#7924: throwIO gets subsumed by a later imprecise exception -----------------------------+---------------------------------------------- Reporter: dmwit | Owner: Type: bug | Status: new Priority: normal | Component: Compiler Version: 7.6.1 | Keywords: Os: Unknown/Multiple | Architecture: Unknown/Multiple Failure: None/Unknown | Blockedby: Blocking: | Related: -----------------------------+---------------------------------------------- The code below exits with exception "Boom" when compiled with no options (the expected behavior, since throwIO should always subsume exceptions that come later in the IO monad), but with a head-of-empty-list exception when compiled with -O. Similar code has shown this problem on 7.2 and 7.4. {{{ {-# LANGUAGE DeriveDataTypeable #-} import Control.Exception (throwIO, Exception) import Control.Monad (when) import Data.Typeable (Typeable) data Boom = Boom deriving (Show, Typeable) instance Exception Boom main = do args <- return [] -- Should throw this exception. when (length args /= 1) (throwIO Boom) -- With -O, instead throws this one from head []. let n = read (head args) when (n < 0) (throwIO Boom) return (fromInteger n :: Int) }}} -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7924 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler