It’s not really related to SQL. This example in R does not work either: df <- data.frame( v1 = sample(5, 10, replace = TRUE), v2 = sample(5,10, replace = TRUE) ) df %>% filter_(~ “v1” == 1) It does not work because you need to pass to filter_ the expression ~ v1 == 1 — … Read more
How folds differ seems to be a frequent source of confusion, so here’s a more general overview: Consider folding a list of n values [x1, x2, x3, x4 … xn ] with some function f and seed z. foldl is: Left associative: f ( … (f (f (f (f z x1) x2) x3) x4) …) … Read more
You typically use it when you want to instantiate something the first time its actually used. This delays the cost of creating it till if/when it’s needed instead of always incurring the cost. Usually this is preferable when the object may or may not be used and the cost of constructing it is non-trivial.
Lazy IO has the problem that releasing whatever resource you have acquired is somewhat unpredictable, as it depends on how your program consumes the data — its “demand pattern”. Once your program drops the last reference to the resource, the GC will eventually run and release that resource. Lazy streams are a very convenient style … Read more
There is much confusion about this issue. The usual reason given is that “repeatedly appending at end of list requires repeated traversals of list and is thus O(n^2)“. But it would only be so simple under strict evaluation. Under lazy evaluation everything is supposed to be delayed, so it begs the question whether there actually … Read more
This is taken from the scala mailing list and gives implementation details of lazy in terms of Java code (rather than bytecode): class LazyTest { lazy val msg = “Lazy” } is compiled to something equivalent to the following Java code: class LazyTest { public int bitmap$0; private String msg; public String msg() { if … Read more
The problem is that you are trying to access a collection in an object that is detached. You need to re-attach the object before accessing the collection to the current session. You can do that through session.update(object); Using lazy=false is not a good solution because you are throwing away the Lazy Initialization feature of hibernate. … Read more
You’re being hit by filter‘s laziness. Change (filter …) to (doall (filter …)) in your recur form and the problem should go away. A more in-depth explanation: The call to filter returns a lazy seq, which materialises actual elements of the filtered seq as required. As written, your code stacks filter upon filter upon filter…, … Read more
Contrary to what others have stated here, this function does not implement the true sieve of Eratosthenes. It does returns an initial sequence of the prime numbers though, and in a similar manner, so it’s okay to think of it as the sieve of Eratosthenes. I was about done explaining the code when mipadi posted … Read more
The evaluation mechanism in Haskell is by-need: when a value is needed, it is calculated, and kept ready in case it is asked for again. If we define some list, xs=[0..] and later ask for its 100th element, xs!!99, the 100th slot in the list gets “fleshed out”, holding the number 99 now, ready for … Read more