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

8 Apr 2011


      Will it be

$with foo <- getFoo
         bar <- getBar


or will it be

$with foo <- getFoo
    $with bar <- getBar


On Friday, April 08, 2011 06:47:22 AM you wrote:
...
I agree with your comments about $let, which is why I'm reluctant to do so.
If no one has any objections, I'll pull your patch and rename to with.
Also, for consistency, I'll use <- instead of =. In other words:
$width foo <- bar
    #{foo}
Michael
On Fri, Apr 8, 2011 at 4:45 PM, Mark Bradley  wrote:
...
$with definitely differentiates it from the way that haskell does let
binding and even makes it more obvious about the scoping (with
bindings in languages like python/javascript work this way), even
hinting that it doesn't to pattern matching.  I'm in favour.
If we were to implement a $let it would probably have to work on the
current scope otherwise it would confuse, and allow for mutual
recursion perhaps.
On Fri, Apr 8, 2011 at 11:40 PM, Michael Snoyman 
wrote:
...
Hi Mark,
Do you have an opinion on the naming, i.e. with versus let? Does anyone
else?
Michael
On Fri, Apr 8, 2011 at 4:35 PM, Mark Bradley 
wrote:
...
...
your latest change makes my original patch not work.  I updated the
pull request to handle this and deal with the foldable forall problem
that arises from piggy backing on the LineForall construct.  It now
uses the LineMaybe to the same effect.
On Fri, Apr 8, 2011 at 8:08 PM, Michael Snoyman 
wrote:
...
Oh, you weren't joking, that really is a small patch. I should have
looked
before writing this email. OK, it's using the inner block approach.
I think
I'm OK including that if we rename it to $with, e.g.:
$with x <- foo bar
#{x}
Michael
On Fri, Apr 8, 2011 at 1:05 PM, Michael Snoyman

wrote:
...
On Fri, Apr 8, 2011 at 2:47 AM, Mark Bradley 
wrote:
> On Thu, Apr 7, 2011 at 11:22 PM, Michael Snoyman <
michael@snoyman.com>
...
...
...
...
> 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.
Actually, forgetting the rest of the discussion here, I think
extending
...
...
...
...
$forall to work on any Foldable is a great idea. Any objections?
> Also if you already have let for free using forall and return, why
not
...
...
...
...
> make a sugared version that compiles down to that?
I haven't looked at your patch yet (thank you btw), but my concern
is that
introducing $let, the same way it's used in Haskell, introduces
scoping
...
...
...
...
issues that we don't otherwise have. $forall and $maybe already add
a significant complexity to deal with the bound variable names,
but at least
it's bound for only the inner block. With $let, we would want it to
be
...
...
...
...
bound
for the remainder of the block most likely. So we'd have two
choices: * Implement a whole bunch of complexity defining and
implementing new scoping rules.
* Have totally different semantics from Haskell.
I'm not sure which approach your patch took. But maybe the problem
was
...
...
...
...
with my choice of name ($let); $with would likely make more sense
for the
inner block approach. But even so, I'm still concerned that this is
complexity without enough reward.
> > 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@gmail.com>
...
...
...
...
> > wrote:
> >> On Thu, Apr 7, 2011 at 10:34 PM, Mark Bradley
> >> 
> >> 
> >> wrote:
> >> > On Thu, Apr 7, 2011 at 7:51 PM, Max Cantor
> >> >  >> > 
> >> > 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, 
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>
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@haskell.org
> >> >>> http://www.haskell.org/mailman/listinfo/web-devel
> >> >>> 
> >> >>> _______________________________________________
> >> >>> web-devel mailing list
> >> >>> web-devel@haskell.org
> >> >>> http://www.haskell.org/mailman/listinfo/web-devel
> >> >> 
> >> >> _______________________________________________
> >> >> web-devel mailing list
> >> >> web-devel@haskell.org
> >> >> http://www.haskell.org/mailman/listinfo/web-devel
> >> > 
> >> > --
> >> > -barkmadley
> >> > sent from an internet enabled device
> >> 
> >> --
> >> -barkmadley
> >> sent from an internet enabled device
> 
> --
> -barkmadley
> sent from an internet enabled device
--
-barkmadley
sent from an internet enabled device
--
-barkmadley
sent from an internet enabled device

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

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