How do I force evaluation?

[Matthias Fischmann]

Yes, that does not look very nice but it should work.  In a private
response, I was also pointed out the alternative solution of using
case together with `seq` to trigger evaluation, which looked like the
default way to me (what else is seq for?):

|         case monstrousDS `seq` True of True -> hPutStr stderr "ok."

I first tried to wrap this in a `force' function, thereby proving that
I still haven't grasped the concept entirely.  (-:

But maybe it is possible to make a primitive of this in the same way
as has been done with assert?


  Anyway thanks for all your help,
   Matthias



"Jan de Wit" <jwit@studenten.net> writes:

> Hi,
> 
> > I want to do something like:
> >
> > | main = do
> > |         monstrousDataStructure <- monstrousComputation
> > |         hPutStr stderr "success!"
> > |         ...
> >
> > The important point is that I want to make sure that the computation
> > has terminated and not failed before I go to `...'.
> If the monstrous data structure has a 'serious' equality (not always
> returning True), you can try:
> 
> | main = do
> |  monstrousDataStructure <- monstrousComputation
> |  if (monstrousDataStructure == monstrousDataStructure) then
> |    hPutStr stderr "success!"
> |  else
> |     hPutStr stderr "failure?"
> 
> The idea is that comparing for equality walks over the entire data
> structure, forcing evaluation because every location has to be inspected.
> 
> Hope this helps,
> Jan de Wit
> 
> 
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Matthias Fischmann | Research Engineer               | +358 (9) 8565 7474
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