[web-devel] [Hamlet] CSS size wrapper

Michael Snoyman michael at snoyman.com
Wed Feb 16 11:02:34 CET 2011

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.

>> 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.


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