From the Spec: 6.12.3 InfixOperations An infix operator can be an arbitrary identifier. Infix operators have precedence and associativity defined as follows. … The associativity of an operator is determined by the operator’s last character. Operators ending in a colon ‘:’ are right-associative. All other operators are left- associative. You can always see how these … Read more
Yes, it’s concat from the Standard Prelude, given by concat :: [[a]] -> [a] concat xss = foldr (++) [] xss If you want to turn [[[a]]] into [a], you must use it twice: Prelude> (concat . concat) [[[1,2],[3]],[[4]]] [1,2,3,4]
Wadler, School of Haskell, LYAH, HaskellWiki, Quora and many more describe the list monad. Compare: (=<<) :: Monad m => (a -> m b) -> m a -> m b for lists with concatMap :: (a -> [b]) -> [a] -> [b] for m = []. The regular (>>=) bind operator has the arguments flipped, … Read more
Here is a pure version using dif/2 which implements sound inequality. dif/2 is offered by B-Prolog, YAP-Prolog, SICStus-Prolog and SWI-Prolog. firstdup(E, [E|L]) :- member(E, L). firstdup(E, [N|L]) :- non_member(N, L), firstdup(E, L). member(E, [E|_L]). member(E, [_X|L]) :- member(E, L). non_member(_E, []). non_member(E, [F|Fs]) :- dif(E, F), non_member(E, Fs). The advantages are that it can also … Read more
How about (defun get-file (filename) (with-open-file (stream filename) (loop for line = (read-line stream nil) while line collect line)))
You can check multiple conditions inside the same square brackets: /Location/Addr[State=”TX” or State=”AL” or State=”MA”] Or if you have a really long list, you can create a list of states and use the contains() function. /Location/Addr[contains(‘TX AL MA’, State)] This will work fine for two-letter state abbreviations. If you want to make it more robust … Read more
I want to propose a lazier version of merge: merge [] ys = ys merge (x:xs) ys = x:merge ys xs For one example use case you can check a recent SO question about lazy generation of combinations. The version in the accepted answer is unnecessarily strict in the second argument and that’s what is … Read more
attrs = [o.attr for o in objs] was the right code for making a list like the one you describe. Don’t try to subclass list for this. Is there something you did not like about that snippet?
In Java terms, Scala’s Seq would be Java’s List, and Scala’s List would be Java’s LinkedList. Note that Seq is a trait, which is similar to Java’s interface, but with the equivalent of up-and-coming defender methods. Scala’s List is an abstract class that is extended by Nil and ::, which are the concrete implementations of … Read more
List already has a slice method – it takes a subset of elements between a start and end index. What you’re looking for is repeated application of the span method: def span(p: (A) ⇒ Boolean): (List[A], List[A]) Which is documented as: Splits this list into a prefix/suffix pair according to a predicate. Note: c span … Read more