Reading "Parallel and Concurrent Programming in Haskell", at the bottom of page 27 the author wants to force the evaluation of a list of Strings: evaluate ( length puzzles ) Why not just evaluate puzzles ? https://github.com/simonmar/parconc-examples/blob/master/sudoku4.hs
On Monday 26 May 2014, 10:00:33, Derek McLoughlin wrote:
Reading "Parallel and Concurrent Programming in Haskell", at the bottom of page 27 the author wants to force the evaluation of a list of Strings:
evaluate ( length puzzles )
Why not just
evaluate puzzles
?
https://github.com/simonmar/parconc-examples/blob/master/sudoku4.hs
Because "evaluate" means "evaluate to weak head normal form", that is, to the outermost constructor or lambda. evaluate puzzles would evaluate the list just so far that it is known whether the list is empty or not. To evaluate something completely, one needs to evaluate a value that depends on the complete structure. Presumably, to determine the length of the list, one needs to evaluate all paths completely to see whether they lead to a valid puzzle, hence the evaluation is forced by demanding the evaluation of the length.
Thanks. Reading back, this is actually explained on page 14 - sorry for
that.
On 26 May 2014 11:22, "Daniel Fischer"
On Monday 26 May 2014, 10:00:33, Derek McLoughlin wrote:
Reading "Parallel and Concurrent Programming in Haskell", at the bottom of page 27 the author wants to force the evaluation of a list of Strings:
evaluate ( length puzzles )
Why not just
evaluate puzzles
?
https://github.com/simonmar/parconc-examples/blob/master/sudoku4.hs
Because "evaluate" means "evaluate to weak head normal form", that is, to the outermost constructor or lambda.
evaluate puzzles
would evaluate the list just so far that it is known whether the list is empty or not.
To evaluate something completely, one needs to evaluate a value that depends on the complete structure. Presumably, to determine the length of the list, one needs to evaluate all paths completely to see whether they lead to a valid puzzle, hence the evaluation is forced by demanding the evaluation of the length. _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners
On Mon, May 26, 2014 at 5:21 PM, Daniel Fischer < daniel.is.fischer@googlemail.com> wrote:
To evaluate something completely, one needs to evaluate a value that depends on the complete structure. Presumably, to determine the length of the list, one needs to evaluate all paths completely to see whether they lead to a valid puzzle, hence the evaluation is forced by demanding the evaluation of the length.
To avoid that presumption, you could also write evaluate $ rnf x whenever x is an instance of NFData. The length function strikes me as a kludgy stand-in for rnf when rnf is really what's meant. -- Kim-Ee
On Mon, May 26, 2014 at 08:21:36PM +0700, Kim-Ee Yeoh wrote:
The length function strikes me as a kludgy stand-in for rnf when rnf is really what's meant.
And depending on the type of the list entries, calling length won't always have the same effect then calling rnf on the list and therefore on all entries of the list. Greetings, Daniel
On Mon, May 26, 2014 at 04:25:35PM +0200, Daniel Trstenjak wrote:
On Mon, May 26, 2014 at 08:21:36PM +0700, Kim-Ee Yeoh wrote:
The length function strikes me as a kludgy stand-in for rnf when rnf is really what's meant.
And depending on the type of the list entries, calling length won't always have the same effect then calling rnf on the list and therefore on all entries of the list.
Though I could imagine a situation where one wants to force the spine of the list but not the elements it contains. In that case rnf would do too much, and 'length' would do the right thing. However, 'length' still feels kludgy here. What is really wanted is a domain-specific language for expressing which parts of a structure should be forced. Such a domain-specific language can be found here: http://hackage.haskell.org/package/parallel-3.2.0.4/docs/Control-Seq.html In particular, forcing the spine of a list but not its elements can be expressed by evaluate (puzzles `using` seqList r0) . -Brent
participants (5)
-
Brent Yorgey -
Daniel Fischer -
Daniel Trstenjak -
Derek McLoughlin -
Kim-Ee Yeoh