Control.Monad.Error.Class

初めてまともに使った。エラー抽象化できるのは気分が良い。

% runghc
{-# LANGUAGE ScopedTypeVariables #-}
import Control.Monad.Error.Class

safeHead :: (Error e, MonadError e m) => [a] -> m a
safeHead (x:_) = return x
safeHead [] = throwError $ strMsg "NG"

main :: IO ()
main = let try = do
             (n :: Int) <- safeHead [3 ..]
             putStrLn $ show n
             (m :: Int) <- safeHead []
             putStrLn $ show m
             return ()
       in try `catchError` (\e -> print e)

【実行結果】
3
user error (NG)