haskell - How to response with HTTP status in custom servant handler? -

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i created custom servant handler

type serviceset = tvar (m.map string [microservice]) type localhandler = readert serviceset io

but failed find way response 404-not-found status code client in following function:

getservice :: string -> localhandler microservice getservice sn =   tvar <- ask   ms <- liftio $     sl <- atomically $       sm <- readtvar tvar       return $ case m.lookup sn sm of         nothing -> []         sl -> sl     let n = length sl     <- randomrio (0, n - 1)     return $ if n == 0       nothing       else . head . drop $ sl   case ms of     nothing -> ??? -- throwerror err404     ms' -> return ms'

how send 404 status code in ????

you need add exceptt stack of monad transforms. right now, readert, there no way encode notion of error being thrown.

{-# language datakinds     #-} {-# language typeoperators #-}  module lib  import control.monad.except import control.monad.reader import data.maybe import data.map import ghc.conc import prelude hiding (lookup) import servant.api import servant.server import system.random  type api =   capture "name" string :> '[json] int  type world =   tvar (map string [int])  type effects =   exceptt servanterr (readert world io)  server :: world -> server api server world =   enter (nat transform)       transform :: effects -> exceptt servanterr io     transform (exceptt foo) =       exceptt $ runreadert foo world  :: string -> effects int sn =   tvar <- ask   ms <- liftio $     sl <- atomically $       sm <- readtvar tvar       return (frommaybe [] (lookup sn sm))     let n = length sl     <- randomrio (0, n - 1)     return $ if n == 0       nothing       else . head . drop $ sl   case ms of     nothing ->       throwerror err404     ms' ->       return ms'

with exceptt servanterr . readert (tvar ...) can throwerror err404, servant catch , use return http 404. natural transformation exceptt servanterr . readert (tvar ...) :~> exceptt servanterr have unwrap , rewrap in order discharge reader effect. in all, not terribly more code.


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