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{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE ImpredicativeTypes #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
module Conftrack.Source (ConfigSource(..), SomeSource(..)) where
import Conftrack.Value (Key, Value(..), ConfigError(..))
import Control.Monad.State (StateT (..))
import Data.Text (Text)
-- | An abstraction over "config sources". This might mean file formats,
-- environment variables, or any other kind of format that can be seen as a
-- key-value store.
class ConfigSource s where
-- | Some sources require state, e.g. to keep track of which values were
-- already read.
type SourceState s
-- | read a single value from the source.
fetchValue :: Key -> s -> StateT (SourceState s) IO (Either ConfigError (Value, Text))
-- | given @s@, determine if any keys are "left over" and were not used.
-- This is used to produce warnings for unknown configuration options;
-- since not all sources can support this, this function's return type
-- includes @Maybe@ and sources are free to return @Nothing@ if they
-- cannot determine if any unknown keys are present.
leftovers :: s -> StateT (SourceState s) IO (Maybe [Key])
-- | An opaque type for any kind of config sources. Values of this type can be
-- acquired from they @Conftrack.Source.*@ modules, or by implementing the
-- 'ConfigSource' type class.
data SomeSource = forall source. ConfigSource source
=> SomeSource (source, SourceState source)
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