Re: [web-devel] Setting variables in hamlet forall

On Thu, Apr 7, 2011 at 11:22 PM, Michael Snoyman wrote:

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}

I was really going out there with my suggestions and examples. The real benefit of a unified approach is that you can extend it to apply to your custom container types. Making it pretty similar to foldable but with an default behaviour when the data structure is empty. Also if you already have let for free using forall and return, why not make a sugared version that compiles down to that?

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 wrote:

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On Thu, Apr 7, 2011 at 10:34 PM, Mark Bradley wrote:

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On Thu, Apr 7, 2011 at 7:51 PM, Max Cantor wrote:

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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

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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:

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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, 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>#{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

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