Problem to show value out of Handler monad in Haskell and Yesod - haskell

Question

I am a beginner in Haskell and I am still studying and I stopped at the problem that has to do with monads.
The problem is that I want to show the comment writer in the template.
I can't do that because when I add a comment from the database it is always in the Handler monad.
Maybe my whole idea is wrong I don't know.
This is the comment entity:
ManComment
text Text sqltype=varchar(500)
created UTCTime sqltype=DateTime
writer UserId Maybe sqltype=varchar(255) default=NULL -- comment writer
manId ManifestationId sqltype=varchar(255) default=NULL
deriving Show Typeable
This is the handler function:
getManDetailsR :: ManifestationId -> Handler Html
getManDetailsR mid = do
(ui, user) <- requireAuthPair
comments <- runDB $ getComFromMan mid
defaultLayout $ do
setTitle "Manifestation details"
$(widgetFile "man-details")
Part of hamlet file (in last line trying to show writer):
$if null comments
<h4>There is not comments!
$else
$forall Entity cid com <- comments
<form method=post action=#{DeleteManCommentR mid cid}>
<li .list-group-item>
<div class="row">
<div class="col-xs-10 col-md-11">
<div>
<div .mic-info> By: <a href=#{ProfileR}>#{getCommentWriter $ com}</a>
I'm trying to get a comment writer here:
getCommentWriter :: ManComment -> Handler Text
getCommentWriter c = do
user <- runDB $ get404 $ fromJust $ manCommentWriter c --get writer from database
let username = userIdent user
return username -- but return puts in Handler monad
Eventually an error occurred:
No instance for (blaze-markup-0.8.2.7:Text.Blaze.ToMarkup
(Handler Text))
arising from a use of ‘toHtml’

Answer 1

you should be able to use it in the same way you use comments there: bind it to a local value and access this in your hamlet-file/code.
Of course you want to pair this up with the comment itself so I'd propose something like this:
getCommentAndWriter :: ManComment -> Handler (Entity ManComment, Text)
getCommentAndWriter c = do
user <- runDB $ get404 $ fromJust $ manCommentWriter c --get writer from database
let username = userIdent user
return (c, username)
getManDetailsR :: ManifestationId -> Handler Html
getManDetailsR mid = do
(ui, user) <- requireAuthPair
cs <- runDB $ getComFromMan mid
comments <- forM cs getCommentAndWriter
defaultLayout $ do
setTitle "Manifestation details"
$(widgetFile "man-details")
$if null comments
<h4>There is not comments!
$else
$forall (Entity cid com, writer) <- comments
<form method=post action=#{DeleteManCommentR mid cid}>
<li .list-group-item>
<div class="row">
<div class="col-xs-10 col-md-11">
<div>
<div .mic-info> By: <a href=#{ProfileR}>#{writer}</a>
Of course you should look into querying both of this together if you can but basically this should work.
PS: if you want to use more fields you probably would want to use a record - here I choose to tuple it up for simplicity.