What is point free style in Functional Programming

问题:

A phrase that I've noticed recently is the concept of "point free" style...

First, there was this question, and also this one.

Then, I discovered here they mention "Another topic that may be worth discussing is the authors' dislike of point free style."

What is "point free" style? Can someone give a concise explanation? Does it have something to do with "automatic" currying?

To get an idea of my level - I've been teaching myself Scheme, and have written a simple Scheme interpreter... I understand what "implicit" currying is, but I don't know any Haskell or ML.

回答1:

Just look at the Wikipedia article to get your definition:

Tacit programming (point-free programming) is a programming paradigm in which a function definition does not include information regarding its arguments, using combinators and function composition [...] instead of variables.

Haskell example:

Conventional (you specify the arguments explicitly):

sum (x:xs) = x + (sum xs)
sum [] = 0

Point-free (sum doesn't have any explicit arguments - it's just a fold with + starting with 0):

 sum = foldr (+) 0

Or even simpler: Instead of g(x) = f(x), you could just write g = f.

So yes: It's closely related to currying (or operations like function composition).

回答2:

Point-free style means that the arguments of the function being defined are not explicitly mentioned, that the function is defined through function composition.

If you have two functions, like

square :: a -> a
square x = x*x

inc :: a -> a
inc x = x+1

and if you want to combine these two functions to one that calculates x*x+1, you can define it "point-full" like this:

f :: a -> a
f x = inc (square x)

The point-free alternative would be not to talk about the argument x:

f :: a -> a
f = inc . square

回答3:

An JavaScript sample:

//not pointfree cause we receive args
var initials = function(name) {
  return name.split(' ').map(compose(toUpperCase, head)).join('. ');
};

const compose = (...fns) => (...args) => fns.reduceRight((res, fn) => [fn.call(null, ...res)], args)[0];
const join = m => m.join();

//pointfree
var initials = compose(join('. '), map(compose(toUpperCase, head)), split(' '));

initials("hunter stockton thompson");
// 'H. S. T'

Reference

回答4:

Point free style means that the code doesn't explicitly mention it's arguments, even though they exist and are being used.

This works in Haskell because of the way functions work.

For instance:

myTake = take

returns a function that takes one argument, therefore there is no reason to explicit type the argument unless you just want too.

回答5:

Here is one example in TypeScript without any other library:

interface Transaction {
  amount: number;
}

class Test {
  public getPositiveNumbers(transactions: Transaction[]) {
    return transactions.filter(this.isPositive);

    //return transactions.filter((transaction: {amount: number} => transaction.amount > 0));
  }

  public getBigNumbers(transactions: Transaction[]) {
    // point-free
    return transactions.filter(this.moreThan(10));

    // not point-free
    // return transactions.filter((transaction: any) => transaction.amount > 10);
  }

  private isPositive(transaction: Transaction) {
    return transactions.amount > 0;
  }

  private moreThan(amount: number) {
    return (transaction: Transaction) => {
      return transactions.amount > amount;
    }
  }
}

You can see point-free style is more "fluent" and easier to read.