Your example of declarative programming above is not an actual program, so it’s not a good example. The main difference is between imperative and declarative. Functional is a particular kind of declarative. C, C++, Java, Javascript, BASIC, Python, Ruby, and most other programming languages are imperative. As a rule, if it has explicit loops (for, … Read more
One way to find all the repeated values in a list is using groupingBy and then filter the values which are > 1. E.g. val list = listOf(“orange”, “apple”, “apple”, “banana”, “water”, “bread”, “banana”) println(list.groupingBy { it }.eachCount().filter { it.value > 1 }) Output {apple=2, banana=2}
Functional programmers generally don’t have a lot of use for diagrams. Many functional programmers (but not all) find that writing down types is a good way to encapsulate the design relationships that OO programmers put into UML diagrams. Because mutable state is rare in functional programs, there are no mutable “objects”, so it is not … Read more
I’m glad you’re starting by asking about a specification rather than implementation first. There are a lot of ideas floating around about what FRP is. From the very start in the early 90’s (when I was working in interactive graphics at Sun Microsystems and then Microsoft Research), it has been about two properties (a) denotative … Read more
Downward funargs are local functions that are not returned or otherwise leave their declaration scope. They only can be passed downwards to other functions from the current scope. Two examples. This is a downward funarg: function () { var a = 42; var f = function () { return a + 1; } foo(f); // … Read more
It’s hard for me to think of many downsides to functional programming. Then again, I am a former chair of the International Conference on Functional Programming, so you may safely assume I am biased. I think the main downsides have to do with isolation and with barriers to entry. Learning to write good functional programs … Read more
One key feature in a functional language is the concept of first-class functions. The idea is that you can pass functions as parameters to other functions and return them as values. Functional programming involves writing code that does not change state. The primary reason for doing so is so that successive calls to a function … Read more
Objects are poor man’s closures. Consider Java. Java is an object-oriented programming language with no language level support for real lexical closures. As a work-around Java programmers use anonymous inner classes that can close over the variables available in lexical scope (provided they’re final). In this sense, objects are poor man’s closures. Closures are poor … Read more
Continuations can be modelled as monads, using CPS, but they are not algebraic effects as they cannot be modelled using Lawvere theories. See Martin Hyland and John Power, 2007, The Category Theoretic Understanding of Universal Algebra: Lawvere Theories and Monads (pdf), ENTCS 172:437-458.
As @ljedrz points out, to make it work you only need to reference the composed functions again: let finally = compose(&*multiply_and_add, &*divide_and_subtract); (Note that in Rust, convention dictates that variable names should be in snake_case) However, we can make this better! Since Rust 1.26, we can use abstract return types (previously featured gated as #![feature(conservative_impl_trait)]). … Read more