Show HN: Omg-Curry – JavaScript currying library | Hacker News Reader

Show HN: Omg-Curry – JavaScript currying library

github.com

11 pointsdebdut16 comments

I had mixed results in the past when I tried to use currying in Javascript. The stack traces were cluttered by references to the currying function and it also masked bugs if you forgot to pass an argument. Without currying, if you pass less arguments than the function expects then you usually get an error right away complaining that the argument is undefined. However, if the function is curried then it silently succeeds. You only notice the problem later, when you try to use the return value and realize that it is actually a function. To make things worse, when this happens the stack trace points to the place that is trying to use the return value instead of pointing to the function call that is the actual source of the problem.

My final impression was that currying works better a language like Haskell that has a strong type system. In a dynamic language it is more problematic.

How about Typescript then?

When I did that it was before Typescript existed. If anyone has tried currying with Typescript I'd love to hear how it went.

JavaScript actually has a pretty ergonomic way of doing currying natively:

  const add = (a) => (b) => a + b;
  
  [1, 2, 3, 4].map(add(2));  // [3, 4, 5, 6]

I like this too (and use a version myself) however this form only supports one argument at a time. E.g. add(1,2) is going to return the curry of add(1), which is not what you want. The OP's function doesn't have that problem.

Something I would really like, although this would require some language-level changes, I think, is the ability to curry arguments in any order, maybe using placeholders. So you could say add(_, 2) and get back a function of one argument, such that if you call it with 1, for example, you invoke add(1, 2).

What it ends up being for normal usage is something like this:

  add(1)(2)

Which is admittedly a little bit clunky, though from a quick read I don't think the library helps with this.

You might be interested in the "pipeline operator" proposal. In addition to pairing nicely with curried functions, it offers a certain amount of re-ordering.

Yeah I think your quick read is off:

  add4(1)(3)(5)(10)
  add4(1)(3,5)(10)
  add4(1,3,5,10)
  // 19

Sorry I'm not that familiar with this. What would be a good use-case for currying?

The example doesn't feel that exciting...

    add4(1)(3)(5)(10)
    add4(1)(3,5)(10)
    add4(1,3,5,10)
    // 19

Currying becomes useful in tandem with functional composition.

Indeed currying shows up most in functional programming world

The add example is a bit boring because all the arguments are the same "kind". Partial evaluation is more interesting if the arguments are more independent. To borrow a contrived example from the Lodash docs, you can use partial evaluation to make a custom greeter function:

https://lodash.com/docs/4.17.15#partial https://lodash.com/docs/4.17.15#curry

    function greet(greeting, name) {
        return greeting + ' ' + name;
    }
 
    var sayHelloTo = _.partial(greet, 'hello');
    sayHelloTo('fred'); // => 'hello fred'

    var sayHi = _.partial(greet, 'hi');
    greetFred('fred'); // => 'hi fred'

Currying works similarly, except that the function by does the partial evaluation automatically if you call it with less arguments.

    var greet = _.curry(function(greeting, name) {
        return greeting + ' ' + name
    })

    var sayHelloTo = greet('hello');
    sayHelloTo('fred'); // => 'hello fred'

    var sayHi = greet('hi');
    greetFred('fred'); // => 'hi fred'

It's nice for any type of list iteration. I'll share my "match" function (for use when filtering) when I'm at my computer tomorrow.

It's tomorrow somewhere…

    /**
     * These functions are intended to be used with array iteration methods
     * -------------------
     * // Example data:
     * let e1 = { a: 1, b:  2, c: "C" };
     * let e2 = { a: 2, b: 12, c: "D" };
     * let myArray = [ e1, e2 ];
     *
     * // Basic usage, matching a constant
     * myArray.filter(match({a: 1})); // [ e1 ]
     * myArray.filter(match({a: 3})); // [ ]
     *
     * // Matching multiple properties
     * myArray.filter(match({ a: 1, c: "C" }));    // [ e1 ]
     * myArray.filter(match({ a: 1, c: "D" }));    // [ ]
     * myArray.filter(matchAny({ a: 1, c: "D" })); // [ e1, e2 ]
     *
     * // If a function is provided, return the result of the function
     * // called with the corresponding property as its argument.
     * let belowTen = n => n < 10;
     * myArray.filter(match({ a: belowTen, b: belowTen })); // [ e1 ]
     * myArray.filter(matchAny({ a: belowTen, b: belowTen })); // [ e1, e2 ]
     *
     */
    export const match = o1 => o2 => Object.entries(o1).every(
      ([k, v]) => typeof v === "function" ? v(o2[k]) : o2[k] === v
    );
    export const matchAny = o1 => o2 => Object.entries(o1).some(
      ([k, v]) => typeof v === "function" ? v(o2[k]) : o2[k] === v
    );

> WTF is Curry?

> A Curry takes arguments one by one unlike functions which takes all arguments at once.

Took the words right out of my brain.

The old "drop a link" routine isn't beloved here at HN, but this xkcd is just so good:

https://xkcd.com/2210/

funny how most react devs were using currying directly when they were calling redux lib.

still think it(redux) was a misplaced usage of currying.

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