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

Michael Snoyman michael at snoyman.com
Tue Jun 14 17:40:16 CEST 2011

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

    [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

    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.


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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