I am trying to understand variable interpolation in Yesod's Hamlet, in particular implementing if-then-else-like logic. Let's say I want to add CSS styling based on what value the integer x has. The x comes from a for-loop in the template, i.e. I do not have access to it from the Haskell code.
Desired result:
<span class="even positive">2</span>
<span class="odd positive">13</span>
<span class="odd non-positive">-1</span>
<span class="even non-positive">0</span>
Attempt at inline if:
<span class="#{if even x then "even" else "odd"} ...">#{x}</span>
Attempt using $with:
$with cls <- (if even x then "even" else "odd")
<span class="#{cls}">#{x}</span>
Neither works; both result in
• Illegal variable name: ‘if’
When splicing a TH expression
Apparently that part of Haskell syntax is not implemented in the #{...} parser. Is there a clean way to do this? I can currently see only one way out, and it is ugly as hell:
$if ((even x) && (x > 0))
<span class="even positive">#{x}</span>
$elseif (even x)
<span class="even non-positive">#{x}</span>
$elseif (x > 0)
<span class="odd positive">#{x}</span>
$else
<span class="odd non-positive">#{x}</span>
Most Haskell syntax isn't supported inside these template expressions. I think that's by design, since you don't want your template to contain complex business logic.
I would write the code as a function on the Haskell end, and call it from the template. Something like:
-- .hs file
altclass, signclass :: Int -> String
altclass x = if even x then "even" else "odd"
signclass x = if x > 0 then "positive" else "non-positive"
-- .hamlet file
<span class="#{altclass x} #{signclass x}">#{x}</span>
Doing it this way properly separates the business logic out of the template, but still allows you to use in-template variables.