[Haskell-cafe] Strictness in do block

[Duncan Coutts]

On Wed, 2006-07-05 at 12:08 +0100, Neil Mitchell wrote:
> Hi,
> 
> > What I want to know is the generic way to force an entire String (or
> > other list, perhaps from hGetContents) to be evaluated (read into RAM, I
> > guess) so the underlying file can be closed.... and do it right now.
> What I have done in the past is to take the length of the string, and
> test the length in some way - although I guess you could call seq on
> the length.

evaluate is for just that purpose.

evaluate (length input)


from the docs:

http://haskell.org/ghc/docs/latest/html/libraries/base/Control-Exception.html#v%3Aevaluate

        evaluate :: a -> IO a
        
        Forces its argument to be evaluated, and returns the result in
        the IO monad. It can be used to order evaluation with respect to
        other IO operations; its semantics are given by
        
           evaluate undefined `seq` return ()  ==> return ()
           catch (evaluate undefined) (\e -> return ())  ==> return ()
        
        NOTE: (evaluate a) is not the same as (a `seq` return a).


Duncan