map, grep and friends

Transform lists with map, filter with grep, fold with [+] — and chain it all with dots.

map transforms every element; grep keeps the ones that match. The * (whatever star) builds a tiny function in place:

say (1..10).map(* ** 2);
say (1..10).grep(* %% 2);

(1 4 9 16 25 36 49 64 81 100)
(2 4 6 8 10)

* ** 2 reads "whatever, squared"; * %% 2 reads "whatever, divisible by 2".

Chains #

Because each method returns a new list, calls chain left to right into little pipelines:

my @words = <pearl onion camelia butterfly bee>;
say @words.grep(*.chars > 5).map(*.uc).sort;
say @words.sort(*.chars).join(' < ');

(BUTTERFLY CAMELIA)
bee < pearl < onion < camelia < butterfly

That second line sorts by a key — here, word length.

Reduction #

Put an operator in [...] and it folds a whole list down to one value:

say [+] 1..100;
say [*] 1..10;
say [max] 3, 41, 9, 26;

5050
3628800
41

Try it #

In one chain: take 1..20, keep the odd numbers, square them, and sum the lot.

say ...;

say [+] (1..20).grep(* % 2).map(* ** 2);

1330