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.\" ========================================================================
.\"
.IX Title "Safe::Isa 3pm"
.TH Safe::Isa 3pm "2018-05-06" "perl v5.26.2" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
Safe::Isa \- Call isa, can, does and DOES safely on things that may not be objects
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 2
\&  use strict;
\&  use warnings;
\&  
\&  { package Foo; sub new { bless({}, $_[0]) } }
\&  { package Bar; our @ISA = qw(Foo); sub bar { 1 } }
\&  
\&  my $foo = Foo\->new;
\&  my $bar = Bar\->new;
\&  my $blam = [ 42 ];
\&  
\&  # basic isa usage \-
\&  
\&  $foo\->isa(\*(AqFoo\*(Aq);  # true
\&  $bar\->isa(\*(AqFoo\*(Aq);  # true
\&  $blam\->isa(\*(AqFoo\*(Aq); # BOOM
\&  
\&  $foo\->can(\*(Aqbar\*(Aq);  # false
\&  $bar\->can(\*(Aqbar\*(Aq);  # true
\&  $blam\->can(\*(Aqbar\*(Aq); # BOOM
\&  
\&  # Safe::Isa usage \-
\&  
\&  use Safe::Isa;
\&  
\&  $foo\->$_isa(\*(AqFoo\*(Aq);  # true
\&  $bar\->$_isa(\*(AqFoo\*(Aq);  # true
\&  $blam\->$_isa(\*(AqFoo\*(Aq); # false, no boom today
\&  
\&  $foo\->$_can(\*(Aqbar\*(Aq);  # false
\&  $bar\->$_can(\*(Aqbar\*(Aq);  # true
\&  $blam\->$_can(\*(Aqbar\*(Aq); # false, no boom today
.Ve
.PP
Similarly:
.PP
.Vb 2
\&  $maybe_an_object\->$_does(\*(AqRoleName\*(Aq); # true or false, no boom today
\&  $maybe_an_object\->$_DOES(\*(AqRoleName\*(Aq); # true or false, no boom today
.Ve
.PP
And just in case we missed a method or two:
.PP
.Vb 2
\&  $maybe_an_object\->$_call_if_object(name => @args);
\&  $maybe_an_object\->$_call_if_can(name => @args);
.Ve
.PP
Or to re-use a previous example for purposes of explication:
.PP
.Vb 3
\&  $foo\->$_call_if_object(isa => \*(AqFoo\*(Aq);  # true
\&  $bar\->$_call_if_object(isa => \*(AqFoo\*(Aq);  # true
\&  $blam\->$_call_if_object(isa => \*(AqFoo\*(Aq); # false, no boom today
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
How many times have you found yourself writing:
.PP
.Vb 1
\&  if ($obj\->isa(\*(AqSomething\*(Aq)) {
.Ve
.PP
and then shortly afterwards cursing and changing it to:
.PP
.Vb 1
\&  if (Scalar::Util::blessed($obj) and $obj\->isa(\*(AqSomething\*(Aq)) {
.Ve
.PP
Right. That's why this module exists.
.PP
Since perl allows us to provide a subroutine reference or a method name to
the \-> operator when used as a method call, and a subroutine doesn't require
the invocant to actually be an object, we can create safe versions of isa,
can and friends by using a subroutine reference that only tries to call the
method if it's used on an object. So:
.PP
.Vb 1
\&  my $isa_Foo = $maybe_an_object\->$_call_if_object(isa => \*(AqFoo\*(Aq);
.Ve
.PP
is equivalent to
.PP
.Vb 7
\&  my $isa_Foo = do {
\&    if (Scalar::Util::blessed($maybe_an_object)) {
\&      $maybe_an_object\->isa(\*(AqFoo\*(Aq);
\&    } else {
\&      undef;
\&    }
\&  };
.Ve
.PP
Note that we don't handle trying class names, because many things are valid
class names that you might not want to treat as one (like say \*(L"Matt\*(R") \- the
\&\f(CW\*(C`is_module_name\*(C'\fR function from Module::Runtime is a good way to check for
something you might be able to call methods on if you want to do that.
.PP
We are careful to make sure that scalar/list context is preserved for the
method that is eventually called.
.SH "EXPORTS"
.IX Header "EXPORTS"
.ie n .SS "$_isa"
.el .SS "\f(CW$_isa\fP"
.IX Subsection "$_isa"
.Vb 1
\&  $maybe_an_object\->$_isa(\*(AqFoo\*(Aq);
.Ve
.PP
If called on an object, calls \f(CW\*(C`isa\*(C'\fR on it and returns the result, otherwise
returns nothing.
.ie n .SS "$_can"
.el .SS "\f(CW$_can\fP"
.IX Subsection "$_can"
.Vb 1
\&  $maybe_an_object\->$_can(\*(AqFoo\*(Aq);
.Ve
.PP
If called on an object, calls \f(CW\*(C`can\*(C'\fR on it and returns the result, otherwise
returns nothing.
.ie n .SS "$_does"
.el .SS "\f(CW$_does\fP"
.IX Subsection "$_does"
.Vb 1
\&  $maybe_an_object\->$_does(\*(AqFoo\*(Aq);
.Ve
.PP
If called on an object, calls \f(CW\*(C`does\*(C'\fR on it and returns the result, otherwise
returns nothing. If the \f(CW\*(C`does\*(C'\fR method does not exist, returns nothing rather
than failing.
.ie n .SS "$_DOES"
.el .SS "\f(CW$_DOES\fP"
.IX Subsection "$_DOES"
.Vb 1
\&  $maybe_an_object\->$_DOES(\*(AqFoo\*(Aq);
.Ve
.PP
If called on an object, calls \f(CW\*(C`DOES\*(C'\fR on it and returns the result, otherwise
returns nothing. On perl versions prior to 5.10.0, the built in core \f(CW\*(C`DOES\*(C'\fR
method doesn't exist. If the method doesn't exist, this will fall back to
calling \f(CW\*(C`isa\*(C'\fR just like the core \f(CW\*(C`DOES\*(C'\fR method.
.ie n .SS "$_call_if_object"
.el .SS "\f(CW$_call_if_object\fP"
.IX Subsection "$_call_if_object"
.Vb 1
\&  $maybe_an_object\->$_call_if_object(method_name => @args);
.Ve
.PP
If called on an object, calls \f(CW\*(C`method_name\*(C'\fR on it and returns the result,
otherwise returns nothing.
.ie n .SS "$_call_if_can"
.el .SS "\f(CW$_call_if_can\fP"
.IX Subsection "$_call_if_can"
.Vb 1
\&  $maybe_an_object\->$_call_if_can(name => @args);
.Ve
.PP
If called on an object, calls \f(CW\*(C`can\*(C'\fR on it; if that returns true, then
calls \f(CW\*(C`method_name\*(C'\fR on it and returns the result; if any condition is false
returns nothing.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
I gave a lightning talk on this module (and curry and Import::Into) at
\&\s-1YAPC::NA 2013\s0 <https://www.youtube.com/watch?v=wFXWV2yY7gE&t=46m05s>.
.SH "AUTHOR"
.IX Header "AUTHOR"
mst \- Matt S. Trout (cpan:MSTROUT) <mst@shadowcat.co.uk>
.SH "CONTRIBUTORS"
.IX Header "CONTRIBUTORS"
None yet. Well volunteered? :)
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (c) 2012 the Safe::Isa \*(L"\s-1AUTHOR\*(R"\s0 and \*(L"\s-1CONTRIBUTORS\*(R"\s0
as listed above.
.SH "LICENSE"
.IX Header "LICENSE"
This library is free software and may be distributed under the same terms
as perl itself.