>>>> (2) Use plain javascript via Julius
>>>>>>>>>>>>> (3) Use the low-level functions in Yesod.Form to write a widget
>>>>>>>>>>>>> (4) Use a pre-existing function that does what I need, but am not
>>>>>>>>>>>>> aware of this functionality
>>>>>>>>>>>>>
>>>>>>>>>>>>> (1) has appeal as it looks like something small I can contribute to
>>>>>>>>>>>>> the project. It will take me some extra time to figure out the
>>>>>>>>>>>>> details. But, I had a look at the other jQuery widgets and they seem
>>>>>>>>>>>>> to provide an approachable model to follow.
>>>>>>>>>>>>>
>>>>>>>>>>>>> (2) This looks like the most straight-forward approach. I'm just
>>>>>>>>>>>>> learning javascript so would have to figure out how to capture values
>>>>>>>>>>>>> in Haskell from the form.
>>>>>>>>>>>>>
>>>>>>>>>>>>> (3) This looks like the most difficult way. I don't think I know
>>>>>>>>>>>>> enough about the low-level functions in Yesod.Form to be able to
>>>>>>>>>>>>> accomplish this in a timely manner.
>>>>>>>>>>>>>
>>>>>>>>>>>>> (4) This is the best scenario. There's already a way to do this right
>>>>>>>>>>>>> now, and I just haven't identified it. If this is the case, I would
>>>>>>>>>>>>> appreciate being pointed in the right direction.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Until informed otherwise, I'm evaluating options 1 and 2. All feedback
>>>>>>>>>>>>> welcomed. Thanks to all who made Yesod possible.
>>>>>>>>>>>>>
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>>>>>>>>>>
>>>>>>>>>
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