How do I tell what type of value is in a Perl variable

Question

How do I tell what type of value is in a Perl variable    x might be a scalar  a ref to an array or a ref to a hash  or maybe other things

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x is always a scalar  The hint is the sigil    any variable  or dereferencing of some other type  starting with   is a scalar   See perldoc perldata for more about data types    A reference is just a particular type of scalar   The built-in function ref will tell you what kind of reference it is  On the other hand  if you have a blessed reference  ref will only tell you the package name the reference was blessed into  not the actual core type of the data  blessed references can be hashrefs  arrayrefs or other things   You can use Scalar  Util  s reftype will tell you what type of reference it is   use Scalar  Util qw reftype    my  x   bless      My  Foo   my  y         print  type of x      ref  x      n   print  type of y      ref  y      n   print  base type of x      reftype  x      n   print  base type of y      reftype  y      n        produces the output   type of x  My  Foo type of y  HASH base type of x  HASH base type of y  HASH   For more information about the other types of references  e g  coderef  arrayref etc   see this question  How can I get Perl  39 s ref   function to return REF  IO  and LVALUE  and perldoc perlref   Note  You should not use ref to implement code branches with a blessed object  e g   ref  a  eq  My  Foo    say  is a Foo object    say  foo not defined    -- if you need to make any decisions based on the type of a variable  use isa  i e if   a- gt isa  My  Foo         or if   a- gt can  foo           Also see polymorphism

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I like polymorphism instead of manually checking for something   use MooseX  Declare   class Foo       use MooseX  MultiMethods       multi method foo  ArrayRef  arg   say  arg is an array        multi method foo  HashRef  arg    say  arg is a hash        multi method foo  Any  arg        say  arg is something else       Foo- gt new- gt foo        arg is an array Foo- gt new- gt foo 40     arg is something else   This is much more powerful than manual checking  as you can reuse your  checks  like you would any other type constraint   That means when you want to handle arrays  hashes  and even numbers less than 42  you just write a constraint for  even numbers less than 42  and add a new multimethod for that case   The  calling code  is not affected   Your type library   package MyApp  Types  use MooseX  Types -declare   gt    EvenNumberLessThan42    use MooseX  Types  Moose qw Num    subtype EvenNumberLessThan42  as Num  where       lt  42  amp  amp       2    0      Then make Foo support this  in that class definition    class Foo       use MyApp  Types qw EvenNumberLessThan42        multi method foo  EvenNumberLessThan42  arg    say  arg is an even number less than 42        Then Foo- gt new- gt foo 40  prints arg is an even number less than 42 instead of arg is something else   Maintainable

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At some point I read a reasonably convincing argument on Perlmonks that testing the type of a scalar with ref or reftype is a bad idea   I don t recall who put the idea forward  or the link   Sorry   The point was that in Perl there are many mechanisms that make it possible to make a given scalar act like just about anything you want   If you tie a filehandle so that it acts like a hash  the testing with reftype will tell you that you have a filehanle   It won t tell you that you need to use it like a hash   So  the argument went  it is better to use duck typing to find out what a variable is   Instead of   sub foo       my  var   shift      my  type   reftype  var       my  result      if   type eq  HASH               result    var- gt  foo             elsif   type eq  ARRAY               result    var- gt  3             else            result    foo              return  result      You should do something like this   sub foo       my  var   shift      my  type   reftype  var       my  result       eval            result    var- gt  foo           1    guarantee a true result if code works            or eval             result    var- gt  3           1            or do            result    foo              return  result      For the most part I don t actually do this  but in some cases I have   I m still making my mind up as to when this approach is appropriate   I thought I d throw the concept out for further discussion   I d love to see comments   Update  I realized I should put forward my thoughts on this approach   This method has the advantage of handling anything you throw at it   It has the disadvantage of being cumbersome  and somewhat strange   Stumbling upon this in some code would make me issue a big fat  WTF    I like the idea of testing whether a scalar acts like a hash-ref  rather that whether it is a hash ref     I don t like this implementation

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ref        Perl provides the ref   function so that you can check the reference type before dereferencing a reference         By using the ref   function you can protect program code that dereferences variables from producing errors when the wrong type of reference is used

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A scalar always holds a single element  Whatever is in a scalar variable is always a scalar  A reference is a scalar value   If you want to know if it is a reference  you can use ref  If you want to know the reference type   you can use the reftype routine from Scalar  Util   If you want to know if it is an object  you can use the blessed routine from Scalar  Util  You should never care what the blessed package is  though  UNIVERSAL has some methods to tell you about an object  if you want to check that it has the method you want to call  use can  if you want to see that it inherits from something  use isa  and if you want to see it the object handles a role  use DOES   If you want to know if that scalar is actually just acting like a scalar but tied to a class  try tied  If you get an object  continue your checks   If you want to know if it looks like a number  you can use looks like number from Scalar  Util  If it doesn t look like a number and it s not a reference  it s a string  However  all simple values can be strings   If you need to do something more fancy  you can use a module such as Params  Validate

[perl] How do I tell what type of value is in a Perl variable?

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