[Haskell-beginners] Options for creating a multiple select form via Yesod
Michael Litchard
michael at schmong.org
Tue Jun 14 02:11:55 CEST 2011
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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