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Discussion on: We Don't Need No Stinking map() or filter()

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Brad Gilbert

Perl 6 doesn't do it this way, because it can't do that easily.

If you tried to implement map with reduce it would hang before producing a single value when called on an infinite sequence.

my @a = (0..Inf).map( *² ); # infinite sequence of all squared values

.say() for @a.head(20); # prints first twenty squares
# sequence of only twenty squared values
my @b = (0..^20).reduce: { @_.head(*-1).Slip, @_.tail² }

.say() for @b; # prints the same twenty squares
# doesn't stop until it is killed or runs out of resources
my @c = (0..Inf).reduce: { @_.head(*-1).Slip, @_.tail² }

.say() for @c.head(20); # never gets to this line of code

You could maybe find a way to pass upwards the number of values to produce:

my @d = (0..Inf).reduce: {
  last if @_ > 20; # stop early

  @_.head(*-1).Slip, @_.tail²

.say() for @d;

The thing is that iteration is done using an object based protocol, so map and grep just return an object with the appropriate functionality.
(Most of the time this is abstracted away so you don't need to know how it works.)

Perl 6 is a functional language while also being a procedural language.
So it can't always make the same sort of guesses about what you mean that pure functional languages can make.