Map Reduce in D

I didn't want to confuse function mapping and dictionaries, so I decided to include reduce.

map is a range (list) based function which applies an operation to each element and their result is a range which lazily applies the operation.

I haven't been referencing how other languages apply a solution, but as this started with Python and their list comprehension is an interesting approach to this.

https://www.pythonforbeginners.com/basics/list-comprehensions-in-python

new_range  = [i + i for i in range(5)   if i % 2 == 0]

import std.range;
auto new_range = iota(5)
                 .filter!(i => i % 2 == 0)
                 .map!(i => i + i);

Putting aside the strange name iota the structure is similar. Personally I'd argue D is more clear about the operation order.

import std.range;
auto new_range = iota(5)
                 .map!(i => i + i)
                 .filter!(i => i % 2 == 0);

In the first situation filtering came before the operation, filtering after has no value since two odds make an even. In Python order of operation is a spiral like C function pointers.

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reduce is an operation on a range which produces a single result.

import std.algorithm;
import std.range;

assert(
       iota(5).fold!((a, b) => a + b)
       == 1 + 2 + 3 + 4);

I realize my example uses fold rather than reduce. This has many more names, but I chose fold in the example as this provides better argument order in chained expressions.