[web-devel] [Hamlet] CSS size wrapper

Michael Snoyman michael at snoyman.com
Wed Feb 16 11:42:26 CET 2011

On Wed, Feb 16, 2011 at 12:37 PM, Dmitry Kurochkin
<dmitry.kurochkin at gmail.com> wrote:
> On Wed, 16 Feb 2011 12:02:34 +0200, Michael Snoyman <michael at snoyman.com> wrote:
>> On Wed, Feb 16, 2011 at 11:49 AM, Dmitry Kurochkin
>> <dmitry.kurochkin at gmail.com> wrote:
>> > On Wed, 16 Feb 2011 06:25:54 +0200, Michael Snoyman <michael at snoyman.com> wrote:
>> >> It looks good. Instead of the mkSize TH function, if you just define
>> >> an IsString instance, then anyone using OverloadedStrings will be able
>> >> to use string literals. I haven't confirmed this yet, but it might
>> >> even be possible to embed those string literals inside Cassius and GHC
>> >> will still apply fromString appropriately.
>> >>
>> >
>> > I was thinking about it. But as I understand, it would not work unless
>> > you explicitly specify the resulting size type. E.g.
>> >
>> >  let s = fromString "100px"
>> >
>> > How would GHC know that in this case fromString for PixelSize should be
>> > used? This would force you to add explicit (s :: PixelSize) type. This
>> > is something I want to avoid: Type should be determined from string
>> > constant. Or am I missing something?
>> No, you're right, TH is the way to go here.
> How is it better to integrate to Hamlet? I guess MkSizeType should go to
> Text/Hamlet/ directory and Size.hs will be part of Text/Cassius.hs.

That sounds fine, frankly it doesn't matter to me where MkSizeType
goes since it won't be an exposed module (right?). And Size.hs's code
would go in Text.Cassius, correct.

>> >> As far as variables inside templates: I personally think that's
>> >> crossing the line again into stuff templates shouldn't be dealing
>> >> with, but I'm open for discussions. Since templates tie in so well
>> >> with Haskell, I just don't think it's worth adding a whole bunch of
>> >> extra code and syntax to make it work.
>> >>
>> >
>> > I understand this is a feature which may be used inappropriately. But
>> > here is mine justification for it:
>> >
>> > Consider you want to write a simple Cassius template:
>> >
>> >  #a
>> >    height: 100px
>> >  #b
>> >    width: 100px
>> >
>> > #a and #b are related and their width and height should be always the
>> > same. So it makes perfect sense to define (commonSize = $(mkSize
>> > "100px")) and use it instead of literals. I know that commonSize is
>> > needed and used only for that Cassius template, but I have to define it
>> > in an external Haskell module. That makes template not self-contained
>> > and harder to read, pollutes Haskell code with declarations that should
>> > be local to a template. IMO this cases illustrates how local template
>> > variables may be the right solution:
>> >
>> >  #{let commonSize = $(mkSize "100px")}
>> >  #a
>> >    height: #{commonSize}
>> >  #b
>> >    width: #{commonSize}
>> Actually, I think this code snippet proves the opposite point. It's
>> not really possible to embed TH in a template. I think adding in a
>> whole bunch of Haskell features to Cassius (et al) will simply start
>> people wishing they *were* programming in Haskell instead of
>> templates. My opinion: keep the templates simple and to the point, put
>> the logic in Haskell where you have more power.
> Sigh. It is possible to use TH in QQ, hence I thought it is possible to
> use it in Hamlet as well.

A QQ block is just a String that needs to be interpreted by a Haskell
function into a "Q Exp". Each and every added feature needs to be
coded from scratch, and as such, will never be done as well as
features included in Haskell itself. That's a large part of my
reluctance to adding everything and the kitchen sink (besides the fact
that I think it's not a good idea for a templating language anyway).


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