s9gf4ult | 5 Oct 12:02 2012
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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.
Roman Cheplyaka | 5 Oct 12:33 2012

Re: does try force evaluation of IO value

* s9gf4ult <at> gmail.com <s9gf4ult <at> gmail.com> [2012-10-05 16:02:03+0600]
> 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-Exception-Base.html#g:7

Roman
s9gf4ult | 5 Oct 12:52 2012
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Re: does try force evaluation of IO value

> 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 ?
Roman Cheplyaka | 5 Oct 13:07 2012

Re: does try force evaluation of IO value

* s9gf4ult <at> gmail.com <s9gf4ult <at> gmail.com> [2012-10-05 16:52:52+0600]
> > 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).
(Continue reading)

s9gf4ult | 5 Oct 13:19 2012
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Re: does try force evaluation of IO value

>   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 ?
Roman Cheplyaka | 6 Oct 01:53 2012

Re: does try force evaluation of IO value

* s9gf4ult <at> gmail.com <s9gf4ult <at> gmail.com> [2012-10-05 17:19:21+0600]
> >   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

Gmane