Cannot set up servant app using stack - "Could not find module ‘Servant’"

Question

I'm trying to set up basic project using servant and stack loosely following official servant tutorial and stack guide. As soon as I add import Servant stack build fails with:

Could not find module ‘Servant’
Use -v to see a list of the files searched for.

I've defined servant as a dependency of this module and stack noticed it as stack dependencies shows:

aeson 0.8.0.2
array 0.5.1.0
attoparsec 0.12.1.6
base 4.8.1.0
binary 0.7.5.0
blaze-builder 0.4.0.1
bytestring 0.10.6.0
bytestring-conversion 0.3.1
case-insensitive 1.2.0.4
containers 0.5.6.2
deepseq 1.4.1.1
dlist 0.7.1.2
double-conversion 2.0.1.0
ghc-prim 0.4.0.0
hashable 1.2.3.3
http-media 0.6.2
http-types 0.8.6
integer-gmp 1.0.0.0
mtl 2.2.1
network-uri 2.6.0.3
parsec 3.1.9
primitive 0.6
scientific 0.3.3.8
servant 0.4.4.2
string-conversions 0.4
syb 0.5.1
template-haskell 2.10.0.0
text 1.2.1.3
tforia-products 0.1.0.0
time 1.5.0.1
transformers 0.4.2.0
unordered-containers 0.2.5.1
utf8-string 1.0.1.1
vector 0.10.12.3

Code of module:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}

module API
    ( ProductAPI
    , apiServer
    ) where

import Data.Aeson
import GHC.Generics
import Servant

data Product = Product
  { id :: String
  , name :: String
  } deriving (Eq, Show, Generic)

instance ToJSON Product

products :: [Product]
products =
  [ Product "id123" "shoes of some guy"
  , Product "id234" "hat of some gal"
  ]

type ProductAPI = "products" :> Get '[JSON] [Product]

apiServer :: Server UserAPI
apiServer = return products

Cabal definition of module:

library
  hs-source-dirs:      src
  exposed-modules:     API
  build-depends:       base >= 4.7 && < 5
                     , servant
                     , aeson
  default-language:    Haskell2010

I have no idea where else I have to define that dependency so stack/cabal is able to pick it up.

Answer 1

The Servant module comes from servant-server, as you can see here in the module list. The handy Servant module reexports everything from the servant package (which has all the types to describe web APIs) as well as the key things for running servant web apps.

Long story short: you need to add the servant-server package to your dependencies as well.