threadsafe_cached_function.p6

#!/usr/bin/env perl6

# The problem:
#  - A (pure) function "getinfo" which takes an ID and calculates a
#    return value based on it.
#  - Calculating the return value is expensive, so we want to cache
#    it and never calculate it twice for the same ID.
#  - The function should be thread-safe, and when called
#    simultaneously with *different* IDs the calculations should not
#    block each other.

# Implementation:

BEGIN (
    my $lock = Lock.new;
    my %info;
);

sub getinfo ($id) {
    my $promise;
    
    $lock.protect: {
        $promise = %info{$id} //= start {
            # expensive calculation here
            sleep 2; rand
        }
    }
    
    await $promise
}

# Testing:

await do for 1, 2, 3, 2, 2, 2, 2, 2, 2 {
    start { say "$_: ", getinfo $_ }
}

# Output notes:
#  - The return values for ID 2 are all the same, proving that the
#    caching works.
#  - The program only takes 2.x seconds if there are three or more
#    CPU cores, which proves that the expensive calculations are
#    done in parallel as they should.