functional-classes

#!/usr/bin/env perl

# Copyright (c) 2019 Christian Jaeger, copying@christianjaeger.ch
# This is free software. See the file COPYING.md that came bundled
# with this file.

use strict;
use warnings;
use warnings FATAL => 'uninitialized';

# find modules from functional-perl working directory (non-installed)
use Cwd 'abs_path';
our ($mydir, $myname);

BEGIN {
    my $location = (-l $0) ? abs_path($0) : $0;
    $location =~ /(.*?)([^\/]+?)_?\z/s or die "?";
    ($mydir, $myname) = ($1, $2);
}
use lib "$mydir/../lib";

use Test::Requires qw(Moo);

# Comparison of imperative and functional approaches to some data
# container:

package ImperativePerson {
    use Moo;

    has name       => is => 'ro';
    has todo_items => is => 'ro';    # the array itself is the mutable part

    sub add_todo {
        my ($self, $item) = @_;
        push @{ $self->{todo_items} }, $item;
    }

}

package FunctionalPerson {

    use FP::List;                    # for `cons`, `cons_`

    use FP::Struct ["name", "todo_items"];

    sub add_todo {
        my ($self, $item) = @_;

        # The sub receives the old todo_items list, and returns a new
        # list with $item prepended:
        #$self->todo_items_update(sub { cons $item, $_[0] })
        # Or use `cons_` which returns a function that prepends
        # cons_'s argument to its own argument:
        $self->todo_items_update(cons_ $item)
    }

    _END_
}

use Test::More;

sub testImperative {
    my $p = ImperativePerson->new(name => "Chris", todo_items => []);
    $p->add_todo("buy bread");
    $p->add_todo("walk dog");
    is_deeply $p->todo_items, ['buy bread', 'walk dog'];
}

use FP::Equal qw(is_equal);    # using FP::Equal's `equal` but still
                               # reporting via Test::More
use FP::List;                  # for `list`

sub testFunctional {
    my $p = FunctionalPerson->new_(name => "Chris", todo_items => list());

    # or, using positional arguments:
    #my $p = FunctionalPerson->new("Chris", list());

    # Each add_todo call returns a new version of the object, any
    # pointer to the old version still sees the original value:
    my $p2 = $p->add_todo("buy bread");
    my $p3 = $p2->add_todo("walk dog");
    is_equal $p3->todo_items, list('walk dog', 'buy bread');
    is_equal $p2->todo_items, list('buy bread');
}

if ($ENV{RUN_TESTS}) {
    testImperative;
    testFunctional;
    done_testing;
} else {
    require FunctionalPerl;
    import FunctionalPerl ":repl";
    repl();
}