[web-devel] [Hamlet] CSS size wrapper

[Dmitry Kurochkin]

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.

> >> 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.

Regards,
  Dmitry

> Michael