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

Michael Litchard michael at schmong.org
Tue Jun 14 10:30:16 CEST 2011 

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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