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List Comprehension in D

Jesse Phillips on December 13, 2019

This post is a language comparison coming out of this great article on list comprehension. D does not have List comprehension. Art...

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Jesse Phillips • Dec 15 '19

@byrro , in response to dev.to/byrro/comment/j0om

Are you seriously going to stop at iota? I rarely utilize this in real world code, it is generally used in examples as a quick way to get a range.

Note that terminology here is different in D. Python uses 'range' as a sequence of numbers, D uses "a set of different things of the same general type."

Is 'Cartesian product' easy to read for someone who has never crossed path with this operation? No, but it does give terminology to read about what it does. Whereas

[(i,j) for j in range(2) for i in range(2)]

Don't get me wrong, you could easily provide a library function in Python, name it and do this under the hood, it is not like the library implementation is some kind of English statement about the desired outcome.

I'm not saying these are the things which make or break Python's readability, but it does contribute to it not being able to have the throne of readability.

Renato Byrro • Dec 16 '19

You seem to love Dlang and that's totally fine.

But the way you're expressing yourself is not helping to convince others about Dlang, if that's your goal. It's not just me.

I'm a fierce defender of freedom of private initiative and expression. But we've got to think about what is our goal and how to better get there.

If I may suggest an idea, why not approach the issue like: "if you love Python XYZ feature, then you'll love Dlang even more"? That would be more appealing for people to try Dlang.

Comment deleted

Jesse Phillips • Dec 16 '19

Just stumbled acrossed this

medium.com/better-programming/how-...

The issue I see with this is chaining multiple operations. In the example he stores the result before the next operation.

Let's look at an alternative way to make it readable.

// Original breakup, but written in D
auto numbers = [1,2,3,4,5,6];

auto odd_numbers = filter!(n=> n % 2 == 1)(numbers);

auto squared_odd_numbers = map!(n=> n * n)(odd_numbers);

// fold is another name used for reduce 
auto total1 = fold!((acc, n)=> acc + n)(squared_odd_numbers, 0);

// chain operation 
auto total2 = numbers
    .filter!(n=> n % 2 == 1)
    .map!(n=> n * n)
    .fold!((acc, n)=> acc + n)(0);

// Name the lambda operation 
alias odds = x => x % 2 == 1;
alias square = x => x * x;
alias sum = (acc, n)=> acc + n;

auto total3 = numbers
    .filter!odds
    .map!square 
    .fold!sum(0);


assert(total1 == total2);
assert(total3 == total2);

The language isn't the driving force for readability here.

If you look closely D does provide usage challenges since I introduced the use of alias. Explaining its need would be more technical than lambda : is that a bad thing? I don't know.

Jesse Phillips • Dec 15 '19 • Edited

Possibly, do you have examples?

What is the Pythonic way? I showed D idioms.