main.cassius

#menu
    ^{transition "all 0.2s ease-out"}

Mainhandler.hs

module Handler.Mainhandler where

import Import
import Service.Theme
import Service.Mixin

getMainR :: Handler Html
getMainR = do
    defaultLayout $ do
        setTitle "Main page"
        $(widgetFile "main")

Mixin.hs

module Service.Mixin where

{-# LANGUAGE QuasiQuotes #-}
import Text.Cassius
import Data.Text

transition :: Text -> Mixin
transition val =
    [cassiusMixin|
        -webkit-transition: #{val}
        -moz-transition: #{val}
        -ms-transition: #{val}
        -o-transition: #{val}
        transition: #{val}
    |]