The Perl Toolchain Summit needs more sponsors. If your company depends on Perl, please support this very important event.


Params::Classify - argument type classification


    use Params::Classify qw(
        is_undef check_undef
        is_string check_string
        is_number check_number
        is_glob check_glob
        is_regexp check_regexp
        is_ref check_ref ref_type
        is_blessed check_blessed blessed_class
        is_strictly_blessed check_strictly_blessed
        is_able check_able);

    $c = scalar_class($arg);

    if(is_undef($arg)) {

    if(is_string($arg)) {
    if(is_number($arg)) {

    if(is_glob($arg)) {
    if(is_regexp($arg)) {

    if(is_ref($arg)) {
    $t = ref_type($arg);
    if(is_ref($arg, "HASH")) {
    check_ref($arg, "HASH");

    if(is_blessed($arg)) {
    if(is_blessed($arg, "IO::Handle")) {
    check_blessed($arg, "IO::Handle");
    $c = blessed_class($arg);
    if(is_strictly_blessed($arg, "IO::Pipe::End")) {
    check_strictly_blessed($arg, "IO::Pipe::End");
    if(is_able($arg, ["print", "flush"])) {
    check_able($arg, ["print", "flush"]);


This module provides various type-testing functions. These are intended for functions that, unlike most Perl code, care what type of data they are operating on. For example, some functions wish to behave differently depending on the type of their arguments (like overloaded functions in C++).

There are two flavours of function in this module. Functions of the first flavour only provide type classification, to allow code to discriminate between argument types. Functions of the second flavour package up the most common type of type discrimination: checking that an argument is of an expected type. The functions come in matched pairs, of the two flavours, and so the type enforcement functions handle only the simplest requirements for arguments of the types handled by the classification functions. Enforcement of more complex types may, of course, be built using the classification functions, or it may be more convenient to use a module designed for the more complex job, such as Params::Validate.

This module is implemented in XS, with a pure Perl backup version for systems that can't handle XS.


This module divides up scalar values into the following classes:

  • undef

  • string (defined ordinary scalar)

  • typeglob (yes, typeglobs fit into scalar variables)

  • regexp (first-class regular expression objects in Perl 5.11 onwards)

  • reference to unblessed object (further classified by physical data type of the referenced object)

  • reference to blessed object (further classified by class blessed into)

These classes are mutually exclusive and should be exhaustive. This classification has been chosen as the most useful when one wishes to discriminate between types of scalar. Other classifications are possible. (For example, the two reference classes are distinguished by a feature of the referenced object; Perl does not internally treat this as a feature of the reference.)


Each of these functions takes one scalar argument (ARG) to be tested, possibly with other arguments specifying details of the test. Any scalar value is acceptable for the argument to be tested. Each is_ function returns a simple truth value result, which is true iff ARG is of the type being checked for. Each check_ function will return normally if the argument is of the type being checked for, or will die if it is not.



Determines which of the five classes described above ARG falls into. Returns "UNDEF", "STRING", "GLOB", "REGEXP", "REF", or "BLESSED" accordingly.

The Undefined Value


Check whether ARG is undef. is_undef(ARG) is precisely equivalent to !defined(ARG), and is included for completeness.



Check whether ARG is defined and is an ordinary scalar value (not a reference, typeglob, or regexp). This is what one usually thinks of as a string in Perl. In fact, any scalar (including undef and references) can be coerced to a string, but if you're trying to classify a scalar then you don't want to do that.


Check whether ARG is defined and an ordinary scalar (i.e., satisfies "is_string" above) and is an acceptable number to Perl. This is what one usually thinks of as a number.

Note that simple ("is_string"-satisfying) scalars may have independent numeric and string values, despite the usual pretence that they have only one value. Such a scalar is deemed to be a number if either it already has a numeric value (e.g., was generated by a numeric literal or an arithmetic computation) or its string value has acceptable syntax for a number (so it can be converted). Where a scalar has separate numeric and string values (see "dualvar" in Scalar::Util), it is possible for it to have an acceptable numeric value while its string value does not have acceptable numeric syntax. Be careful to use such a value only in a numeric context, if you are using it as a number. "scalar_num_part" in Scalar::Number extracts the numeric part of a scalar as an ordinary number. (0+ARG suffices for that unless you need to preserve floating point signed zeroes.)

A number may be either a native integer or a native floating point value, and there are several subtypes of floating point value. For classification, and other handling of numbers in scalars, see Scalar::Number. For details of the two numeric data types, see Data::Integer and Data::Float.

This function differs from looks_like_number (see "looks_like_number" in Scalar::Util; also "looks_like_number" in perlapi for a lower-level description) in excluding undef, typeglobs, and references. Why looks_like_number returns true for undef or typeglobs is anybody's guess. References, if treated as numbers, evaluate to the address in memory that they reference; this is useful for comparing references for equality, but it is not otherwise useful to treat references as numbers. Blessed references may have overloaded numeric operators, but if so then they don't necessarily behave like ordinary numbers. looks_like_number is also confused by dualvars: it looks at the string portion of the scalar.



Check whether ARG is a typeglob.



Check whether ARG is a regexp object.

References to Unblessed Objects


Check whether ARG is a reference to an unblessed object. If it is, then the referenced data type can be determined using ref_type (see below), which will return a string such as "HASH" or "SCALAR".


Returns undef if ARG is not a reference to an unblessed object. Otherwise, determines what type of object is referenced. Returns "SCALAR", "ARRAY", "HASH", "CODE", "FORMAT", or "IO" accordingly.

Note that, unlike ref, this does not distinguish between different types of referenced scalar. A reference to a string and a reference to a reference will both return "SCALAR". Consequently, what ref_type returns for a particular reference will not change due to changes in the value of the referent, except for the referent being blessed.

is_ref(ARG, TYPE)
check_ref(ARG, TYPE)

Check whether ARG is a reference to an unblessed object of type TYPE, as determined by "ref_type". TYPE must be a string. Possible TYPEs are "SCALAR", "ARRAY", "HASH", "CODE", "FORMAT", and "IO".

References to Blessed Objects


Check whether ARG is a reference to a blessed object. If it is, then the class into which the object was blessed can be determined using "blessed_class".

is_blessed(ARG, CLASS)
check_blessed(ARG, CLASS)

Check whether ARG is a reference to a blessed object that claims to be an instance of CLASS (via its isa method; see "isa" in perlobj). CLASS must be a string, naming a Perl class.


Returns undef if ARG is not a reference to a blessed object. Otherwise, returns the class into which the object is blessed.

ref (see "ref" in perlfunc) gives the same result on references to blessed objects, but different results on other types of value. blessed_class is actually identical to "blessed" in Scalar::Util.


Check whether ARG is a reference to a blessed object, identically to "is_blessed". This exists only for symmetry; the useful form of is_strictly_blessed appears below.

is_strictly_blessed(ARG, CLASS)
check_strictly_blessed(ARG, CLASS)

Check whether ARG is a reference to an object blessed into CLASS exactly. CLASS must be a string, naming a Perl class. Because this excludes subclasses, this is rarely what one wants, but there are some specialised occasions where it is useful.


Check whether ARG is a reference to a blessed object, identically to "is_blessed". This exists only for symmetry; the useful form of is_able appears below.

is_able(ARG, METHODS)
check_able(ARG, METHODS)

Check whether ARG is a reference to a blessed object that claims to implement the methods specified by METHODS (via its can method; see "can" in perlobj). METHODS must be either a single method name or a reference to an array of method names. Each method name is a string. This interface check is often more appropriate than a direct ancestry check (such as "is_blessed" performs).


Probably ought to handle something like Params::Validate's scalar type specification system, which makes much the same distinctions.


Data::Float, Data::Integer, Params::Validate, Scalar::Number, Scalar::Util


Andrew Main (Zefram) <>


Copyright (C) 2004, 2006, 2007, 2009, 2010, 2017 Andrew Main (Zefram) <>

Copyright (C) 2009, 2010 PhotoBox Ltd


This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.