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The Functional DLang Garden provides a variety of snippets that can be used to learn D or as a quick reference.
All samples are valid code and automatically tested on every run. Contributions are welcome.
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Increment elements

With map we can call a custom function for every element

import std.algorithm: map;
auto result = [1, 2, 3].map!(a => a + 1);
assert(result.equal([2 ,3, 4]));

Reduce to minimum

With reduce we can apply a function with a starting value and call it with the memo and the current value for all values.

import std.algorithm: min, max, reduce;
auto result = [3, 2, 1].reduce!min;
assert(result == 1);
result = [3, 2, 1].reduce!max;
assert(result == 3);

Filter elements

We can also filter our input range with custom functions

import std.algorithm: count, filter;
import std.string: indexOf;

auto result = ["hello", "world"]
                .filter!(a => a.indexOf("wo") >= 0)
assert(result == 1);

Reverse sort

Sort accepts a pred template, which means we can just pass our own

import std.algorithm: sort;

auto result = [1, 3, 2].sort!"a > b";
assert(result.equal([3, 2, 1]));

Split string to ints

A pretty common pattern is to read user input. Splitter is usually lazily evaluated.

import std.algorithm: map, splitter;
import std.array: array;
import std.conv: to;

auto result = "1 3 2".splitter().map!(to!int);
assert(result.equal([1, 3, 2]));

Count number of unique elements

A typical example from Unix is to sort and count the number of unique lines. In D we can do the same!

import std.algorithm: count, sort, uniq;
auto result = [1, 3, 2, 2, 3].sort().uniq.count;
assert(result == 3);

Pairwise sum

We split our input range into chunks of the size two and calculate the sum for each.

import std.algorithm: map, sum;
import std.range: chunks;

auto result = [1, 2, 3, 4].chunks(2).map!(sum);
assert(result.equal([3, 7]));

Count chars

This approach requires sorting the array. Use a dict (below) - it doesn't require sorting.

import std.array: array;
import std.algorithm: sort, group;
import std.typecons: tuple; // create tuples manually

auto result = "ABCA".array.sort().group.array;
auto expected = [tuple('A', 2),
                 tuple('B', 1),
                 tuple('C', 1)];
assert(expected == cast(typeof(expected)) result);

List k-mer

We can iterate pairwise over all k-mer and list them. The syntax to convert a tuple back to list is a bit hard to figure out.

import std.algorithm: map;
import std.array: array, join;
import std.range: dropOne, only, save, zip;
import std.conv: to;

auto arr = "AGCGA".array;
auto result =
assert(result == "AG,GC,CG,GA");

Count k-mer with defaultdict

We iterate over all pairs of the string and increment a key in our dictionary. In D a new key is automatically created once it is accessed for the first time. The syntax to convert a tuple back to list is a bit hard to figure out.

import std.array: array;
import std.algorithm: each, map;
import std.range: dropOne, only, save, zip;
import std.conv: to;

int[string] d;
auto arr = "AGAGA".array;
   .each!(a => d[a]++);
assert(d == ["AG": 2, "GA": 2]);

Filter by index

With enumerate we get an index which we can use to filter

import std.algorithm: filter, map;
import std.range: enumerate;

auto result = [3, 4, 5]
                .filter!(a => a[0] != 1)
assert(result.equal([3, 5]));

Sum up even indexed number

With enumerate we get an index with which we can remove all odd numbers.

import std.algorithm: filter, map, sum;
import std.range: enumerate;

auto result = [3, 4, 5]
                .filter!(a => a[0] % 2 == 0)
assert(result == 8);

Most common word

Yet another good example of group.

import std.algorithm: group, map, sort;
import std.array: split;
import std.string: toLower;
import std.typecons: tuple;

string text = "Two times two equals four";
auto result = text
                .split(' ')
assert(result.equal([tuple("equals", 1u), tuple("four", 1u),
                     tuple("times", 1u),  tuple("two", 2u)]));

Format a range pipeline

reverseArgs can be used to continue using the result in a UFCS pipe

import std.algorithm : filter, map, sum;
import std.range : iota;
import std.format : format;
import std.functional : reverseArgs;

auto res = 6.iota
  .filter!(a => a % 2) // 0 2 4
  .map!(a => a * 2) // 0 4 8
  .reverseArgs!format("Sum: %d");
assert(res == "Sum: 18");

Lazy parser

With cumulativeFold a lazy, stack-based parser can be written

import std.algorithm : cumulativeFold, equal, map, until;
import std.range : zip;
auto input = "foo()bar)";
auto result = input.cumulativeFold!((count, r){
        case '(':
        case ')':
    return count;
})(1).zip(input).until!(e => e[0] == 0).map!(e => e[1]);


This is a good example how expressive functional programming can be. Also note that the second base case is not necessary.

import std.algorithm: filter;
import std.array: array;

int[] qs(int[] arr) {
    if(!arr.length) return [];
    if(arr.length == 1) return arr; // optional
    return qs(arr.filter!(a => a < arr[0]).array) ~ arr[0] ~ qs(arr[1..$].filter!(a => a >= arr[0]).array);
assert(qs([3, 2, 1, 4]) == [1, 2, 3, 4]);
assert(qs([1]) == [1]);
assert(qs([]) == []);