haskell - Adding response header in Servant -


i trying figure out how add cors response header in servant (basically, set response header "access-control-allow-origin: *"). wrote small test case below addheader function errors out. appreciate figuring out error below.

code:

{-# language cpp           #-} {-# language datakinds     #-} {-# language derivegeneric #-} {-# language typefamilies  #-} {-# language typeoperators #-} {-# language overloadedstrings #-} module main  import data.aeson import ghc.generics import network.wai import servant import network.wai.handler.warp (run) import control.monad.trans.either import control.monad.io.class (liftio) import control.monad (when, (<$!>)) import data.text t import data.configurator c import data.maybe import system.exit (exitfailure)  data user = user   { name              :: t.text   , password          :: t.text   } deriving (eq, show, generic)  instance tojson user instance fromjson user  type token = t.text  type userapi = "grant" :> reqbody '[json] user :> post '[json] (headers '[header "access-control-allow-origin" t.text] token)  userapi :: proxy userapi userapi = proxy  authuser :: user -> bool authuser u = case (password u) of     "somepass" -> true     _     -> false  server :: server userapi server = users     users :: user -> eithert servanterr io token         users u = case (authuser u) of           true -> return $ addheader "*" $ ("ok" :: token)           false -> return $ addheader "*" $ ("notok" :: token)  app ::  application app  = serve userapi server  main :: io () main = run 8081 app

this error get:

src/test.hs:43:10:     couldn't match type ‘headers                            '[header "access-control-allow-origin" text] text’                    ‘text’     expected type: server userapi       actual type: user -> eithert servanterr io token     in expression: users     in equation ‘server’:         server           = users                         users :: user -> eithert servanterr io token               users u                 = case (authuser u) of {                     true -> return $ addheader "*" $ ("something" :: token)                     false -> return $ addheader "*" $ ("something" :: token) }  src/test.hs:46:28:     couldn't match type ‘text’ ‘headers '[header h v0] text’     in expression: addheader "*"     in second argument of ‘($)’, namely       ‘addheader "*" $ ("something" :: token)’     in expression: return $ addheader "*" $ ("something" :: token)  src/test.hs:47:29:     couldn't match type ‘text’ ‘headers '[header h1 v1] text’     in expression: addheader "*"     in second argument of ‘($)’, namely       ‘addheader "*" $ ("something" :: token)’     in expression: return $ addheader "*" $ ("something" :: token)

i have working version simpler api (simple get) works. but, userapi of above type, errors out. addheader function type seems agree type signature way think it. missing here or won't error out this.

madjar suggested this, expand upon it: addheader changes return type:

x :: int x = 5  y :: headers '[header "someheader" string] int y = addheader "headerval" y

in case, means have update type of users binding return either servanterr io (headers '[header "access-control-allow-origin" t.text] token

more generally, can use :kind! server userapi in ghci see type synonym expands - that's helpful servant!


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