When starting a Snap app, the -e command line flag indicates what runtime environment to run in. The default is devel which will load all the devel.cfg Snaplet config files. The Environment module adds a splice to conditionally render a node list depending on the string passed to -e on the command line.

Here's an example:

 initApp :: SnapletInit App App
 initApp = makeSnaplet "app" "An snaplet example application." Nothing $ do
     h <- nestSnaplet "heist" heist $ heistInit "templates"
     addEnvironmentSplices h
     return $ App h

Then, in the Heist templates:

 <ifEnvironment name="devel">
   <link href="static/css/bootstrap.min.css" rel="stylesheet">
   <link href="static/css/font-awesome.min.css" rel="stylesheet">
 </ifEnvironment>
 <ifEnvironment name="prod">
   <link href="http://cdn/css/bootstrap.min.css" rel="stylesheet">
   <link href="http://cdn/css/font-awesome.min.css" rel="stylesheet">
 </ifEnvironment>

The <ifEnvironment name="prod"> block will only render if the command line was passed -e prod.