Intra subsite links

If you copy the “Hello world” subsite from the book, you should be able to run it using runghc (if not, something else is wrong—check the versions listed at the bottom of this page).

However, as soon as you insert a link inside the subsite, everything goes terribly awry. For instance, if you replace the definition of getSubRootR with:

getSubRootR = defaultLayout [whamlet|<a href=@{SubRootR}>Link to self|]

You are liable to get something like:

simple.hs:16:38:
    Could not deduce (master ~ HelloSub)
    from the context (Yesod master)
      bound by the type signature for
                 getSubRootR :: Yesod master => GHandler HelloSub master RepHtml
      at simple.hs:16:1-71
      `master' is a rigid type variable bound by
               the type signature for
                 getSubRootR :: Yesod master => GHandler HelloSub master RepHtml
               at simple.hs:16:1
    Expected type: Route master
      Actual type: Route HelloSub
    In the first argument of `\ u_a3g6
                                -> urender_a3g5 u_a3g6 []', namely
      `SubRootR'
    In the first argument of `toHtml', namely
      `\ u_a3g6 -> urender_a3g5 u_a3g6 [] SubRootR'

This is because SubRootR is of type Route HelloSub, but Yesod is expecting the routes to be the same type as master. One way to fix this problem is to use getRouteToMaster (edit: renamed getRouteToParent after Yesod 1.2) to transform SubRootR into type Route master:

getSubRootR = do
  toMaster <- getRouteToMaster
  defaultLayout [whamlet|<a href=@{toMaster SubRootR}>Link to self|]

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Based on a conversation with Michael Snoyman

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This material was tested with GHC 7.4.1 and yesod-platform 1.0.4.2.