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

David McBride dmcbride at neondsl.com
Thu Jun 16 00:59:52 CEST 2011 

I haven't finished.  I was making the changes he suggested, but found
a problem.  I sent him a question but he's probably a busy guy.

I don't see the changes in his github page, where are you getting this
from?  If you have found them, it is just a matter of taking the
package, cabal configure, cabal install and then it should be
available for all your coding needs.

On Wed, Jun 15, 2011 at 6:40 PM, Michael Litchard <michael at schmong.org> wrote:
> 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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