Clojure is not statically typed, so doesn’t need the strict this/that/whatever type declarations that are necessary in haskell (and, I gather, Scala). If you want to return a string, you return a string; if you return nil instead, that’s okay too. “Functional” does not correspond exactly to “strict compile-time typing”. They are orthogonal concepts, and … Read more
You might check out how Martin Erwig’s Haskell functional graph library does things. For instance, its shortest-path functions are all pure, and you can see the source code for how it’s implemented. Another option, like fmark mentioned, is to use an abstraction which allows you to implement pure functions in terms of state. He mentions … Read more
I am going to answer for a medium-sized project in the conditions that I am familiar with, that is between 100K and 1M lines of source code and up to 10 developers. This is what we are using now, for a project started two months ago in August 2013. Build system and code organization: one … Read more
TL;DR A functional implementation can be faster than your original procedural implementation, in certain cases. Why is the functional style so much slower than the imperative style? Is there some problem with the functional implementation which is causing such a huge slowdown? As Matthieu M. already pointed out, the important thing to note is that … Read more
Purity is only violated if you observe the exception, and make a decision based on it that changes the control flow. Actually throwing an exception value is referentially transparent — it is semantically equivalent to non-termination or other so-called bottom values. If a (pure) function is not total, then it evaluates to a bottom value. … Read more
I think @Vijay’s answer is the best one so far and I just intend to Erlangify it. Pairs (cons cells) in Erlang are written as [Head|Tail] and nil is written as []. There is no restriction as to what the head and tail are but if you use the tail to chain more cons cells … Read more
Yes, actually you can take some code and convert every function call—and every return—into a tail call. What you end up with is called continuation-passing style, or CPS. For example, here’s a function containing two recursive calls: (define (count-tree t) (if (pair? t) (+ (count-tree (car t)) (count-tree (cdr t))) 1)) And here’s how it … Read more
The Little Schemer teaches recursion really well, and it’s fun and simple to read. I also liked The Scheme Programming Language for a broader introduction into the language.
There’s no community consensus on what are the essential concepts in functional programming. In Why Functional Programming Matters (PDF), John Hughes argues that they are higher-order functions and lazy evaluation. In Wearing the Hair Shirt: A Retrospective on Haskell, Simon Peyton Jones says the real essential is not laziness but purity. Richard Bird would agree. … Read more
The arity of a function is stored in the metadata of the var. (:arglists (meta #’str)) ;([] [x] [x & ys]) This requires that the function was either defined using defn, or the :arglists metadata supplied explicitly.