Re: [web-devel] [Hamlet] CSS size wrapper

16 Feb 2011


      On Wed, Feb 16, 2011 at 11:49 AM, Dmitry Kurochkin
 wrote:
...
On Wed, 16 Feb 2011 06:25:54 +0200, Michael Snoyman  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.
...
...
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.

Michael