MIT licensed by Daniel YU
Maintained by Daniel YU

Module documentation for 0.3.3

This version can be pinned in stack with:salak-0.3.3@sha256:acc240e9f70be4eb20fe20671f1036c1a212ccdf4b0212f591b93ce27e0ee267,2530


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Configuration (re)loader in Haskell.






This library defines a universal procedure to load configurations and parse properties, also supports reload configuration files.

Parse Functions

HasSalak monad provide a unified function require to parse properties. Here are some examples.

a :: Bool              <- require "bool.key"
b :: Maybe Int         <- require "int.optional.key"
c :: Either String Int <- require "int.error.key"
d :: IO Int            <- require "int.reloadable.key" -- This property can be changed by reloading configurations.

Load Strategy

We can load configurations from command lines, environment, configuration files such as yaml or toml etc., and we may want to have our own strategies to load configurations from multiply sources and overwrite properties by orders of these sources.

PropConfig defines a common loading strategy:

  1. loadCommandLine
  2. loadEnvironment
  3. loadConfFiles
  4. load file from folder application.dir if defined
  5. load file from current folder if enabled
  6. load file from home folder if enabled
  7. file extension matching, support yaml or toml or any other loader.

Load earlier has higher priorities. Priorities cannot be changed.

For command lines and environment,

CommandLine:  --package.a.enabled=true
Environment: PACKAGE_A_ENABLED: false



export TEST_CONFIG_NAME=daniel

Current Directory: salak.yaml

  name: noop
  dir: ls

Current Directory: salak.toml

data Config = Config
  { name :: Text
  , dir  :: Maybe Text
  , ext  :: Int
  } deriving (Eq, Show)

instance Monad m => FromProp m Config where
  fromProp = Config
    <$> "user" ? pattern "[a-z]{5,16}"
    <*> "pwd"
    <*> "ext" .?= 1

main = runSalakWith "salak" (YAML :|: TOML) $ do
  c :: Config <- require "test.config"
  lift $ print c

GHCi play

λ> :set -XFlexibleInstances -XMultiParamTypeClasses -XOverloadedStrings
λ> import Salak
λ> import Data.Default
λ> import Data.Text(Text)
λ> data Config = Config { name :: Text, dir  :: Maybe Text, ext  :: Int} deriving (Eq, Show)
λ> instance Monad m => FromProp m Config where fromProp = Config <$> "user" <*> "dir" <*> "ext" .?= 1
λ> runSalak def (require "") :: IO Config
Config {name = "daniel", dir = Nothing, ext = 1}


  • Recover placeholder
  • Add git pull support.
  • Add automatic reloading.
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