scala - Why does spray routing formFields take a val instead of a type -

i trying set basic server using spray routing , use type represent 1 of forms puts.

i using formfields('some, 'fields).as(thing) notation described in documentation

at first glance assumed .as taking taking type , instantiating object of type using input defined in fromfields.

my example constructed this:

type updatepasswordrequest = tuple3[string, string, string]  startserver(interface, port) {   path("user" / intnumber) { useremail =>     put {       formfields('password, 'password2, 'key).as(updatepasswordrequest) { req =>         //...       }     }   }  } 

after getting rather confused error couldn't resolve symbol updatepasswordrequest changed type val , @ least compiles.

what happening here? why expecting val? can pass type parameter function?