[Haskell-cafe] To seq or not to seq, that is the question

[Edward Z. Yang]

Excerpts from Tom Ellis's message of Sat Mar 09 00:34:41 -0800 2013:
> I've never looked at evaluate before but I've just found it's haddock and
> given it some thought.
> 
>     http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Exception-Base.html#v:evaluate
> 
> Since it is asserted that
> 
>     evaluate x = (return $! x) >>= return
> 
> is it right to say (on an informal level at least) that evaluating an IO
> action to WHNF means evaluating it to the outermost >>= or return?

Sure.

Prelude> let x = undefined :: IO a
Prelude> x `seq` ()
*** Exception: Prelude.undefined
Prelude> (x >>= undefined) `seq` ()
()

> > For non-IO monads, since everything is imprecise anyway, it doesn't
> > matter.
> 
> Could you explain what you mean by "imprecise"?

Imprecise as in imprecise exceptions, http://research.microsoft.com/en-us/um/people/simonpj/papers/imprecise-exn.htm

Edward