Make extras accessors safer

Even though it's "guaranteed" that values will be present because we set
them, nothing stops end-users from using these functions on Creds values
created by other plugins! Since that seems common, it would be
irresponsible of us to remain so unsafe.

README.md

@@ -44,16 +44,17 @@ authenticate creds = do
         eGitHubUser :: Either String GitHubUser
         eGitHubUser = getUserResponseJSON creds
 
-        -- Avert your eyes
+        -- Avert your eyes, simplified example
+        Just accessToken = getAccessToken creds
         Right githubUser = eGitHubUser
 
     -- Or make followup requests using our access token
-    runGitHub (getAccessToken creds) $ userRepositories githubUser
+    runGitHub accessToken $ userRepositories githubUser
 
     -- Or store it for later
     insert User
         { userIdent = credsIdent creds
-        , userAccessToken = getAccessToken creds
+        , userAccessToken = accessToken
         }
 ```
 

package.yaml

@@ -20,13 +20,13 @@ library:
   dependencies:
     - aeson >=0.6 && <1.3
     - bytestring >=0.9.1.4
+    - errors
     - hoauth2 >=1.3.0 && <1.6
     - http-client >=0.4.0 && <0.6
     - http-conduit >=2.0 && <3.0
     - http-types >=0.8 && <0.10
     - microlens
     - random
-    - safe
     - safe-exceptions
     - text >=0.7 && <2.0
     - transformers >=0.2.2 && <0.6

src/Yesod/Auth/OAuth2.hs

@@ -23,13 +23,14 @@ module Yesod.Auth.OAuth2
     , getUserResponseJSON
     ) where
 
+import Control.Error.Util (note)
+import Control.Monad ((<=<))
 import Data.Aeson (FromJSON, eitherDecode)
 import Data.ByteString.Lazy (ByteString, fromStrict)
 import Data.Text (Text)
 import Data.Text.Encoding (encodeUtf8)
 import Network.HTTP.Conduit (Manager)
 import Network.OAuth.OAuth2
-import Safe (fromJustNote)
 import Yesod.Auth
 import Yesod.Auth.OAuth2.Dispatch
 import Yesod.Core.Widget
@@ -62,22 +63,14 @@ authOAuth2Widget widget name oauth getCreds =
     login tm = [whamlet|<a href=@{tm $ oauth2Url name}>^{widget}|]
 
 -- | Read from the values set via @'setExtra'@
---
--- This is unsafe.
---
-getAccessToken :: Creds m -> AccessToken
-getAccessToken = AccessToken
-    . fromJustNote "yesod-auth-oauth2 bug: credsExtra without accessToken"
-    . lookup "accessToken" . credsExtra
+getAccessToken :: Creds m -> Maybe AccessToken
+getAccessToken =
+    (AccessToken <$>) . lookup "accessToken" . credsExtra
 
 -- | Read from the values set via @'setExtra'@
---
--- This is unsafe.
---
-getUserResponse :: Creds m -> ByteString
-getUserResponse = fromStrict . encodeUtf8
-    . fromJustNote "yesod-auth-oauth2 bug: credsExtra without userResponse"
-    . lookup "userResponse" . credsExtra
+getUserResponse :: Creds m -> Maybe ByteString
+getUserResponse =
+    (fromStrict . encodeUtf8 <$>) . lookup "userResponse" . credsExtra
 
 -- | Read from the values set via @'setExtra'@, decode as JSON
 --
@@ -85,4 +78,5 @@ getUserResponse = fromStrict . encodeUtf8
 -- errors.
 --
 getUserResponseJSON :: FromJSON a => Creds m -> Either String a
-getUserResponseJSON = eitherDecode . getUserResponse
+getUserResponseJSON =
+    eitherDecode <=< note "userResponse key not present" . getUserResponse