Currying & Partial Functions

Clojure does not support automatic currying, (+3) would result in applying + to 3, resulting with number 3 instead of a function that adds 3 as in Haskell. Therefore, in Clojure we use partial that enables the equivalent behavior.

  1. (defn sum
  2.   "Sum two numbers together"
  3.   [number1 number2]
  4.   (+ number1 number2))
  6. (sum 1 2)
  7. ;; => 3

If you try and evaluate sum with a single value then you get an arity exception

  1. (sum 1)
  2. ;; => clojure.lang.ArityException
  3. ;; => Wrong number of args (1) passed to: functional-concepts/sum

If we did need to call sum with fewer than the required arguments, for example if we are mapping sum over a vector, then we can use partial to help us call the sum function with the right number of arguments.

Lets add the value 2 to each element in our collection

  1. (map (partial sum 2) [1 3 5 7 9])

Using functions on more arguments than they can normally take

The reduce function can only work on a single collection as an argument (or a value and a collection), so an error occurs if you wish to reduce over multiple collections.

  1. (reduce + [1 2 3 4])
  2. ;; => 10
  4. (reduce + [1 2 3 4] [5 6 7 8])
  5. ;; returns an error due to invalid arguments

However, by using partial we can take one collection at once and return the result of reduce on each of those collections.

  1. (map (partial reduce +) [[1 2 3 4] [5 6 7 8]])

In the above example we mapp the partial reduce function over each element of the vector, each element being a collection.

Using partial to set a default value

We can use the partial function to create a default message that can be just given just the custom part. For example, if we want to have a default welcome message but include a custom part to the message at the end.

First we would define a function that combines parts of the message together.

  1. (defn join-strings
  2.   "join one or more strings"
  3.   [& args]
  4.   (apply str args))

The [& args] argument string says take all the arguments passed and refer to them by the name args. Its the & character that has the semantic meaning, so any name after the & can be used, although args is common if there is no domain specific context involved.

We can simply call this function with all the words of the message.

  1. (join-strings "Hello" " " "Clojure" " " "world")
  2. ;; ⇒ "Hello Clojure world"

Now we define a name called wrap-message that can be used to wrap the start of our message. This name binds to a partial function call to join-strings wich send that function the default message and any custom message you add when evaluationg wrap-message

  1. (def wrap-message (partial join-strings "Hello Clojurians in "))
  3. (wrap-message) 
  4. ;; ⇒ "Hello Clojurians in "
  6. (wrap-message "London")
  7.  ;; => "Hello Clojurians in London"

Currying in clojure

Currying is the process of taking some function that accepts multiple arguments, and turning it into a sequence of functions, each accepting a single argument. Or put another way, to transform a function with multiple arguments into a chain of single-argument functions.

Currying relies on having fixed argument sizes, whereas Clojure gets a lot of flexibility from variable argument lengths (variable arity).

Clojure therefore has the partial function gives results similar to currying, however the partical function also works with variable functions.

Partial refers to supplying some number of arguments to a function, and getting back a new function that takes the rest of the arguments and returns the final result

One advantage of partial is to avoid having to write your own anonymous functions

Useful references