lazy-evaluation

问题 When can i cleanup objects stored in static variables in C#? I have a static variable that is lazily initialized: public class Sqm { private static Lazy<Sqm> _default = new Lazy<Sqm>(); public static Sqm Default { get { return _default.Value; } } } Note : That i've just changed Foo to be a static class. It doesn't change the question in any way if Foo is static or not. But some people are convinced that there is no way that an instance of Sqm could be constructed without first constructing an

问题 Drawing on the discussion on conditional dplyr evaluation I would like conditionally execute a step in pipeline depending on whether the reference column exists in the passed data frame. Example The results generated by 1) and 2) should be identical. Existing column # 1) mtcars %>% filter(am == 1) %>% filter(cyl == 4) # 2) mtcars %>% filter(am == 1) %>% { if("cyl" %in% names(.)) filter(cyl == 4) else . } Unavailable column # 1) mtcars %>% filter(am == 1) # 2) mtcars %>% filter(am == 1) %>% {

问题 I have the following code which provokes an error and writes a dump of all frames using dump.frames() as proposed e. g. by Hadley Wickham: a <- -1 b <- "Hello world!" bad.function <- function(value) { log(value) # the log function may cause an error or warning depending on the value } tryCatch( { a.local.value <- 42 bad.function(a) bad.function(b) }, error = function(e) { dump.frames(to.file = TRUE) }) When I restart the R session and load the dump to debug the problem via load(file = "last

问题 prefix(_ maxLength:) returns a type-erased Sequence in the following code EXAMPLE : public struct CycleIterator <Base: Sequence>: IteratorProtocol { public typealias Element = Base.Element private var sequence: Base private var iterator: Base.Iterator internal init (_ sequence: Base) { self.sequence = sequence self.iterator = self.sequence.makeIterator() } public mutating func next () -> Element? { var next = self.iterator.next() if next == nil { self.iterator = self.sequence.makeIterator()

问题 prefix(_ maxLength:) returns a type-erased Sequence in the following code EXAMPLE : public struct CycleIterator <Base: Sequence>: IteratorProtocol { public typealias Element = Base.Element private var sequence: Base private var iterator: Base.Iterator internal init (_ sequence: Base) { self.sequence = sequence self.iterator = self.sequence.makeIterator() } public mutating func next () -> Element? { var next = self.iterator.next() if next == nil { self.iterator = self.sequence.makeIterator()

问题 I'm writing a Clojure implementation of this coding challenge, attempting to find the average length of sequence records in Fasta format: >1 GATCGA GTC >2 GCA >3 AAAAA For more background see this related StackOverflow post about an Erlang solution. My beginner Clojure attempt uses lazy-seq to attempt to read in the file one record at a time so it will scale to large files. However it is fairly memory hungry and slow, so I suspect that it's not implemented optimally. Here is a solution using

问题 Take the following expression: FALSE AND (expression) Will MySQL evaluate the expression or just move on as soon as it sees FALSE ? Some background context-- I wanted to speed up a query by doing: JOIN... ON (indexed_column1=indexed_column2 AND non_indexed_column_a=non_indexed_column_b) For background on why I'm doing this query see this answer If it's going to always evaluate non_indexed_column_a=non_indexed_column_b then no time is saved with that. 回答1: The MySQL query optimizer uses

问题 The documentation of seq function says the following: A note on evaluation order: the expression seq a b does not guarantee that a will be evaluated before b . The only guarantee given by seq is that the both a and b will be evaluated before seq returns a value. In particular, this means that b may be evaluated before a . If you need to guarantee a specific order of evaluation, you must use the function pseq from the "parallel" package. So I have a lazy version of sum function with

问题 Unlike other unsafe* operations, the documentation for unsafeInterleaveIO is not very clear about its possible pitfalls. So exactly when is it unsafe? I would like to know the condition for both parallel/concurrent and the single threaded usage. More specifically, are the two functions in the following code semantically equivalent? If not, when and how? joinIO :: IO a -> (a -> IO b) -> IO b joinIO a f = do !x <- a !x' <- f x return x' joinIO':: IO a -> (a -> IO b) -> IO b joinIO' a f = do !x

问题 public class myClass { public myClass(String InstanceName) { Name = InstanceName; } public String Name { get; set; } } // Now using myClass lazily I have: Lazy<myClass> myLazy; Console.WriteLine(myLazy.Value.Name); My question is how to pass InstanceName to myClass constructor when we are using a lazy instance ? 回答1: Try this: Lazy<myClass> myLazy = new Lazy<myClass>(() => new myClass(InstanceName)); Remember that the expression is evaluated lazily, so if you change the value of the variable