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Composable loggers for PureScript.


A logger receives records and potentially performs some effects. You can create a logger from any function r -> m Unit for any r and m.

Unlike most other logging libraries, purescript-logger has no separate concepts "loggers" and "handlers". Instead, loggers can be composed into larger loggers using the Semigroup instance. Loggers can also be transformed using cmap (for transforming records) and cfilter (for filtering records). An example use case might be the following:

data Level = Debug | Info | Warning | Error
derive instance eqLevel :: Eq Level
derive instance ordLevel :: Ord Level

type Entry =
  { time   :: DateTime
  , level  :: Level
  , fields :: Map String String

fileLogger :: Path -> Logger Effect Entry
fileLogger path = Logger \entry -> ?todo {- append entry to file -}

main = do
  let debugLogger      = fileLogger "debug.log"      # cfilter (\e -> e.level == Debug)
  let productionLogger = fileLogger "production.log" # cfilter (\e -> e.level /= Debug)
  let logger = debugLogger <> productionLogger

  time <- now
  log logger {time, level: Info, fields: Map.singleton "msg" "boot"}