doall is all you need. Just because the seq has type LazySeq doesn’t mean it has pending evaluation. Lazy seqs cache their results, so all you need to do is walk the lazy seq once (as doall does) in order to force it all, and thus render it non-lazy. seq does not force the entire … 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
Mostly because it can be more efficient — values don’t need to be computed if they’re not going to be used. For example, I may pass three values into a function, but depending on the sequence of conditional expressions, only a subset may actually be used. In a language like C, all three values would … 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
Python is actually behaving as defined. Three separate functions are created, but they each have the closure of the environment they’re defined in – in this case, the global environment (or the outer function’s environment if the loop is placed inside another function). This is exactly the problem, though – in this environment, i is … Read more
I needed this as well so I just took the source code from b93 and put it in a “util” class. I had to modify it slightly to work with the current API. For reference here’s the working code (take it at your own risk…): public static<A, B, C> Stream<C> zip(Stream<? extends A> a, Stream<? … 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
With dplyr >= 0.7.0, you can use pull() to get a vector from a tbl. library(dplyr, warn.conflicts = FALSE) db <- src_sqlite(tempfile(), create = TRUE) iris2 <- copy_to(db, iris) vec <- pull(iris2, Species) head(vec) #> [1] “setosa” “setosa” “setosa” “setosa” “setosa” “setosa”
The difference between them is, that a val is executed when it is defined whereas a lazy val is executed when it is accessed the first time. scala> val x = { println(“x”); 15 } x x: Int = 15 scala> lazy val y = { println(“y”); 13 } y: Int = <lazy> scala> x … Read more
It’s a strictness declaration. Basically, it means that it must be evaluated to what’s called “weak head normal form” when the data structure value is created. Let’s look at an example, so that we can see just what this means: data Foo = Foo Int Int !Int !(Maybe Int) f = Foo (2+2) (3+3) (4+4) … Read more