I’ve been spending a good amount of time lately working on 4clojure in my free time. For those who don’t know, 4clojure is archive of problems for practicing Clojure. It accepts solutions in Clojure, runs the test cases and gives you feedback right in the browser.

Today I’ll share my experience on solving an interesting problem and seeing how we can use Clojure’s vast repertoire of functions in the standard library to make the code elegant and idiomatic. I’ll start with how I solved the problem and then contrast it with how knowing the standard library (& being clever) helps.

An excerpt of the problem is stated below -

Your friend Joe is always whining about Lisps using the prefix notation for math. Show him how you could easily write a function that does math using the infix notation. Assume a simple calculator that does not do precedence and instead just calculates left to right.

Quite simply, you need to write a calculator which takes an infix expression in a language that evaluates only prefix or polish notation by default.

Here are a few test cases for the same -

(= 42 (__ 38 + 48 - 2 / 2))
(= 72 (__ 20 / 2 + 2 + 4 + 8 - 6 - 10 * 9))


Let’s start with a naive solution. We’ll use what I like to call the Swiss Army knife of flow control in Clojure - loop and recur.

(defn infix [& exprs]
  (loop [[x & xs] exprs 
         acc 0 
         oper +]
        (nil? x) acc
        (number? x) (recur xs (oper acc x) nil)
        :else (recur xs acc x))))
; testing the function        
(infix 38 + 48 - 2 / 2)
;=> 42

The solution is quite self-explanatory. We start with defining our variadic function using &. Next, we provide a set of bindings to loop to begin the recursion. We use the [x & xs] syntax to bind the first and the rest of the expression list. Lastly, we set up conditions using cond. When we encounter a number, we recurse by updating the binding for acc. The else clause indicates the case where we get a operator wherein we simply update the oper binding.


Another interesting way to do it is by noticing that an infix expression is actually made up of sub-expressions, which could be converted to prefix and evaluated one by one. For example 32 + 42 - 4 is ((32 + 42) - 4). Our strategy here can be to pick first 3 tokens, evaluate them and recursively evaluate the rest of the expression. The code for that is below -

(defn infix 
  ([x op y] (op x y))
  ([x op y & xs]
    (apply infix (cons (infix x op y) xs))))

Here we define two sets of expressions that will be returned by the function depending on whether it is passed 3 or more arguments. The case when we have more than 3, we simply extract first three, recursively call the function with those three and build a list out of the result. The clever trick here is apply which is required to make infix work on a list. To demonstrate, (apply (infix '(34 + 53)) is same as (infix 34 + 53).


This is a really neat way of solving the problem. To lead you to the insight, lets look at infix expressions from another perspective - 38 (+ 48) (- 2) (/ 2). If you set aside the first element, you can see that each sub-expression is one third of a prefix expression. So somehow if we can get the 38 to evaluate with (+ 48) and use that result to carry out this operation recursively, we are done!

To split into groups of two, we can use partition.

(partition 2 (rest '(38 + 48 - 2 / 2)))
; => ((+ 48) (- 2) (/ 2))

Ok, now lets use the immensely powerful reduce to evaluate the list. But what about the first element? Remember, reduce also takes an accumulator as the starting point for reduction so that’s where we can accommodate the first elem. With that, we have our function.

(defn infix [x & expr] 
  (reduce (fn [acc [oper y]] (oper acc y))
           x (partition 2 expr)))


I hope this demonstrates the power of functions available in clojure.core. Using them you can build elegant and idiomatic solutions to most of your problems. Having a good handle on the standard library would mean that you would rarely have to resort to primitives such as loop and recur. Till next time!