[web-devel] [Hamlet] CSS size wrapper

[Dmitry Kurochkin]

On Wed, 16 Feb 2011 12:42:26 +0200, Michael Snoyman <michael at snoyman.com> wrote:
> 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.
> 

Yeah, MkSizeType module is not exported.

Patch for Hamlet attached.

> >> >> 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).
> 

Thanks for explanation. I think I am convinced for now :)

Regards,
  Dmitry

> Michael
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