[Haskell-cafe] guided seq (Was: Interruptible threads with IO loops)

[Joachim Breitner]

Dear Wren,

Am Mittwoch, den 21.12.2011, 13:04 -0500 schrieb wren ng thornton:
> On 12/21/11 4:31 AM, Joachim Breitner wrote:
> > This would be particularly handy when with, for example
> >          snd tuple `evaluateUntilItDoesNotNeed` tuple
> > where the tuple is unevaluated in the right component (and where we
> > don’t want to force this just now), but retains something large in the
> > left component (that we want to become GCable).
> 
> Can't you already do that with:
> 
>      let (_,x) = tuple in ...x...
> 
> or
> 
>      case tuple of (_,x) -> ...x...
> 
> ?
> 
> The tuple will be evaluated far enough that we can access its second 
> component, and then we're free to discard the tuple itself provided it's 
> not referenced elsewhere. The above would only be strict in x if the use 
> sites are strict.
> 
> Or do you mean that you want something with the semantics of the above, 
> but with a syntactic form that enables us to abstract out the ellipses?

Your first example would not work, because the tuple is only evaluated
when x is evaluated, which might be later than the point where I want
the left component of the tuple to be GCed:

Prelude Debug.Trace> let (_,x) = trace "some tuple" (error "a", error "b") in const () x
()

The second case works in this particular instance:

Prelude Debug.Trace> case  trace "some tuple" (error "a", error "b") of (_,x) -> const () x
some tuple
()


But assume the function is not snd but rather a library provided
function that you have no control over and that you know returns one
component of the tuple, but you don’t know which (depending on other
parameters, perhaps). The advantage of a `evaluateUntilItDoesNotNeed`
would be that it can be combined with code that was written without
having such considerations in mind.

Greetings,
Joachim
        

-- 
Joachim "nomeata" Breitner
  mail at joachim-breitner.de  |  nomeata at debian.org  |  GPG: 0x4743206C
  xmpp: nomeata at joachim-breitner.de | http://www.joachim-breitner.de/

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