does try force evaluation of IO value
if i have some lazy io action, assuming (maction :: IO a) which may raise exceptions while running, will "try" force evaluation of "a" to determine is exception raised or not ? Sorry for my broken english.
* s9gf4ult@gmail.com
if i have some lazy io action, assuming (maction :: IO a) which may raise exceptions while running, will "try" force evaluation of "a" to determine is exception raised or not ? Sorry for my broken english.
No, it won't force the return value. To do that, use evaluate http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Exc... Roman
No, it won't force the return value. To do that, use evaluate http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Exc eption-Base.html#g:7 But what is happening when exception is raised when consuming value from result of "try" ? try has signature try :: Exception e => IO a -> IO (Either e a) so it must return either exception value either evaluated value and must not raise exceptions, but how can "try" return (Right a) wthout making sure that action will not raise any exception ?
* s9gf4ult@gmail.com
No, it won't force the return value. To do that, use evaluate http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Exc eption-Base.html#g:7 But what is happening when exception is raised when consuming value from result of "try" ? try has signature try :: Exception e => IO a -> IO (Either e a) so it must return either exception value either evaluated value and must not raise exceptions, but how can "try" return (Right a) wthout making sure that action will not raise any exception ?
There's no promise that the returned lazy value won't throw any exceptions. Example: Prelude Control.Exception> r <- try (return $ error "bam") :: IO (Either SomeException Int) Prelude Control.Exception> r Right *** Exception: bam try will only catch exceptions that arise from executing the IO action. If you need to be sure that no exception will be raised, you need to force it yourself, like this: Prelude Control.Exception> r <- try (evaluate $ error "bam") :: IO (Either SomeException Int) Prelude Control.Exception> r Left bam If the return type is a lazy structure, you may also need to use deepseq ('evaluate' only evaluates to WHNF). Roman
* s9gf4ult@gmail.com
Prelude Control.Exception> r <- try (return $ error "bam") :: IO (Either SomeException Int) Prelude Control.Exception> r Right *** Exception: bam Wow, this is realy breaking news for me. Where can i read more about this magic ?
See chapter 19 of Real World Haskell, especially section "Laziness and Exception Handling". http://book.realworldhaskell.org/read/error-handling.html Roman
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Roman Cheplyaka -
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