feat: add Config module for YAML config loading

package.yaml

@@ -46,6 +46,7 @@ dependencies:
   - time
   - wai
   - warp
+  - yaml
 
 library:
   source-dirs: src

roux-server.cabal

@@ -18,6 +18,7 @@ library
   exposed-modules:
       Data.CookLang
       Roux
+      Roux.Config
       Roux.CooklangPrint
       Roux.Html
       Roux.NYTimes
@@ -58,6 +59,7 @@ library
     , time
     , wai
     , warp
+    , yaml
   default-language: Haskell2010
 
 executable check-recipes
@@ -95,6 +97,7 @@ executable check-recipes
     , time
     , wai
     , warp
+    , yaml
   default-language: Haskell2010
 
 executable roux-server
@@ -133,6 +136,7 @@ executable roux-server
     , unix
     , wai
     , warp
+    , yaml
   default-language: Haskell2010
 
 test-suite roux-server-test
@@ -179,4 +183,5 @@ test-suite roux-server-test
     , time
     , wai
     , warp
+    , yaml
   default-language: Haskell2010

src/Roux/Config.hs

@@ -0,0 +1,72 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE OverloadedStrings #-}
+
+{- | Configuration for Roux, loaded from a YAML config file.
+
+Default location: ./roux-config.yaml in the working directory.
+Overridable via --config-file CLI flag.
+-}
+module Roux.Config (
+    RouxConfig (..),
+    AnthropicConfig (..),
+    loadConfig,
+) where
+
+import Data.Text (Text)
+import qualified Data.Yaml as Y
+import qualified Fleece.Aeson as Fleece
+import qualified Fleece.Aeson.Decoder as FleeceD
+import Fleece.Core (constructor, object, optionalNullable, required, text, (#+))
+import qualified Fleece.Core as FC
+import System.Directory (doesFileExist)
+
+-- | Top-level config.
+data RouxConfig = RouxConfig
+    { rcAnthropic :: Maybe AnthropicConfig
+    }
+    deriving stock (Eq, Show)
+
+-- | Anthropic API configuration.
+data AnthropicConfig = AnthropicConfig
+    { acApiKey :: !Text
+    , acBaseUrl :: !(Maybe Text) -- default: https://api.anthropic.com
+    , acModel :: !(Maybe Text) -- default: claude-sonnet-4-20250514
+    }
+    deriving stock (Eq, Show)
+
+-- | Fleece schema for AnthropicConfig.
+anthropicConfigSchema :: FC.Schema Fleece.Decoder AnthropicConfig
+anthropicConfigSchema =
+    object $
+        constructor AnthropicConfig
+            #+ required "api_key" acApiKey text
+            #+ optionalNullable FC.EmitNull "base_url" acBaseUrl text
+            #+ optionalNullable FC.EmitNull "model" acModel text
+
+-- | Fleece schema for RouxConfig.
+rouxConfigSchema :: FC.Schema Fleece.Decoder RouxConfig
+rouxConfigSchema =
+    object $
+        constructor RouxConfig
+            #+ optionalNullable FC.EmitNull "anthropic" rcAnthropic anthropicConfigSchema
+
+{- | Load config from a YAML file. Returns config with nothing set if the file
+doesn't exist (LLM features will be unavailable but the server still runs).
+Exits with an error if the file exists but is malformed.
+-}
+loadConfig :: FilePath -> IO RouxConfig
+loadConfig path = do
+    exists <- doesFileExist path
+    if not exists
+        then pure (RouxConfig Nothing)
+        else do
+            result <- Y.decodeFileEither path
+            case result of
+                Left parseErr ->
+                    fail $ "Failed to parse " <> path <> ": " <> show parseErr
+                Right val ->
+                    case FleeceD.fromValue rouxConfigSchema val of
+                        Right cfg -> pure cfg
+                        Left decodeErr ->
+                            fail $
+                                "Invalid config format in " <> path <> ": " <> show decodeErr