🤖What’s slack-log?
A command line app which automatically makes this👇:
YAMAMOTO Yuji (山本悠滋)
2021-11-07 Haskell Day 2021
Compared to Slack Archive, whose goal is same:
slack-log save:
slack-log generate-html:
Look into the cabal file!
app directory contains slack-log command’s source code, and there is Main.hs, which contains the main function
Main.hs from the beginning by the first line of Slack Web API callsFirst of all, open app/Main.hs:
import UI.Butcher.Monadic ({- ... snip ... -})
main :: IO ()
main = do
-- ... snip ...
mainFromCmdParserWithHelpDesc $ \helpDesc -> do
addHelpCommand helpDesc
addCmd "save" $ addCmdImpl saveCmd
addCmd "generate-html" $ do
-- ... snip ...
addCmdImpl $ generateHtmlCmd onlyIndex
addCmd "paginate-json" $ addCmdImpl paginateJsonCmdslack-log saveAdd the subcommand save here
slack-log save command availablesaveCmd is the actually executed IO actionsaveCmd Function (Overview)Divided into 3 parts:
saveCmd Function (Load Config 1)saveCmd Function (Load Config 2)saveCmd = do
-- ... --
apiConfig <- Slack.mkSlackConfig . slackApiToken =<< failWhenLeft =<< decodeEnv
-- ... --saveCmd Function (Load Config 3)saveCmd Function (Save Users List)saveUsersList function
saveUsersList to see how slack-log calls Slack’s Web APIsaveUsersList FunctionsaveUsersList :: ReaderT Slack.SlackConfig IO ()
saveUsersList = do
result <- Slack.usersList
-- ... Save users list extracted from the `result` ...Slack.usersList is the function imported from the slack-web package
usersList though it’s referred to as Slack.usersList by Haskell’s qualified import featureReaderT・runReaderT (1)The point here is:
ReaderT automatically passes the argument given by runReaderT to all functions listed in the do expressionsReaderT・runReaderT (2)Expressions using ReaderT I referred before are typed as following:
runReaderT :: ReaderT r IO a
-> r -> IO a
usersList :: ReaderT r IO (Response ListRsp)
saveUsersList :: ReaderT r IO ()runReaderT receives r (in slack-log, instantiated as SlackConfig), then transforms a Monad called (somewhat longer named) “ReaderT r IO” into just IOReaderT・runReaderT (3)Expressions using ReaderT I referred before are typed as following:
runReaderT :: ReaderT r IO a
-> r -> IO a
usersList :: ReaderT r IO (Response ListRsp)
saveUsersList :: ReaderT r IO ()usersList, all functions to call Slack’s Web API are typed as a “ReaderT r IO” Monad function
saveUsersList is also typed as a “ReaderT r IO” function tooMonad function, the caller needs to be the same type of Monad function tooReaderT?Monads’ features in a single do syntax by creating a composed Monad
do syntax of “ReaderT r IO”, both “Reader r” and IO are availableIO would be popular enough. How about “Reader r”?Reader?“Reader r a” is just an equivalent (newtype-ed) type of “r -> a”:
do as a Monad!Reader r” make functions a Monad?
doReader’s doExample: A function defined NOT using do of “Reader r”:
Reader’s doExample: Define a function equivalent to the last slide’s using do of “Reader r”:
someFunc :: Reader Arg [Result]
someFunc = do
r1 <- f1
r2 <- f2
r3 <- f3
return $ r1 ++ r2 ++ r3
-- To use it, apply runReader to it with an argument👇
runReader someFunc argarg only once to use runReader!Reader does in dorunReader to every function listed in the do
Reader by manually passing argsReader provides is sometimes important: e.g. Creating a embedded DSLusersList function👆 is virtually same with 👇
ReaderT?SlackConfig to every function calling Slack’s Web API such as usersList
ReaderT SlackConfig IO where possibleReaderT is used for the “ReaderT Pattern”, the most popular use case of Monad Transformers
usersList) are typed to use with the “ReaderT Pattern”
ReaderT Pattern”. Sorry!ReaderT?It’s easy enough to convert between “ReaderT SlackConfig IO a” and “SlackConfig -> IO a”
Libraries not directly related to the “ReaderT Pattern” should NOT force users to use ReaderT so that newbies don’t get confused
Actual type of usersList:
Its simplified version:
ReaderT (Recap 1)Lines to save users list (repeated):
ReaderT (Recap 2)With runReaderT and ReaderT,
becomes:
ReaderT (Recap 3)With runReaderT and ReaderT,
saveUsersList :: Slack.SlackConfig -> IO ()
saveUsersList apiConfig = do
result <- Slack.usersList apiConfig
-- ... Save users list extracted from the `result` ...becomes:
rio package?
ReaderT Pattern” more easily!path package?
butcher package, it easily parses command line argumentsslack-web package, learn how to use the ReaderT Monad Transformer!
(`runReaderT` apiConfig) someApiFunc without understanding ReaderT