2010/1/24 Stephen Tetley
Hi Sergey
Are you sure you always want return to write as well?
'return' for the standard' MTL writer monad produces _a_ within the monad, but does no "writing":
instance (Monoid w) => Monad (Writer w) where return a = Writer (a, mempty) m >>= k = Writer $ let (a, w) = runWriter m (b, w') = runWriter (k a) in (b, w `mappend` w')
Text.XHtml.Transitional has the noHtml function which would be the analogue to mempty.
Best wishes
Stephen
Mmm, yes.. it is not what i want actually. Seems, that my return-with-writing will break monad laws from "Real world Haskell". Now its clear, that i should use simple Writer Html (). Now code looks like: nested :: (Html -> Html) -> (Writer Html ()) -> Html nested a x = a << ( execWriter $ x ) buildPage :: Writer Html () buildPage = do tell $ header `nested` do tell $ meta ! [ strAttr "http-equiv" "Content-Type", strAttr "content" "text/html", strAttr "http-equiv" "text/html; charset=UTF-8" ] tell $ thetitle << "PageTitle" tell $ css_link ! [ href "css/blueprint/screen.css", media "screen, projection"] tell $ css_link ! [ href "css/blueprint/print.css", media "print"] tell $ css_link ! [ href "css/custom.css" ] tell $ body `nested` do tell $ noHtml -- ... and so on Thanks for warnings! -- Sergey