terminology

What is the meaning of the concepts 'covariance' and 'contravariance'? Given 2 classes, Animal and Elephant (which inherits from Animal ), my understanding is that you would get a run-time errors if you try and put an Elephant into an array of Animals, and this happens because Elephant is "bigger" (more specific) than Animal. But could you place an Animal into an array of Elephant, seeing how Elephant is guaranteed to contain the Animal properties? You have it backwards. You can add an Elephant to an Animal array because it is an Animal, and it's guaranteed to have all of the methods an Animal

I've heard it referred to as a stream , as an infinite list , and sometimes even as a lazy sequence . What is the correct term for the following pattern? (Clojure code shown) (def first$ first) (defn second$ [str] (cond (empty? str) () true ((first (rest str))))) (defn stream-builder [next_ n] (cons n (cons (fn [] (stream-builder next_ (next_ n))) ()))) (defn stream [str n] (cond (= 0 n) () true (cons (first$ str) (stream (second$ str) (- n 1))))) (def odd (stream-builder (fn [n] (+ 2 n))1)) (println (stream odd 23)) > (1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45) Short

I'm reading the book: Haskell School of Expression and on page 56, at the beginning of chapter 5, I read the terms "polymorphic data types" and "polymorphic types". Are these two terms refer to the same concept? Are they synonyms ? Or is there any difference between the two? If yes, what? A type (in Haskell) is a piece of syntax which can meaningfully be put right of :: to classify an expression left of :: . Each syntactic component of a type is itself classified by a kind , where the kind of types (which classify expressions) is * . Some people are happy to use the word "type" to refer to any

What is the difference between function template and template function? Tim Sylvester The term "function template" refers to a kind of template. The term "template function" is sometimes used to mean the same thing, and sometimes to mean a function instantiated from a function template. This ambiguity is best avoided by using "function template" for the former and something like "function template instance" or "instance of a function template" for the latter. Note that a function template is not a function. The same distinction applies to "class template" versus "template class". From this FAQ

Hi i am a beginner in computer vision and i wish to know what exactly is the difference between a homography and affine tranformation, if you want to find the translation between two images which one would you use and why?. From papers and definitions I found online, I am yet to find the difference between them and where one is used instead of the other. Thanks for your help. I have set it down in the terms of a layman. Homography A homography, is a matrix that maps a given set of points in one image to the corresponding set of points in another image. The homography is a 3x3 matrix that maps