yesod - Could not deduce (blaze-markup-0.6.3.0:Text.Blaze.ToMarkup Day) arising from a use of ‘toHtml’ -

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i'm trying use yesod build simple web site , i'm starting code max tagher's excellent intro on youtube, yesodscreencast. i've forked code github, , add date posting indicate when published, i'm running problem can't quite figure out given low experience haskell , beginner's experience yesod. i've been unable find answer via googleplex.

yesod provides native dayfield in yesod.form.fields, thought needed add postdate field in blogpost following config/models using day:

blogpost     title text     postdate day     article markdown

and add blogpostform in postnew.hs:

blogpostform :: aform handler blogpost blogpostform = blogpost              <$> areq textfield     (bfs ("title" :: text)) nothing             <*> areq dayfield      (bfs ("postdate" :: day)) nothing             <*> areq markdownfield (bfs ("article" :: text)) nothing

when compiles following error message:

handler/home.hs:16:11:     not deduce (blaze-markup-0.6.3.0:text.blaze.tomarkup day)       arising use of ‘tohtml’     context (persistentity blogpost)       bound pattern constructor                  entity :: forall record.                            persistentity record =>                            key record -> record -> entity record,                in lambda abstraction       @ handler/home.hs:16:11-34     in first argument of ‘aswidgett ghc.base.. towidget’, namely       ‘tohtml (blogpostpostdate post_apzp)’     in stmt of 'do' block:       (aswidgett ghc.base.. towidget)         (tohtml (blogpostpostdate post_apzp))     in expression:       { (aswidgett ghc.base.. towidget)              ((blaze-markup-0.6.3.0:text.blaze.internal.preescapedtext                ghc.base.. data.text.pack)                 "<h4><li><a href=\"");            (geturlrenderparams             >>=               (\ urender_apzq                  -> (aswidgett ghc.base.. towidget)                       (tohtml                          (\ u_apzr -> urender_apzq u_apzr [] (postdetailsr id_apzo)))));            (aswidgett ghc.base.. towidget)              ((blaze-markup-0.6.3.0:text.blaze.internal.preescapedtext                ghc.base.. data.text.pack)                 "\">");            (aswidgett ghc.base.. towidget)              (tohtml (blogpostpostdate post_apzp));            .... }

if change day text, works expect. i'm not sure why yesod can't deal day since has dayfield in yesod.form.fields expect handle this. figure simple, can't seem determine need fix error.

it appears there no instance tomarkup date datatype.

you supply instance yourself:

instance tomarkup date   tomarkup = tomarkup . show

which turns date string , converts markup. if default show instance doesn't fit needs supply formatter , put in place of show.


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