Currying and Partial Function Application (PFA)

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Making Your Program Spicy
Currying
and
Partial Function Application (PFA)
Dhaval Dalal
@softwareartisan
https://dhavaldala...

• Refers to the phenomena of rewriting a N-arg function
to a nest of functions, each taking only 1-arg at a time
Currying
...

Currying
For each arg, there is
another nested function,
that takes a arg and
returns a function taking
the subsequent arg...

Currying
• Function type associates towards right.
• In other words, arrow groups towards right.
Function<String, Function...

Why Currying?
• Facilitates arguments to be applied from diﬀerent
scopes.
Function<Integer, Function<Integer, Integer>> ad...

But, how does that
make programming
spicier?

• Let’s say we have a Customer repository.
• Now, we want to allow authorised calls to repo. So,
Let’s write an authorise ...

• We also have a Request.
• Lets see them in action…
class Request { 
public final int id; 
private Map<String, String> pa...

• Requests vary, however the CustomerRepository is
same.

• Can we avoid repeated injection of the repo?
Solution 1
Option...

• Requests vary, however the CustomerRepository is
same.

• Can we avoid repeated injection of the repo?
Solution 1
Option...

Solution 2
class Authoriser {
public
Function<Request, Optional<Customer>> authorise(CustomerRepository repo) {
return req...

Need some more
spice?

interface Transaction { }
interface ApprovalStrategy {
boolean approve(List<Transaction> ts);
//…
}
class Clearing {
priva...

Curried DI
interface Transaction { }
interface ApprovalStrategy {
boolean approve(List<Transaction> ts);
//…
}
class Clear...

Making Our Own Curry
<T, U, R>
Function<T, Function<U, R>> curry(BiFunction<T, U, R> fn) {
return t -> u -> fn.apply(t, u)...

Uncurry or Tuple it!
<T, U, R>
BiFunction<T, U, R> uncurry(Function<T, Function<U, R>> fn) {
return (t, u) -> fn.apply(t)....

Currying in Scala
scala> def merge1(x: String, y: Int) = x + y.toString
merge1: (x: String, y: Int)String
scala> val merge...

Curry Uncurry
scala> def merge1(x: String, y: Int) = x + y.toString
merge1: (x: String, y: Int)String
scala> val merge1F =...

PFA in Scala
// Partially Applied Function or Partial Function Application (PFA) 
def greet(salutation: String, fname: Str...

Re-shaping Third Party
Library Functions
• In third-party libraries we may never be able to
modify the code.

• Use Curry...

Currying in Haskell
greet :: String -> String -> String -> String 
greet salutation fname lname = salutation ++ ". " ++ fn...

Currying in Haskell
Prelude> :type (*)
(*) :: Num a => a -> a -> a
Prelude> multiply2 = (*2)
Prelude> :type multiply2
mult...

PFA in Clojure
(defn new-person [salutation f-name l-name] 
(println salutation f-name l-name)) 
 
(new-person "Mr." "Dhav...

PFA in Clojure
• What if we want to ﬁx the third arg - last name
instead of the 1st?
(def new-dalal (partial new-person sa...

Lambda to the rescue
• We wrap this in a lambda!
(def new-dalal (fn [sal fname] 
(new-person sal fname "Dalal")))
(new-dal...

Currying in Groovy
def filterList = { filter, list -> list.findAll(filter) } 
def even = { it % 2 == 0 } 
def evens = filt...

Currying in Groovy
• And if you need to curry from right-to-left, use rcurry()
def merge = { x, y, z -> 
println "arg0: x ...

Reﬂections
• You curry strictly from left-to-right.

• We don’t have to provide all the arguments to the
function at one g...

References
• Code Jugalbandi (http://codejugalbandi.org)

• Exploring Functional Programming - Ryan Lemmer and
Dhaval Dala...

Thank - You!
https://github.com/DhavalDalal/FunctionalConference-2019-Currying-PFA-Demo

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