From type/Supply

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method produce(Supply:D: &with --> Supply:D)

Creates a "producing" supply with the same semantics as List.produce.

my $supply = Supply.from-list(1..5).produce({$^a + $^b});
$supply.tap(-> $v { say "$v" }); # OUTPUT: «1␤3␤6␤10␤15␤»

From type/Any

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multi method produce(Any:U: & --> Nil)
multi method produce(Any:D: &with)
multi sub produce (&with, +list)

This is similar to reduce, but returns a list with the accumulated values instead of a single result.

<10 5 3>.reduce( &[*] ).say ; # OUTPUT: «150␤» 
<10 5 3>.produce( &[*] ).say; # OUTPUT: «(10 50 150)␤» 

The last element of the produced list would be the output produced by the .reduce method.

If it's a class, it will simply return Nil.

From type/List

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multi sub    produce(&with, *@values)
multi method produce(List:D: &with)

Generates a list of all intermediate "combined" values along with the final result by iteratively applying a function which knows how to combine two values.

If @values contains just a single element, a list containing that element is returned immediately. If it contains no elements, an exception is thrown, unless &with is an operator with a known identity value.

If &with is the function object of an operator, its inherent identity value and associativity is respected - in other words, (VAL1, VAL2, VAL3).produce(&[OP]) is the same as VAL1 OP VAL2 OP VAL3 even for operators which aren't left-associative:

# Raise 2 to the 81st power, because 3 to the 4th power is 81 
[2,3,4].produce(&[**]).say;        # OUTPUT: «(4 81 2417851639229258349412352)␤» 
say produce &[**], (2,3,4);        # OUTPUT: «(4 81 2417851639229258349412352)␤» 
say [\**] (2,3,4);                 # OUTPUT: «(4 81 2417851639229258349412352)␤» 
 
# Subtract 4 from -1, because 2 minus 3 is -1 
[2,3,4].produce(&[-]).say;         # OUTPUT: «(2 -1 -5)␤» 
say produce &[-], (2,3,4);         # OUTPUT: «(2 -1 -5)␤» 
say [\-] (2,3,4);                  # OUTPUT: «(2 -1 -5)␤»

A triangle metaoperator [\ ] provides a syntactic shortcut for producing with an infix operator:

# The following all do the same thing... 
my @numbers = (1,2,3,4,5);
say produce { $^a + $^b }, @numbers;
say produce * + *, @numbers;
say produce &[+], @numbers; # operator does not need explicit identity 
say [\+] @numbers;          # most people write it this way

The visual picture of a triangle [\ is not accidental. To produce a triangular list of lists, you can use a "triangular comma":

[\,] 1..5;
# ( 
# (1) 
# (1 2) 
# (1 2 3) 
# (1 2 3 4) 
# (1 2 3 4 5) 
# )

Since produce is an implicit loop, it responds to next, last and redo statements inside &with:

say (2,3,4,5).produce: { last if $^a > 7; $^a + $^b }; # OUTPUT: «(2 5 9)␤»