nested configs which are optional

1 files changed, 12 insertions, 0 deletions

diff --git a/src/Conftrack.hs b/src/Conftrack.hs
index b15fa02..ab830ff 100644
--- a/src/Conftrack.hs
+++ b/src/Conftrack.hs
@@ -16,6 +16,7 @@ module Conftrack
   , readOptionalValue
   , readRequiredValue
   , readNested
+  , readNestedOptional
   , SomeSource
   , ConfigError(..)
   , Key(..)
@@ -145,6 +146,17 @@ readNested (Key prefix') = Fetch $ \s1 -> do
   (config, s2) <- nested (s1 { fetcherPrefix = fetcherPrefix s1 <> NonEmpty.toList prefix' })
   pure (config, s2 { fetcherPrefix = fetcherPrefix s1 })
 
+readNestedOptional :: forall a. Config a => Key -> Fetch (Maybe a)
+readNestedOptional (Key prefix) = Fetch $ \s1 -> do
+  let (Fetch nested) = readConfig @a
+  (config, s2) <- nested (s1 { fetcherPrefix = fetcherPrefix s1 <> NonEmpty.toList prefix })
+
+  if null (fetcherErrors s2)
+    then pure (Just config, s2 { fetcherPrefix = fetcherPrefix s1 })
+    -- TODO: resetting errors like this makes configKeysOf less useful. Perhaps move nested errors to warnings?
+    else pure (Nothing, s2 { fetcherPrefix = fetcherPrefix s1
+                           , fetcherErrors = fetcherErrors s1 })
+
 
 collectUnused :: [SomeSource] -> IO [Warning]
 collectUnused sources = do