You'll need the AdminR constructor: AdminR AdminRootR.

It doesn't work in the scaffolded site, whereas it works as expected in a single file site like https://github.com/yesodweb/yesod/wiki/Hierarchical-routes-and-breadcrumbs

I created a new scaffolded site, used add-handler to create Handler/Admin.hs, and changed the route in config/routes to

/admin AdminR:

    / AdminRootR GET

yesod devel results in Application.hs:27:1: Not in scope: data constructor `AdminRootR'

changing that to

/admin AdminRootR GET

runs without errors.

If you use
-ddump-splices when compiling, GHC will spit out the generated
code/data types, which should give a good idea of what's going on
under the surface.

I can't, it results in 

Import.hs:10:2: lexical error at character 'i'

Using ghc -cpp -ddump-splices main.hs 

results in Model.hs:13:1: Parse error: naked expression at top level

Arthur