[web-devel] Setting variables in hamlet forall

[Michael Snoyman]

A few points:

1) The cost is twofold: making Hamlet more complex from a user perspective,
and making the codebase more complex. I'm not a fan of either, unless it's
really justified.

2) I'm not really certain how your example below works as far as
disambiguating Maybe versus [] (i.e., $maybe versus $forall), but if we're
willing to go in this direction, you already have $let for free:

$forall foo <- foos
    $forall foobar <- return $ bar foo
        #{foobar}

Here, return would be for the [] instance of Monad. We could also use
$maybe, using the Maybe instance of Monad.

Michael

On Thu, Apr 7, 2011 at 3:46 PM, Mark Bradley <barkmadley at gmail.com> wrote:

> On Thu, Apr 7, 2011 at 10:34 PM, Mark Bradley <barkmadley at gmail.com>
> wrote:
> > On Thu, Apr 7, 2011 at 7:51 PM, Max Cantor <mxcantor at gmail.com> wrote:
> >> put me in the opposed category.
> >>
> >> You can just as easily put:
> >>  let formId rs = fromMaybe "" $ lookup $...
> >>
> >> in the haskell function that loads the hamlet file then you just have to
> put
> >>  #{formId rs}
> >>
> >> in the hamlet.  I think adding syntax should be done only when very
> necessary.  seems like a very small win here at a big cost.
> >
> > Where is the cost? Most of the effort would be just glueing together
> > some pieces of existing code. Given that there are already two places
> > where hamlet does variable binding, adding a third will not hurt it,
> > or perhaps a single more expressive form of variable binding is
> > required. Something like monadic bind (>>=) where you can bind
> > non-monadic values using the identity monad.
>
> An example:
>
> $bind row <- rs
>    $bind formId <- Identity $ fromMaybe "" $ IntMap.lookup $ getInt
> "form_id" row
>        <td>#{formId counties}
>        <td>#{formId customers}
>
> It could also be possible to do else cases where it didn't bind:
>
> -- list bind
> $bind row <- rs
>    -- identity bind
>    $bind formId <- Identity $ fromMaybe "" $ IntMap.lookup $ getInt
> "form_id" row
>        <td>#{formId counties}
>        <td>#{formId customers}
>        -- maybe bind
>        $bind someValue <- someMaybeValue
>            <div>content
>        -- maybe value was Nothing
>        $nobind
>            <div>other content
>    -- not possible with identity bind possible place for error/warning
>    $nobind
>        <div>This should not happen!
>
> -- empty list
> $nobind
>    <div>i left my content in my other pants
>
>
>
> >
> >>
> >> yes, if you have a situation where many handlers are calling the same
> hamlet file, there might be some duplication, but then you can always raise
> the formId function to a top-level function.
> >>
> >> max
> >>
> >>  On Apr 7, 2011, at 5:15 PM, Michael Snoyman wrote:
> >>
> >>> I've been very hesitant about adding more features to Hamlet,
> especially ones that are already implemented in Haskell. That's been my
> reasoning for avoiding any kind of variable definitions until now. However,
> this does seem like a compelling use case.
> >>>
> >>> I don't think it would make sense to limit it to foralls: it makes as
> much sense in maybes, and I think it would be confusing if it only applied
> in some cases. As for syntax, how about:
> >>>
> >>> $forall row <- rs
> >>>     $let formId = fromMaybe "" $ IntMap.lookup $ getInt "form_id" row
> >>>     ...
> >>>
> >>> I'm not 100% sold on this yet, what does everyone else think?
> >>>
> >>> One last note: I'm probably going to be announcing a feature freeze on
> Yesod 0.8 *very* soon, and making a beta release to Yackage so that people
> can test. If you have any last-minute input, now's the time. I'm planning on
> giving the beta test period about a week, and then releasing to Hackage.
> >>>
> >>> Michael
> >>>
> >>> On Thu, Apr 7, 2011 at 2:57 AM, <vagif.verdi at gmail.com> wrote:
> >>> I noticed a pattern that in hamlet $forall i often retrieve the same
> value
> >>> from a map, Sometimes 3,4 times.
> >>>
> >>>    $forall row <- rs
> >>>            <td><a href=@{FormR (getInt "form_id" row)}>#{getStr
> "form_name"
> >>> row}
> >>>            <td>#{getStr "docname" row}
> >>>            ...
> >>>            <td>#{fromMaybe "" (IntMap.lookup (getInt "form_id" row)
> >>> counties)}
> >>>            <td>#{fromMaybe "" (IntMap.lookup (getInt "form_id" row)
> >>> customers)}
> >>>
> >>> Would it be possible to allow let statement in forall for often used
> values ?
> >>>
> >>> Regards,
> >>> Vagif Verdi
> >>>
> >>> _______________________________________________
> >>> web-devel mailing list
> >>> web-devel at haskell.org
> >>> http://www.haskell.org/mailman/listinfo/web-devel
> >>>
> >>> _______________________________________________
> >>> web-devel mailing list
> >>> web-devel at haskell.org
> >>> http://www.haskell.org/mailman/listinfo/web-devel
> >>
> >>
> >> _______________________________________________
> >> web-devel mailing list
> >> web-devel at haskell.org
> >> http://www.haskell.org/mailman/listinfo/web-devel
> >>
> >
> >
> >
> > --
> > -barkmadley
> > sent from an internet enabled device
> >
>
>
>
> --
> -barkmadley
> sent from an internet enabled device
>
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