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DRAFT: Synopsis 16: IO / Name Services


    Largely, the authors of the related Perl 5 docs.
    Larry Wall <>
    Mark Stosberg <>
    Tim Nelson <>
    Daniel Ruoso <>


    Created: 12 Sep 2006

    Last Modified: 19 Nov 2009
    Version: 23

This is a draft document. Many of these functions will work as in Perl 5, except we're trying to rationalize everything into roles. For now you can assume most of the important functions will automatically be in the * namespace. However, with IO operations in particular, many of them are really methods on an IO handle, and if there is a corresponding global function, it's merely an exported version of the method.


Overridable IO handles

In Perl 6, there are the standard IO handles, and any number of overriding inner filehandles for the same symbol.

The standard handles are our old familiar friends (with new names). Standard input changed from STDIN to $*IN, standard output changed from STDOUT to $*OUT, and standard error changed from STDERR to $*ERR. In Perl 6 these symbols represent more of a concept than a given filehandle, since the meaning is contextually determined. The process's version of these handles live in the PROCESS:: namespace, which is more global than the per-interpreter GLOBAL:: namespace.

When no explicit filehandle is used, the standard IO operators are defined in terms of the dynamic variables. So the print function prints to $*OUT, while warn warns to $*ERR. The lines() term inputs from $*ARGFILES which defaults to $*IN in the absence of any filenames. So any given dynamic scope (interpreter, thread, function or method call) may redefine the current meaning of any of those filehandles within the dynamic scope of itself and of its called routines.

So to put it another way, when you write something like

    say "Howdy, world!"

the say function looks for the current meaning of $*OUT, and takes the closest definition it can find in its callers. If none of the callers have overridden the definition, it looks in the interpreter's GLOBAL namespace. If the interpreter hasn't overridden the meaning, it takes the meaning from PROCESS. In essence, any dynamic scope in Perl 6 is allowed to do IO redirection much like a Unix shell does with its subprocesses, albeit with a different syntax:

        temp $*OUT = open $newfile, :w;
        foo() # all stdout goes to $newfile
    # stdout reverts to outer scope's definition

Roles and Classes

The roles and classes that define most of the functionality for IO are defined in S32-setting-library/IO.pod. The main functions used are listed in S29 with references to S32-setting-library/IO.pod.

Special Quoting Syntax

The use of filenames requires a special quoting syntax. It works as follows:


Both of the above result in the same IO::Path object.

The quote characters can be any of the usual ones, although / is probably a bad choice for filenames.

The code shown above returns a Path object (or a descendant thereof).

Naturally you can also ask for interpolation in filenames:


There are a number of special adverbs that can be applied to the file quoting operator. Most of these are filesystem-specific. They confine what can be included in a filename.

Any path that starts with a "/" is considered an absolute path, otherwise the path is considered relative.

When creating a path with qp{}, the Path.Encoding attribute is set to $?ENC, unless the :bin modifier (see below) is used.

Default constraints

The default constraints can be set with the "use path" pragma, for example:

    use path :posix;
    use path :modern;
    use path :local;

The default when in strict mode is "use path :posix", whereas the default in lax mode is "use path :local".

:posix constraints

The :modern set of constraints paths are portable POSIX paths (see POSIX.1-2008 sections 4.7 and 3.276). If platform/filesystem specific behavior is needed, specific constraints should be applied as needed (see below).

The default constraints are to only allow "/" as separator and only allows portable POSIX filenames. That means A-Z, a-z, 0-9, <period>, <underscore>, and <hyphen>. Additionally, hyphen may not be the first character in the path.

Any path that starts with a "/" is considered an absolute path, otherwise the path is considered relative.


In addition to the POSIX constraints above, the path should fit into 8 characters, followed by a full stop, and then three more characters. Only the one full stop should appear in the filename. Additionally, no path may be longer than 64 characters.


This is :win on a Windows platform, :unix on a Unix platform, etc. Note that this is specifically not portable between platforms with different constraint sets.

:modern constraints

The :modern set of constraints are the same as :posix, except that they may contain any UTF-8 character, rather than just those listed.

:win constraints

We allow Windows style paths so converting and maintaining code on this platform is not a pain.

my Path $path = p:win{C:\Program Files\MS Access\file.file};

Note that this specifically excludes the backslash quoting usually used with q{}.

:unix constraints

For Unix specific behavior we have a p:unix{} literal. Here the only limits are what is defined by the locale and the filesystem type. So we won't be able to write full Unicode if locale is set to Latin1.

my Path $path = p:unix{/usr/src/bla/myfile?:%.file};

:bin constraints (no constraints at all)

If the above are causing problems, p:bin{} can be used as no checking is done here. However, this leaves the Path.Encoding attribute undefined, which means that certain features of Path will remain unavailable unless this attribute is set.

Other constraints

It is expected that other sets of constraints, such as VMS, DOS, and old-style Mac will be provided by modules.


The global variable $*CWD is a Path object with certain special properties, mainly:

 * It must check whether the path exists before changing its value
 * It can only be assigned an absolute path; if you want to make relative 
   changes, use the Array interface

$*CWD is specific to the current thread, unlike %*ENV<PATH> or the native getcwd/chdir path, which are both usually process-wide.

The variable is used in at least these cases:

 * When a Path object is created, if the string it is given is not an
   absolute path, then $*CWD is prepended to it.
 * When a subprocess is executed, it is executed with the current
   thread's $*CWD as its working directory.

Name Services

User role

    role User {
        has $username; # Username (some descendants(?) may want to implement a real $name)
        has $id;  # User ID
        has $dir; # Home directory for files
    method User new($Username?, $UID?) {...}

Creates a new User object, fetching the information either by username or user ID.

    method write() {...}

Tries to write the current User object to the user database. This may well fail.


When converted to a Str, returns $username.


When converted to a Num, returns $uid.

OS::Unix::User role

    role OS::Unix::User does User {
        has $password;
        has $gid;
        has $gecos;
        has $shell;

All the information is naturally fetched from the system via getpwuid, getpwnam, or the like.

Group role

    role Group {
        has $name;
        has $id;
        has @members;
    method Group new(:$Name, :$ID);
    method write();

Tries to write the group entry into the system group database.

OS::Unix::NameServices role

The NameServices role has a bunch of functions that between them will return the whole Name Services database between them, as lists of objects. The lists are specifically intended to be lazy.

    role NameServices {
        method List of User     users()     {...} # getpwent, setpwent, endpwent
        method List of Group    groups()    {...} # getgrent, setgrent, endgrent
        method List of Service  services()  {...} # getservent, setservent, endservent
        method List of Protocol protocols() {...} # getprotoent, setprotoent, endprotoent
        method List of Network  networks()  {...} # getnetent, setnetent, endnetent
        method List of Host     hosts()     {...} # gethostent, sethostent, endhostent


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