706ba61 · distributed-process

19-- outputs

20--

21-- > Hello world

22−-- > *** Exception: user error (Pattern match failure in do expression at tests/Traced.hs:107:3-9)

22+-- > *** Exception: user error (Pattern match failure in do expression at Traced.hs:187:3-9)

23-- > Trace:

24-- > 0 Left 2

25-- > 1 Left 1

36--

37-- The advantage of this idiom is that it gives you line number information when the guard fails:

38--

39−-- > *Traced> test3

39+-- > *Traced> test2

40−-- > *** Exception: user error (Pattern match failure in do expression at tests/Traced.hs:103:3-6)

40+-- > *** Exception: user error (Pattern match failure in do expression at Traced.hs:193:3-6)

41-- > Trace:

42−-- > 0 False

43−-- > 1 Left 1

42+-- > 0 Left 1

44module Traced ( MonadS(..)

43module Traced ( MonadS(..)

45 , return

44 , return

46 , (>>=)

45 , (>>=)

54import qualified Prelude

53import qualified Prelude

55import Data.String (fromString)

54import Data.String (fromString)

56import Control.Exception (catches, Handler(..), SomeException, throw, Exception(..), IOException)

55import Control.Exception (catches, Handler(..), SomeException, throw, Exception(..), IOException)

56+import Control.Applicative ((<$>))

57import Data.Typeable (Typeable)

58import Data.Maybe (catMaybes)

59import Data.List (intersperse)

98 trace :: a -> Maybe String

99

100instance (Trace a, Trace b) => Trace (Either a b) where

101− trace (Left x) = trace x

101+ trace (Left x) = ("Left " ++) <$> trace x

102− trace (Right y) = trace y

102+ trace (Right y) = ("Right " ++) <$> trace y

103

104instance Trace a => Trace [a] where

105 trace as = Just $ "[" ++ (concat . intersperse "," . catMaybes . map trace $ as) ++ "]"