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Raku port of Perl's Object::Trampoline module 1.50.4


# direct object creation
my $dbh = DBIish.connect( ... );
my $sth = $dbh.prepare( 'select foo from bar' );

if $need-to-execute {
    $sth.execute; # execute
else {
    say "database handle and statement executed without need";

# with delayed object creation
use Object::Trampoline;
my $dbh = Object::Trampoline.connect( DBIish, ... );
my $sth = Object::Trampoline.prepare( $dbh, 'select foo from bar' );

if $need-to-execute {
    $sth.execute; # create $dbh, then $sth, then execute
else {
    say "no database handle opened or statement executed";
LEAVE .disconnect with $dbh;  # only disconnects if connection was made

# alternate setup way, more idiomatic Raku
my $dbh = trampoline { DBIish.connect: ... }
my $sth = trampoline { $dbh.prepare: 'select foo from bar' }

# lazy default values for attributes in objects
class Foo {
    has $.bar = trampoline { say "delayed init"; "bar" }
my $foo = Foo.new;
say $foo.bar;  # delayed init; bar


This module tries to mimic the behaviour of Perl's Object::Trampolinee module as closely as possible in the Raku Programming Language.

There are times when constructing an object is expensive but you are not sure yet you are going to need it. In that case it can be handy to delay the creation of the object. But then your code may become much more complicated.

This module allows you to transparently create an intermediate object that will perform the delayed creation of the original object when any method is called on it.

The original Perl was is to call any method on the Object::Trampoline class, with as the first parameter the object to call that method on when it needs to be created, and the other parameters the parameters to give to that method then.

The alternate, more idiomatic Raku way, is to call the trampoline subroutine with a code block that contains the code to be executed to create the final object. This can also be used to serve as a lazy default value for a class attribute.

To make it easier to check whether the actual object has been created, you can check for .defined or booleaness of the object without actually creating the object. This can e.g. be used when wanting to disconnect a database handle upon exiting a scope, but only if an actual connection has been made (to prevent it from making the connection only to be able to disconnect it).


Due to the lexical scope of use in Raku, it is not really possible to mimic the behaviour of Object::Trampoline::Use.

Due to the nature of the implementation, it is not possible to use smartmatch (aka ~~) on a trampolined object. This is because smartmatching a trampolined object does not call any methods on it. Fixing that by using a Proxy would break the .defined and .Bool functionality.

You can also not call .WHAT on a trampolined object. This is because .WHAT is internally generated as a call to nqp::what, and it is as yet impossible to provide candidates for nqp:: ops.


Elizabeth Mattijsen liz@raku.rocks

Source can be located at: https://github.com/lizmat/Object-Trampoline . Comments and Pull Requests are welcome.

If you like this module, or what I’m doing more generally, committing to a small sponsorship would mean a great deal to me!


Copyright 2018, 2019, 2020, 2021, 2023 Elizabeth Mattijsen

This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.

Re-imagined from the Perl version as part of the CPAN Butterfly Plan. Perl version originally developed by Steven Lembark.