[Haskell-cafe] does try force evaluation of IO value

[Roman Cheplyaka]

* 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).

Roman