Originally posted on yazeedb.com
Eric Elliott’s exceptional Composing Software series is initially what got me excited about functional programming. It's a must-read.
At one point in the series, he mentioned currying. Both computer science and mathematics agree on the definition:
Currying turns multi-argument functions into unary (single argument) functions.
Curried functions take many arguments one at a time. So if you have
greet = (greeting, first, last) => `${greeting}, ${first} ${last}`;
greet('Hello', 'Bruce', 'Wayne'); // Hello, Bruce Wayne
Properly currying greet
gives you
curriedGreet = curry(greet);
curriedGreet('Hello')('Bruce')('Wayne'); // Hello, Bruce Wayne
This 3-argument function has been turned into three unary functions. As you supply one parameter, a new function pops out expecting the next one.
Properly?
I say "properly currying" because some curry
functions are more flexible in their usage. Currying's great in theory, but invoking a function for each argument gets tiring in JavaScript.
Ramda's curry
function lets you invoke curriedGreet
like this:
// greet requires 3 params: (greeting, first, last)
// these all return a function looking for (first, last)
curriedGreet('Hello');
curriedGreet('Hello')();
curriedGreet()('Hello')()();
// these all return a function looking for (last)
curriedGreet('Hello')('Bruce');
curriedGreet('Hello', 'Bruce');
curriedGreet('Hello')()('Bruce')();
// these return a greeting, since all 3 params were honored
curriedGreet('Hello')('Bruce')('Wayne');
curriedGreet('Hello', 'Bruce', 'Wayne');
curriedGreet('Hello', 'Bruce')()()('Wayne');
Notice you can choose to give multiple arguments in a single shot. This implementation's more useful while writing code.
And as demonstrated above, you can invoke this function forever without parameters and it’ll always return a function that expects the remaining parameters.
How's This Possible?
Mr. Elliot shared a curry
implementation much like Ramda's. Here’s the code, or as he aptly called it, a magic spell:
const curry = (f, arr = []) => (...args) =>
((a) => (a.length === f.length ? f(...a) : curry(f, a)))([...arr, ...args]);
Umm… 😐
Yeah, I know... It's incredibly concise, so let's refactor and appreciate it together.
This Version Works the Same
I've also sprinkled debugger
statements to examine it in Chrome Developer Tools.
curry = (originalFunction, initialParams = []) => {
debugger;
return (...nextParams) => {
debugger;
const curriedFunction = (params) => {
debugger;
if (params.length === originalFunction.length) {
return originalFunction(...params);
}
return curry(originalFunction, params);
};
return curriedFunction([...initialParams, ...nextParams]);
};
};
Open your Developer Tools and follow along!
Let's Do This!
Paste greet
and curry
into your console. Then enter curriedGreet = curry(greet)
and begin the madness.
Pause on line 2
Inspecting our two params we see originalFunction
is greet
and initialParams
defaulted to an empty array because we didn’t supply it. Move to the next breakpoint and, oh wait… that’s it.
Yep! curry(greet)
just returns a new function that expects 3 more parameters. Type curriedGreet
in the console to see what I’m talking about.
When you’re done playing with that, let’s get a bit crazier and do
sayHello = curriedGreet('Hello')
.
Pause on line 4
Before moving on, type originalFunction
and initialParams
in your console. Notice we can still access those 2 parameters even though we’re in a completely new function? That’s because functions returned from parent functions enjoy their parent’s scope.
Real-life inheritance
After a parent function passes on, they leave their parameters for their kids to use. Kind of like inheritance in the real life sense.
curry
was initially given originalFunction
and initialParams
and then returned a “child” function. Those 2 variables weren’t disposed of yet because maybe that child needs them. If he doesn’t, then that scope gets cleaned up because when no one references you, that’s when you truly die.
Ok, back to line 4…
Inspect nextParams
and see that it’s ['Hello']
…an array? But I thought we said curriedGreet(‘Hello’)
, not curriedGreet(['Hello'])
!
Correct: we invoked curriedGreet
with 'Hello'
, but thanks to the rest syntax, we’ve turned 'Hello'
into ['Hello']
.
Y THO?!
curry
is a general function that can be supplied 1, 10, or 10,000,000 parameters, so it needs a way to reference all of them. Using the rest syntax like that captures every single parameter in one array, making curry
's job much easier.
Let’s jump to the next debugger
statement.
Line 6 now, but hold on.
You may have noticed that line 12 actually ran before the debugger
statement on line 6. If not, look closer. Our program defines a function called curriedFunction
on line 5, uses it on line 12, and then we hit that debugger
statement on line 6. And what’s curriedFunction
invoked with?
[...initialParams, ...nextParams];
Yuuuup. Look at params
on line 5 and you’ll see ['Hello']
. Both initialParams
and nextParams
were arrays, so we flattened and combined them into a single array using the handy spread operator.
Here’s where the good stuff happens.
Line 7 says “If params
and originalFunction
are the same length, call greet
with our params and we’re done.” Which reminds me…
JavaScript functions have lengths too
This is how curry
does its magic! This is how it decides whether or not to ask for more parameters.
In JavaScript, a function’s .length
property tells you how many arguments it expects.
greet.length; // 3
iTakeOneParam = (a) => {};
iTakeTwoParams = (a, b) => {};
iTakeOneParam.length; // 1
iTakeTwoParams.length; // 2
If our provided and expected parameters match, we’re good, just hand them off to the original function and finish the job!
That’s baller 🏀
But in our case, the parameters and function length are not the same. We only provided ‘Hello’
, so params.length
is 1, and originalFunction.length
is 3 because greet
expects 3 parameters: greeting, first, last
.
So what happens next?
Well since that if
statement evaluates to false
, the code will skip to line 10 and re-invoke our master curry
function. It re-receives greet
and this time, 'Hello'
, and begins the madness all over again.
That’s recursion, my friends.
curry
is essentially an infinite loop of self-calling, parameter-hungry functions that won’t rest until their guest is full. Hospitality at its finest.
Back at line 2
Same parameters as before, except initialParams
is ['Hello']
this time. Skip again to exit the cycle. Type our new variable into the console, sayHello
. It’s another function, still expecting more parameters, but we’re getting warmer…
Let’s turn up the heat with sayHelloToJohn = sayHello('John')
.
We’re inside line 4 again, and nextParams
is ['John']
. Jump to the next debugger on line 6 and inspect params
: it’s ['Hello', 'John']
! 🙀
Why, why, why?
Because remember, line 12 says “Hey curriedFunction
, he gave me 'Hello'
last time and ‘John’
this time. Take them both in this array [...initialParams, ...nextParams]
.”
Now curriedFunction
again compares the length
of these params
to originalFunction
, and since 2 < 3
we move to line 10 and call curry
once again! And of course, we pass along greet
and our 2 params, ['Hello', 'John']
We’re so close, let’s finish this and get the full greeting back!
sayHelloToJohnDoe = sayHelloToJohn('Doe')
I think we know what happens next.
The Deed Is Done
greet
got his parameters, curry
stopped looping, and we’ve received our greeting: Hello, John Doe
.
Play around with this function some more. Try supplying multiple or no parameters in one shot, get as crazy as you want. See how many times curry
has to recurse before returning your expected output.
curriedGreet('Hello', 'John', 'Doe');
curriedGreet('Hello', 'John')('Doe');
curriedGreet()()('Hello')()('John')()()()()('Doe');
Many thanks to Eric Elliott for introducing this to me, and even more thanks to you for appreciating curry
with me. Until next time!
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