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Data::Dumper::Interp - interpolate Data::Dumper output into strings for human consumption


  use open IO => ':locale';
  use Data::Dumper::Interp;

  @ARGV = ('-i', '/file/path');
  my %hash = (abc => [1,2,3,4,5], def => undef);
  my $ref = \%hash;
  my $obj = bless {}, "Foo::Bar";

  # Interpolate variables in strings with Data::Dumper output
  say ivis 'FYI ref is $ref\nThat hash is: %hash\nArgs are @ARGV';

    # -->FYI ref is {abc => [1,2,3,4,5], def => undef}
    #    That hash is: (abc => [1,2,3,4,5], def => undef)
    #    Args are ("-i","/file/path")

  # Label interpolated values with "expr="
  say dvis '$ref\nand @ARGV';

    #-->ref={abc => [1,2,3,4,5], def => undef}
    #   and @ARGV=("-i","/file/path")

  # Functions to format one thing
  say vis $ref;      # {abc => [1,2,3,4,5], def => undef}
  say vis \@ARGV;    # ["-i", "/file/path"]  # any scalar
  say avis @ARGV;    # ("-i", "/file/path")
  say hvis %hash;    # (abc => [1,2,3,4,5], def => undef)

  # Format a reference with abbreviated referent address
  say visr $ref;     # HASH<457:1c9>{abc => [1,2,3,4,5], ...}

  # Just abbreviate a referent address or arbitrary number
  say addrvis refaddr($ref);  # 457:1c9
  say addrvis $ref;           # HASH<457:1c9>
  say addrvis $obj;           # Foo::Bar<984:ef8>

  # Stringify objects
  { use bigint;
    my $struct = { debt => 999_999_999_999_999_999.02 };
    say vis $struct;
      # --> {debt => (Math::BigFloat)999999999999999999.02}

    # But if you do want to see object internals...
    say visnew->viso($struct);
      # --> {debt => bless({...lots of stuff...},'Math::BigInt')}

    # These do the same thing
    say visnew->Objects(0)->vis($struct);
    { local $Data::Dumper::Interp::Objects=0; say vis $struct; }
    say viso $struct;   # 'viso' is not exported by default

  # Wide characters are readable
  use utf8;
  my $h = {msg => "My language is not ASCII ☻ ☺ 😊 \N{U+2757}!"};
  say dvis '$h' ;
    # --> h={msg => "My language is not ASCII ☻ ☺ 😊 ❗!"}

  #-------- OO API --------

  say visnew->MaxStringwidth(50)->Maxdepth($levels)->vis($datum);

  say Data::Dumper::Interp->new()

  #-------- UTILITY FUNCTIONS --------
  say u($might_be_undef);  # $_[0] // "undef"
  say quotekey($string);   # quote if not a valid bareword
  say qsh($string);        # quote if needed for /bin/sh
  say qshpath($pathname);  # shell quote excepting ~ prefix
  say "Runing this: ", qshlist(@command_and_args);

  system "ls -ld ".join(" ",map{ qshpath }
                            ("/tmp", "~sally/My Documents", "~"));


This Data::Dumper wrapper optimizes output for human consumption and avoids side-effects which interfere with debugging.

The namesake feature is interpolating Data::Dumper output into strings. Simple functions are also provided to format a scalar, array, or hash.

Internally, Data::Dumper is called to visualize (i.e. format) data with pre- and post-processing to "improve" the results:

  • One line if possible, else folded to terminal with, WITHOUT newline.

  • Safely printable Unicode characters appear as themselves.

  • Objects like Math:BigInt etc. are stringified (by default).

  • "Virtual" values behind overloaded deref operators are shown.

  • Data::Dumper bugs^H^H^H^Hquirks are circumvented.

See "DIFFERENCES FROM Data::Dumper".

Utilities are also provided to quote strings for /bin/sh.


ivis 'string to be interpolated'

Returns the argument with variable references and escapes interpolated as in in Perl double-quotish strings, but using Data::Dumper to format variable values.

$var is replaced by its value, @var is replaced by "(comma, sparated, list)", and %hash by "(key => value, ...)" . Complex expressions with indexing, dereferences, slices and method calls are also recognized.

Expressions are evaluated in the caller's context using Perl's debugger hooks, and may refer to almost any lexical or global visible at the point of call (see "LIMITATIONS").

IMPORTANT: The argument must be single-quoted to prevent Perl from interpolating it beforehand.

dvis 'string to be interpolated'

The 'd' is for "debugging". Like ivis but labels expansions with "expr=" and shows spaces visibly as '·'. Other debug-oriented formatting may also occur (TBD).


avis LIST


vis formats a single scalar ($_ if no argument is given) and returns the resulting string.

avis formats an array (or any list) as comma-separated values in parenthesis.

hvis formats key => value pairs in parenthesis.


Variations of the above five functions have extra characters in their names to imply certain options. For example visq is like vis but shows strings in single-quoted form (implied by the 'q' suffix).

There are no fixed function names; you can use any combination of characters in any order, prefixed or suffixed to the primary name with optional '_' separators. The function will be generated when it is imported* or called as a method.

The available modifier characters are:

    l - omit parenthesis to return a bare list with "avis" or "hvis"; omit quotes from a string formatted by "vis".

    o - show object internals (see Objects);

    r - show abbreviated addresses of refs (see Refaddr).

    <NUMBER> - limit structure depth to <NUMBER> levels (see Maxdepth).

    See Useqq for more info about these:

    c - Show control characters as "Control Picture" characters

    C - condense strings of repeated characters

    d - ("debug-friendly") Condense strings; show spaces as middle-dot if STDOUT is utf-encoding

    h - show numbers > 9 in hexadecimal

    O - Optimize for strings containing binary octets.

    q - show strings 'single quoted' if possible

      With q Data::Dumper is called with Useqq(0), but depending on the version of Data::Dumper the result may be "double quoted" anyway if wide characters are present.

    u - show numbers with underscores between groups of three digits

Functions must be imported explicitly unless they are imported by default (see list below).

To avoid having to import functions in advance, you can use them as methods and import only the visnew function:

  use Spreadsheet::Edit::Interp qw/visnew/;
  say visnew->vis($struct);
  say visnew->visrq($my_object);
  say visnew->avis(@ARGV);
  say visnew->avis2lrq(@ARGV);

(visnew creates a new object. Non-existent methods are auto-generated via the AUTOLOAD mechanism).

Functions imported by default

 ivis  dvis    vis  avis  hvis
 ivisq dvisq   visq avisq hvisq rvis rvisq

 addrvis addrvisl
 u quotekey qsh qshlist qshpath

* To save memory, only stub declarations with prototypes are generated for imported functions. Bodies are generated when actually used via the AUTOLOAD mechanism. The :debug import tag prints messages chronicling these events.

Showing Abbreviated Addresses

addrvis REF_or_NUMBER

Returns a string showing an address in both decimal and hexadecimal, but abbreviated to only the last few digits.

The number of digits starts at 3 and increases over time if necessary to keep new results unambiguous.

For REFs, the result is like "HASH<457:1c9>" or "Package::Name<457:1c9>".

If the argument is a plain number, just the abbreviated address is returned, e.g. "<457:1c9>".

"undef" is returned if the argument is undefined. Croaks if the argument is defined but not a number or reference.

addrvis_digits(NUMBER) forces a minimum width and addrvis_forget() discards past values and resets to 3 digits.

addrvisl REF_or_NUMBER

Like addrvis but omits the <angle brackets>.




These create an object initialized from the global configuration variables listed below. No arguments are permitted. visnew is simply a shorthand.

All the functions described above and any variations may be called as methods on an object (when not called as a method the functions create a new object internally).

For example:

   $msg = visnew->Foldwidth(40)->avis(@ARGV);

returns the same string as

   local $Data::Dumper::Interp::Foldwidth = 40;
   $msg = avis @ARGV;

"Variations" can be called similarly, for example

   $msg = visnew->Foldwidth(40)->vis_r2($x); # show addresses; Maxdepth 2

Configuration Variables / Methods

These work the same way as variables/methods in Data::Dumper.

Each config method has a corresponding global variable in package Data::Dumper::Interp which provides the default value.

When a config method is called without arguments the current value is returned, and when called with an argument the value is changed and the object is returned so that calls can be chained.



Longer strings are truncated and Truncsuffix appended. MaxStringwidth=0 (the default) means no limit.


Defaults to the terminal width at the time of first use.



Objects([ list of classnames ])

A false value disables special handling of objects (that is, blessed things) and internals are shown as with Data::Dumper.

A "1" (the default) enables for all objects, otherwise only for the specified class name(s) or derived classes.

When enabled, object internals are never shown. The class and abbreviated address are shown as with addrvis e.g. "Foo::Bar<392:0f0>", unless the object overloads the stringification ('""') operator, or array-, hash-, scalar-, or glob- deref operators; in that case the first overloaded operator found will be evaluated, the object replaced by the result, and the check repeated.

By default, "(classname)" is prepended when an overloaded operator is evaluated to make clear what happened.

Objects({objects => VALUE, show_classname => BOOL})

This form, passing a hashref, allows control of whether "(classname)" is prepended to the result from an overloaded operator.

If the show_classname value is false, then overload results will appear unadorned, i.e. they will look as if the overload result was the original value.

The objects value indicates whether and for which classes special object handling is enabled (false, "1", "classname" or [list of classnames]).


The default sorts numeric substrings in keys by numerical value, e.g. "A.20" sorts before "A.100". See Data::Dumper documentation.


0 means generate 'single quoted' strings when possible.

1 means generate "double quoted" strings as-is from Data::Dumper. Non-ASCII charcters will be shown as hex or octal escapes.

Otherwise generate "double quoted" strings enhanced according to option keywords given as a :-separated list, e.g. Useqq("unicode:controlpics"). The avilable options are:


Printable ("graphic") characters are shown as themselves rather than hex escapes, and '\n', '\t', etc. are shown for ASCII control codes.


Show ASCII control characters using single "control picture" characters: '␤' is shown for newline instead of '\n', and similarly ␀ ␇ ␈ ␛ ␌ ␍ ␉ for \0 \a \b \e \f \r \t.

Every character occupies the same space with a fixed-width font, but the tiny "control picures" can be hard to read; to see traditional \n etc. while still seeing wide characters as themselves, set Useqq to just "unicode";


Optimize for viewing binary strings (i.e. strings of octets, not "wide" characters). Octal escapes are shown instead of \n, \r, etc.


Space characters are shown as '·' (Middle Dot).


Repeated sequences in strings are shown as "⸨charxrepcount⸩". For example

  vec(my $s, 31, 1) = 1;
  say unpack "b*", $s;
  say visnew->Useqq("unicode:condense")->visl(unpack "b*", $s);

Show numbers with '_' seprating groups of 3 digits.


Show using Perl's qq{...} or qqX...Y syntax, rather than "double quotes".

The default is Useqq('unicode') except for dvis which also enables 'condense' and possibly 'spacedots'. Functions/methods with 'q' in their name force Useqq(0);






See Data::Dumper documentation.




Returns the argument ($_ by default) if it is defined, otherwise the string "undef".


quotekey SCALAR

Returns the argument ($_ by default) if it is a valid bareword, otherwise a "quoted string".


qsh $string

The string ($_ by default) is quoted if necessary for parsing by the shell (/bin/sh), which has different quoting rules than Perl. On Win32 quoting is for

If the string contains only "shell-safe" ASCII characters it is returned as-is, without quotes.

If the argument is a ref but is not an object which stringifies, then vis() is called and the resulting string quoted. An undefined value is shown as undef without quotes; as a special case to avoid ambiguity the string 'undef' is always "quoted".

qshpath $might_have_tilde_prefix

Like qsh except that an initial ~ or ~username is left unquoted. Useful with bash or csh.

qshlist @items

Format e.g. a shell command and arguments, quoting when necessary.

Returns a string with the items separated by spaces.


Interpolated Strings

ivis and dvis evaluate expressions in the user's context using Perl's debugger support ('eval' in package DB -- see perlfunc). This mechanism has some limitations:

@_ may show incorrect values except immediately after sub entry. For example after "shift" @_ will appear to still have the original arguments.

A lexical ("my") sub creates a closure, and variables in visible scopes which are not actually referenced by your code may not exist in the closure; an attempt to display them with ivis will fail. For example:

    our $global;
    sub outerfunc {
      my sub inner {
        say dvis '$global'; # croaks with "Error interpolating '$global'"
        # my $x = $global;  # ... unless this is un-commented
Multiply-referenced items

If a structure contains several refs to the same item, the first ref will be visualized by showing the referenced item as you might expect.

However subsequent refs will look like $VAR1->place where place is the location of the first ref in the overall structure. This is how Data::Dumper indicates that the ref is a copy of the first ref and thus points to the same datum. "$VAR1" is an artifact of how Data::Dumper would generate code using its "Purity" feature. Data::Dumper::Interp simply passed through these annotations.

However with Refaddr(true), multiple references to the same thing will all show the address of the referenced thing.

The special "_" stat filehandle may not be preserved

Data::Dumper::Interp queries the operating system to obtain the window size to initialize $Foldwidth, if it is not already defined; this may change the "_" filehandle. After the first call (or if you pre-set $Foldwidth), the "_" filehandle will not change across calls.


Results differ from plain Data::Dumper output in the following ways (most of these can be controlled via Config options):

  • A final newline is never included.

    Everything is shown on a single line if possible, otherwise wrapped to your terminal width (or $Foldwidth), with indented structure levels.

  • Printable Unicode characters appear as themselves instead of \x{ABCD}.

    Note: If your data contains 'wide characters', you should use open IO => ':locale'; or otherwise arrange to encode the output for your terminal. You'll also want use utf8; if your Perl source contains characters outside the ASCII range.

    Undecoded binary octets (e.g. data read from a 'binmode' file) will still be escaped as individual bytes.

  • Depending on options, spaces·may·be·shown·visibly and '␤' may be shown for newline (and similarly for other ASCII controls).

    "White space" characters in qr/compiled regex/ are shown as \t, \n etc.

  • The internals of objects are not shown by default.

    If stringifcation is overloaded it is used to obtain the object's representation. For example, bignum and bigrat numbers are shown as easily readable values rather than "bless( {...}, 'Math::...')".

    Stingified objects are prefixed with "(classname)" to make clear what happened.

    The "virtual" value of objects which overload a dereference operator (@{} or %{}) is displayed instead of the object's internals.

  • Hash keys are sorted treating numeric "components" numerically. For example "A.20" sorts before "A.100".

  • Punctuation variables such as $@, $!, and $?, are preserved over calls.

  • Numbers and strings which look like numbers are kept distinct when displayed, i.e. "0" does not become 0 or vice-versa. Floating-point values are shown as numbers not 'quoted strings' and similarly for stringified objects.

    Although such differences might be immaterial to Perl during execution, they may be important when communicating to a human.




Jim Avera (jim.avera AT gmail)


Public Domain or CC0.