How extract information from my model on the handler in yesod? - entity

Question

my question is about how can I extract from a list of query result the year of each entity Post.
My model is this:
Post
name Text
content Text
date UTCTime default=CURRENT_TIME
deriving Show
And my handler looks like this:
getYearR :: Integer -> Handler Html
getYearR year = do
posts <- runDB $ selectList [] [Desc PostDate]
years <- --some function with posts for extract the list of years
defaultLayout $ do
aDomId <- newIdent
setTitle "Blog"
$(widgetFile "homepage")
Maybe the solution is related with some basic knowledge about monads but I don't know how that works for this situation.
Thanks for your time and I really appreciate your help

Answer 1

First I created a convenience function to extract the year. I referenced these docs on getting the Day from UTCTime, and these docs on getting the year from the Day.
import Data.Time
getYear :: UTCTime -> Integer
getYear time =
let (year,_,_) = toGregorian $ utctDay time
in year
Using this function, I can get a list of years by extracting the Haskell record from the Entity using entityVal, extract the UTCTime using postDate, then extract the year using my getYear function
let years = map (getYear . postDate . entityVal) posts
If you're using the list of years in Hamlet, you could skip creating the intermediate years value and just iterate through posts:
<ul>
$forall Entity id post <- posts
<li>
<h4>The year is #{getYear $ postDate post}</a>