Perl6::Bible::A02(3) User Contributed Perl Documentation Perl6::Bible::A02(3)NAME
Apocalypse_02 - Bits and Pieces
AUTHOR
Larry Wall <larry@wall.org>
VERSION
Maintainer: Larry Wall <larry@wall.org>
Date: 3 May 2001
Last Modified: 4 Dec 2004
Number: 2
Version: 4
Here's Apocalypse 2, meant to be read in conjunction with Chapter 2 of
the Camel Book. The basic assumption is that if Chapter 2 talks about
something that I don't discuss here, it doesn't change in Perl 6. (Of
course, it could always just be an oversight. One might say that people
who oversee things have a gift of oversight.)
Before I go further, I would like to thank all the victims, er,
participants in the RFC process. (I beg special forgiveness from those
whose brains I haven't been able to get inside well enough to
incorporate their ideas). I would also like to particularly thank
Damian Conway, who will recognize many of his systematic ideas here,
including some that have been less than improved by my meddling.
Here are the RFCs covered:
RFC PSA Title
--- --- -----
Textual
005 cdr Multiline Comments for Perl
102 dcr Inline Comments for Perl
Types
161 adb Everything in Perl Becomes an Object
038 bdb Standardise Handling of Abnormal Numbers Like Infinities and NaNs
043 bcb Integrate BigInts (and BigRats) Support Tightly With the Basic Scalars
192 ddr Undef Values ne Value
212 rrb Make Length(@array) Work
218 bcc C<my Dog $spot> Is Just an Assertion
Variables
071 aaa Legacy Perl $pkg'var Should Die
009 bfr Highlander Variable Types
133 bcr Alternate Syntax for Variable Names
134 bcc Alternative Array and Hash Slicing
196 bcb More Direct Syntax for Hashes
201 bcr Hash Slicing
Strings
105 aaa Remove "In string @ must be \@" Fatal Error
111 aaa Here Docs Terminators (Was Whitespace and Here Docs)
162 abb Heredoc Contents
139 cfr Allow Calling Any Function With a Syntax Like s///
222 abb Interpolation of Object Method Calls
226 acr Selective Interpolation in Single Quotish Context
237 adc Hashes Should Interpolate in Double-Quoted Strings
251 acr Interpolation of Class Method Calls
252 abb Interpolation of Subroutines
327 dbr C<\v> for Vertical Tab
328 bcr Single Quotes Don't Interpolate \' and \\
Files
034 aaa Angle Brackets Should Not Be Used for File Globbing
051 ccr Angle Brackets Should Accept Filenames and Lists
Lists
175 rrb Add C<list> Keyword to Force List Context (like C<scalar>)
Retracted
010 rr Filehandles Should Use C<*> as a Type Prefix If Typeglobs Are Eliminated
103 rr Fix C<$pkg::$var> Precedence Issues With Parsing of C<::>
109 rr Less Line Noise - Let's Get Rid of @%
245 rr Add New C<empty> Keyword to DWIM for Clearing Values
263 rr Add Null() Keyword and Fundamental Data Type
Atoms
Perl 6 programs are notionally written in Unicode, and assume Unicode
semantics by default even when they happen to be processing other
character sets behind the scenes. Note that when we say that Perl is
written in Unicode, we're speaking of an abstract character set, not
any particular encoding. (The typical program will likely be written in
UTF-8 in the West, and in some 16-bit character set in the East.)
Molecules
RFC 005: Multiline Comments for Perl
I admit to being prejudiced on this one -- I was unduly influenced at a
tender age by the rationale for the design of Ada, which made a good
case, I thought, for leaving multiline comments out of the language.
But even if I weren't blindly prejudiced, I suspect I'd look at the
psychology of the thing, and notice that much of the time, even in
languages that have multiline comments, people nevertheless tend to use
them like this:
/*
* Natter, natter, natter.
* Gromish, gromish, gromish.
*/
The counterargument to that is, of course, that people don't always do
that in C, so why should they have to do it in Perl? And if there were
no other way to do multiline comments in Perl, they'd have a stronger
case. But there already is another way, albeit one rejected by this RFC
as "a workaround."
But it seems to me that, rather than adding another kind of comment or
trying to make something that looks like code behave like a comment,
the solution is simply to fix whatever is wrong with POD so that its
use for commenting can no longer be considered a workaround. Actual
design of POD can be put off till Apocalypse 26, but we can speculate
at this point that the rules for switching back and forth between POD
and Perl are suboptimal for use in comments. If so, then it's likely
that in Perl 6 we'll have a rule like this: If a "=begin MUMBLE"
transitions from Perl to POD mode then the corresponding "=end MUMBLE"
should transition back (without a "=cut" directive).
Note that we haven't defined our "MUMBLE"s yet, but they can be set up
to let our program have any sort of programmatic access to the data
that we desire. For instance, it is likely that comments of this kind
could be tied in with some sort of literate (or at least, semiliterate)
programming framework.
RFC 102: Inline Comments for Perl
I have never much liked inline comments -- as commonly practiced they
tend to obfuscate the code as much as they clarify it. That being said,
"All is fair if you predeclare." So there should be nothing preventing
someone from writing a lexer regex that handles them, provided we make
the lexer sufficiently mutable. Which we will. (As it happens, the
character sequence ""/*"" will be unlikely to occur in standard Perl 6.
Which I guess means it is likely to occur in nonstandard Perl 6. ":-)"
A pragma declaring nonstandard commenting would also allow people to
use "/* */" for multiline comments, if they like. (But I still think
it'd be better to use POD directives for that, just to keep the text
accessible to the program.)
Built-In Data Types
The basic change here is that, rather than just supporting scalars,
arrays and hashes, Perl 6 supports opaque objects as a fourth
fundamental data type. (You might think of them as pseudo-hashes done
right.) While a class can access its object attributes any way it
likes, all external access to opaque objects occurs through methods,
even for attributes. (This guarantees that attribute inheritance works
correctly.)
While Perl 6 still defaults to typeless scalars, Perl will be able to
give you more performance and safety as you give it more type
information to work with. The basic assumption is that homogenous data
structures will be in arrays and hashes, so you can declare the type of
the scalars held in an array or hash. Heterogenous structures can still
be put into typeless arrays and hashes, but in general Perl 6 will
encourage you to use classes for such data, much as C encourages you to
use structs rather than arrays for such data.
One thing we'll be mentioning before we discuss it in detail is the
notion of "properties." (In Perl 5, we called these "attributes," but
we're reserving that term for actual object attributes these days, so
we'll call these things "properties.") Variables and values can have
additional data associated with them that is "out of band" with respect
to the ordinary typology of the variable or value. For now, just think
of properties as a way of adding ad hoc attributes to a class that
doesn't support them. You could also think of it as a form of class
derivation at the granularity of the individual object, without having
to declare a complete new class.
[Update: We're now calling compile-time properties "traits".]
RFC 161: Everything in Perl Becomes an Object.
This is essentially a philosophical RFC that is rather short on detail.
Nonetheless, I agree with the premise that all Perl objects should act
like objects if you choose to treat them that way. If you choose not to
treat them as objects, then Perl will try to go along with that, too.
(You may use hash subscripting and slicing syntax to call attribute
accessors, for instance, even if the attributes themselves are not
stored in a hash.) Just because Perl 6 is more object-oriented
internally, does not mean you'll be forced to think in object-oriented
terms when you don't want to. (By and large, there will be a few places
where OO-think is more required in Perl 6 than in Perl 5. Filehandles
are more object-oriented in Perl 6, for instance, and the special
variables that used to be magically associated with the currently
selected output handle are better specified by association with a
specific filehandle.)
RFC 038: Standardise Handling Of Abnormal Numbers Like Infinities and NaNs
This is likely to slow down numeric processing in some locations.
Perhaps it could be turned off when desirable. We need to be careful
not to invent something that is guaranteed to run slower than IEEE
floating point. We should also try to avoid defining a type system that
makes translation of numeric types to Java or C# types problematic.
That being said, standard semantics are a good thing, and should be the
default behavior.
RFC 043: Integrate BigInts (and BigRats) Support Tightly With the Basic
Scalars
This RFC suggests that a pragma enables the feature, but I think it
should probably be tied to the run-time type system, which means it's
driven more by how the data is created than by where it happens to be
stored or processed. I don't see how we can make it a pragma, except
perhaps to influence the meaning of "int" and "num" in actual
declarations further on in the lexical scope:
use bigint;
my int $i;
might really mean
my bigint $i;
or maybe just
my int $i is bigint;
since representation specifications might just be considered part of
the "fine print." But the whole subject of lexically scoped variable
properties specifying the nature of the objects they contain is a bit
problematic. A variable is a sort of mini-interface, a contract if you
will, between the program and the object in question. Properties that
merely influence how the program sees the object are not a problem --
when you declare a variable to be constant, you're promising not to
modify the object through that variable, rather than saying something
intrinsically true about the object. (Not that there aren't objects
that are intrinsically constant.)
Other property declarations might need to have some say in how
constructors are called in order to guarantee consistency between the
variable's view of the object, and the nature of the object itself. In
the worst case we could try to enforce consistency at run time, but
that's apt to be slow. If every assignment of a "Dog" object to a
"Mammal" variable has to check to see whether "Dog" is a "Mammal", then
the assignment is going to be a dog.
So we'll have to revisit this when we're defining the relationship
between variable declarations and constructors. In any event, if we
don't make Perl's numeric types automatically promote to big
representations, we should at least make it easy to specify it when you
want that to happen.
[Update: The "Int" type automatically upgrades to arbitrary precision
internally. The "int" type does not.]
RFC 192: Undef Values ne Value
I've rejected this one, because I think something that's undefined
should be considered just that, undefined. I think the standard
semantics are useful for catching many kinds of errors.
That being said, it'll hopefully be easy to modify the standard
operators within a particular scope, so I don't think we need to think
that our way to think is the only way to think, I think.
RFC 212: Make "length(@array)" Work
Here's an oddity, an RFC that the author retracted, but that I accept,
more or less. I think "length(@array)" should be equivalent to
"@array.length()", so if there's a "length" method available, it should
be called.
The question is whether there should be a "length" method at all, for
strings or arrays. It almost makes more sense for arrays than it does
for strings these days, because when you talk about the length of a
string, you need to know whether you're talking about byte length or
character length. So we may split up the traditional length function
into two, in which case we might end up with:
$foo.chars
$foo.bytes
@foo.elems
Or some such. Whatever the method names we choose, differentiating them
would be more powerful in supplying context. For instance, one could
envision calling "@foo.bytes" to return the byte length of all the
strings. That wouldn't fly if we overloaded the method name.
Even "chars($foo)" might not be sufficiently precise, since, depending
on how you're processing Unicode, you might want to know how long the
string is in actual characters, not counting combining characters that
don't take extra space. But that's a topic for later.
[Update: There is no "length" function. There are "bytes", "codes",
"graphs", and "langs" methods for the various Unicode support levels.
(The "chars" method returns one of those values depending on the
current Unicode support level.) Arrays and hashes report number of
elements with the "elems" method.]
RFC 218: "my Dog $spot" Is Just an Assertion
I expect that a declaration of the form:
my Dog $spot;
is merely an assertion that you will not use $spot inconsistently with
it being a "Dog". (But I mean something different by "assertion" than
this RFC does.) This assertion may or may not be tested at every
assignment to $spot, depending on pragmatic context. This bare
declaration does not call a constructor; however, there may be forms of
declaration that do. This may be necessary so that the variable and the
object can pass properties back and forth, and in general, make sure
they're consistent with each other. For example, you might declare an
array with a multidimensional shape, and this shape property needs to
be visible to the constructor, if we don't want to have to specify it
redundantly.
On the other hand, we might be able to get assignment sufficiently
overloaded to accomplish the same goal, so I'm deferring judgment on
that. All I'm deciding here is that a bare declaration without
arguments as above does not invoke a constructor, but merely tells the
compiler something.
[Update: The constructor may be called using the ".=new()" construct.]
Other Decisions About Types
Built-in object types will be in all uppercase: "INTEGER", "NUMBER",
"STRING", "REF", "SCALAR", "ARRAY", "HASH", "REGEX" and "CODE".
Corresponding to at least some of these, there will also be lowercase
intrinsic types, such as "int", "num", "str" and "ref". Use of the
lowercase typename implies you aren't intending to do anything fancy
OO-wise with the values, or store any run-time properties, and thus
Perl should feel free to store them compactly. (As a limiting case,
objects of type "bit" can be stored in one bit.) This distinction
corresponds roughly to the boxed/unboxed distinction of other computer
languages, but it is likely that Perl 6 will attempt to erase the
distinction for you to the extent possible. So, for instance, an "int"
may still be used in a string context, and Perl will convert it for
you, but it won't cache it, so the next time you use it as a string, it
will have to convert again.
[Update: The object types are no longer all caps, but "Int", "Num",
"Str", etc.]
The declared type of an array or hash specifies the type of each
element, not the type of an array or hash as a whole. This is justified
by the notion that an array or hash is really just a strange kind of
function that (typically) takes a subscript as an argument and returns
a value of a particular type. If you wish to associate a type with the
array or hash as a whole, that involves setting a "tie" property. If
you find yourself wishing to declare different types on different
elements, it probably means that you should either be using a class for
the whole heterogenous thing, or at least declare the type of array or
hash that will be a base class of all the objects it will contain.
Of course, untyped arrays and hashes will be just as acceptable as they
are currently. But a language can only run so fast when you force it to
defer all type checking and method lookup till run time.
The intent is to make use of type information where it's useful, and
not require it where it's not. Besides performance and safety, one
other place where type information is useful is in writing interfaces
to other languages. It is postulated that Perl 6 will provide enough
optional type declaration syntax that it will be unnecessary to write
XS-style glue in most cases.
Variables
RFC 071: Legacy Perl $pkg'var Should Die
I agree. I was unduly influenced by Ada syntax here, and it was a
mistake. And although we're adding a properties feature into Perl 6
that is much like Ada's attribute feature, we won't make the mistake of
reintroducing a syntax that drives highlighting editors nuts. We'll try
to make different mistakes this time.
RFC 009: Highlander Variable Types
I basically agree with the problem this RFC is trying to solve, but I
disagree with the proposed solution. The basic problem is that, while
the idiomatic association of $foo[$bar] with @foo rather than $foo
worked fine in Perl 4, when we added recursive data structures to Perl
5, it started getting in the way notationally, so that initial funny
character was trying to do too much in both introducing the "root" of
the reference, as well as the context to apply to the final subscript.
This necessitated odd looking constructions like:
$foo->[1][2][3]
This RFC proposes to solve the dilemma by unifying scalar variables
with arrays and hashes at the name level. But I think people like to
think of $foo, @foo and %foo as separate variables, so I don't want to
break that. Plus, the RFC doesn't unify &foo, while it's perfectly
possible to have a reference to a function as well as a reference to
the more ordinary data structures.
So rather than unifying the names, I believe all we have to do is unify
the treatment of variables with respect to references. That is, all
variables may be thought of as references, not just scalars. And in
that case, subscripts always dereference the reference implicit in the
array or hash named on the left.
This has two major implications, however. It means that Perl
programmers must learn to write @foo[1] where they used to write
$foo[1]. I think most Perl 5 people will be able to get used to this,
since many of them found the current syntax a bit weird in the first
place.
The second implication is that slicing needs a new notation, because
subscripts no longer have their scalar/list context controlled by the
initial funny character. Instead, the context of the subscript will
need to be controlled by some combination of:
1. Context of the entire term.
2. Appearance of known list operators in the subscript, such as comma
or range.
3. Explicit syntax casting the inside of the subscript to list or
scalar context.
4. Explicit declaration of default behavior.
One thing that probably shouldn't enter into it is the run-time type of
the array object, because context really needs to be calculated at
compile time if at all possible.
In any event, it's likely that some people will want subscripts to
default to scalars, and other people will want them to default to
lists. There are good arguments for either default, depending on
whether you think more like an APL programmer or a mere mortal.
[Update: Subscripts are always list context, but it's trivial to force
scalar context with either of the "+" or "~" unary operators.]
There are other larger implications. If composite variables are thought
of as scalar references, then the names @foo and %foo are really scalar
variables unless explicitly dereferenced. That means that when you
mention them in a scalar context, you get the equivalent of Perl 5's
"\@foo" and "\%foo". This simplifies the prototyping system greatly, in
that an operator like "push" no longer needs to specify some kind of
special reference context for its first argument -- it can merely
specify a scalar context, and that's good enough to assume the
reference generation on its first argument. (Of course, the function
signature can always be more specific if it wants to. More about that
in future installments.)
There are also implications for the assignment operator, in that it has
to be possible to assign array references to array variables without
accidentally invoking list context and copying the list instead of the
reference to the list. We could invent another assignment operator to
distinguish the two cases, but at the moment it looks as though bare
variables and slices will behave as lvalues just as they do in Perl 5,
while lists in parentheses will change to a binding of the right-hand
arguments more closely resembling the way Perl 6 will bind formal
arguments to actual arguments for function calls. That is to say,
@foo = (1,2,3);
will supply an unbounded list context to the right side, but
(@foo, @bar) = (@bar, @foo)
will supply a context to the right side that requests two scalar values
that are array references. This will be the default for unmarked
variables in an lvalue list, but there will be an easy way to mark
formal array and hash parameters to slurp the rest of the arguments
with list context, as they do by default in Perl 5.
(Alternately, we might end up leaving the ordinary list assignment
operator with Perl 5 semantics, and define a new assignment operator
such as ":=" that does signatured assignment. I can argue that one both
ways.)
[Update: We ended up with a ":=" binding operator.]
Just as arrays and hashes are explicitly dereferenced via subscripting
(or implicitly dereferenced in list context), so too functions are
merely named but not called by &foo, and explicitly dereferenced with
parentheses (or by use as a bare name without the ampersand (or both)).
The Perl 5 meanings of the ampersand are no longer in effect, in that
ampersand will no longer imply that signature matching is suppressed --
there will be a different mechanism for that. And since &foo without
parens doesn't do a call, it is no longer possible to use that syntax
to automatically pass the @_ array -- you'll have to do that explicitly
now with "foo(@_)".
Scalar variables are special, in that they may hold either references
or actual "native" values, and there is no special dereference syntax
as there is for other types. Perl 6 will attempt to hide the
distinction as much as possible. That is, if $foo contains a native
integer, calling the "$foo.bar" method will call a method on the built-
in type. But if $foo contains a reference to some other object, it will
call the method on that object. This is consistent with the way we
think about overloading in Perl 5, so you shouldn't find this behavior
surprising. It may take special syntax to get at any methods of the
reference variable itself in this case, but it's OK if special cases
are special.
RFC 133: Alternate Syntax for Variable Names
This RFC has a valid point, but in fact we're going to do just the
opposite of what it suggests. That is, we'll consider the funny
characters to be part of the name, and use the subscripts for context.
This works out better, because there's only one funny character, but
many possible forms of dereferencing.
RFC 134: Alternative Array and Hash Slicing
We're definitely killing Perl 5's slice syntax, at least as far as
relying on the initial character to determine the context of the
subscript. There are many ways we could reintroduce a slicing syntax,
some of which are mentioned in this RFC, but we'll defer the decision
on that till Apocalypse 9 on Data Structures, since the interesting
parts of designing slice syntax will be driven by the need to slice
multidimensional arrays.
[Update: There is no Apocalypse 9, but there is a Synopsis 9 that
covers these matters.]
For now we'll just say that arrays can have subscript signatures much
like functions have parameter signatures. Ordinary one-dimensional
arrays (and hashes) can then support some kind of simple slicing syntax
that can be extended for more complicated arrays, while allowing
multidimensional arrays to distinguish between simple slicing and
complicated mappings of lists and functions onto subscripts in a manner
more conducive to numerical programming.
On the subject of hash slices returning pairs rather than values, we
could distinguish this with special slice syntax, or we could establish
the notion of a hashlist context that tells the slice to return pairs
rather than just values. (We may not need a special slice syntax for
that if it's possible to typecast back and forth between pair lists and
ordinary lists.)
[Update: Slicing to get a pairlist can be done by attaching a ":p"
modifier to the subscript.]
RFC 196: More Direct Syntax for Hashes
This RFC makes three proposals, which we'll consider separately.
Proposal 1 is "that a hash in scalar context evaluate to the number of
keys in the hash." (You can find that out now, but only by using the
"keys()" function in scalar context.) Proposal 1 is OK if we change
"scalar context" to "numeric context," since in scalar context a hash
will produce a reference to the hash, which just happens to numify to
the number of entries.
We must also realize that some implementations of hash might have to go
through and count all the entries to return the actual number.
Fortunately, in boolean context, it suffices to find a single entry to
determine whether the hash contains anything. However, on hashes that
don't keep track of the number of entries, finding even one entry might
reset any active iterator on the hash, since some implementations of
hash (in particular, the ones that don't keep track of the number of
entries) may only supply a single iterator.
[Update: You may also call ".elems" to be more explicit.]
Proposal 2 is "that the iterator in a hash be reset through an explicit
call to the "reset()" function." That's fine, with the proviso that it
won't be a function, but rather a method on the HASH class.
Proposal 3 is really about "sort" recognizing pairs and doing the right
thing. Defaulting to sorting on $^a[0] cmp $^b[0] is likely to be
reasonable, and that's where a pair's key would be found. However, it's
probable that the correct solution is simply to provide a default
string method for anonymous lists that happens to produce a decent key
to sort on when "cmp" requests a string representation of either of its
arguments. The "sort" itself should probably just concentrate on
memoizing the returned strings so they don't have to be recalculated.
[Update: The "sort" interface has been completely revamped since this
was written. This will eventually appear in S29, but as of now it's
just in the perl6-language archives.]
RFC 201: Hash Slicing
This RFC proposes to use "%" as a marker for special hash slicing in
the subscript. Unfortunately, the "%" funny character will not be
available for this use, since all hash refs will start with "%".
Concise list comprehensions will require some other syntax within the
subscript, which will hopefully generalize to arrays as well.
Other Decisions About Variables
Various special punctuation variables are gone in Perl 6, including all
the deprecated ones. (Non-deprecated variables will be replaced by some
kind of similar functionality that is likely to be invoked through some
kind of method call on the appropriate object. If there is no
appropriate object, then a named global variable might provide similar
functionality.)
Freeing up the various bracketing characters allows us to use them for
other purposes, such as interpolation of expressions:
"$(expr)" # interpolate a scalar expression
"@(expr)" # interpolate a list expression
[Update: Those forms are gone now. Expression interpolation is done
via closure.]
$#foo is gone. If you want the final subscript of an array, and "[-1]"
isn't good enough, use "@foo.end" instead.
Other special variables (such as the regex variables) will change from
dynamic scoping to lexical scoping. It is likely that even $_ and @_
will be lexically scoped in Perl 6.
Names
In Perl 5, lexical scopes are unnamed and unnameable. In Perl 6, the
current lexical scope will have a name that is visible within the
lexical scope as the pseudo class "MY", so that such a scope can, if it
so chooses, delegate management of its lexical scope to some other
module at compile time. In normal terms, that means that when you use a
module, you can let it import things lexically as well as packagely.
Typeglobs are gone. Instead, you can get at a variable object through
the symbol table hashes that are structured much like Perl 5's. The
variable object for $MyPackage::foo is stored in:
%MyPackage::{'$foo'}
Note that the funny character is part of the name. There is no longer
any structure in Perl that associates everything with the name ""foo"".
Perl's special global names are stored in a special package named ""*""
because they're logically in every scope that does not hide them. So
the unambiguous name of the standard input filehandle is $*STDIN, but a
package may just refer to $STDIN, and it will default to $*STDIN if no
package or lexical variable of that name has been declared.
[Update: We did s/STD// on those, so standard input is now just $*IN.]
Some of these special variables may actually be cloned for each lexical
scope or each thread, so just because a name is in the special global
symbol table doesn't mean it always behaves as a global across all
modules. In particular, changes to the symbol table that affect how the
parser works must be lexically scoped. Just because I install a special
rule for my cool new hyperquoting construct doesn't mean everyone else
should have to put up with it. In the limiting case, just because I
install a Python parser, it shouldn't force other modules into a maze
of twisty little whitespace, all alike.
Another way to look at it is that all names in the ""*"" package are
automatically exported to every package and/or outer lexical scope.
Literals
Underscores in Numeric Literals
Underscores will be allowed between any two digits within a number.
RFC 105: Remove "In string @ must be \@" Fatal Error
Fine.
[Update: The interpolation rules for arrays have been completely
revised. A bare array name no longer interpolates--you have to say
@foo[].]
RFC 111: Here Docs Terminators (Was Whitespace and Here Docs)
Fine.
RFC 162: Heredoc contents
I think I like option (e) the best: remove whitespace equivalent to the
terminator.
By default, if it has to dwim, it should dwim assuming that hard tabs
are 8 spaces wide. This should not generally pose a problem, since most
of the time the tabbing will be consistent throughout anyway, and no
dwimming will be necessary. This puts the onus on people using
nonstandard tabs to make sure they're consistent so that Perl doesn't
have to guess.
Any additional mangling can easily be accomplished by a user-defined
operator.
RFC 139: Allow Calling Any Function With a Syntax Like s///
Creative quoting will be allowed with lexical mutataion, but we can't
parse "foo(bar)" two different ways simultaneously, and I'm unwilling
to prevent people from using parens as quote characters. I don't see
how we can reasonably have new quote operators without explicit
declaration. And if the utility of a quote-like operator is sufficient,
there should be little relative burden in requiring such a declaration.
The form of such a declaration is left to the reader as an exercise in
function property definition. We may revisit the question later in this
series. It's also possible that a quote operator such as "qx//" could
have a corresponding function name like "quote:qx" that could be
invoked as a function.
RFC 222: Interpolation of Object Method Calls
I've been hankering for methods to interpolate for a long time, so I'm
in favor of this RFC. And it'll become doubly important as we move
toward encouraging people to use accessor methods to refer to object
attributes outside the class itself.
I have one "but," however. Since we'll switch to using "." instead of
"->", I think for sanity's sake we may have to require the parentheses,
or ""$file.$ext"" is going to give people fits. Not to mention
""$file.ext"".
[Update: Nowadays we also require brackets on array interpolations and
braces on hash interpolations. See S3 for more.]
RFC 226: Selective Interpolation in Single Quotish Context.
This proposal has much going for it, but there are also difficulties,
and I've come close to rejecting it outright simply because the single-
quoting policy of Perl 5 has been successful. And I think the proposal
in this RFC for "\I"..."\E" is ugly. (And I'd like to kill "\E" anyway,
and use bracketed scopings.)
However, I think there is a major "can't get there from here" that we
could solve by treating interpolation into single quotes as something
hard, not something easy. The basic problem is that it's too easy to
run into a "\$" or "\@" (or a "\I" for that matter) that wants to be
taken literally. I think we could allow the interpolation of arbitrary
expressions into single-quoted strings, but only if we limit it to an
unlikely sequence where three or more characters are necessary for
recognition. The most efficient mental model would seem to be the idea
of embedding one kind of quote in another, so I think this:
\q{stuff}
will embed single-quoted stuff, while this:
\qq{stuff}
will embed double-quoted stuff. A variable could then be interpolated
into a single-quoted string by saying:
\qq{$foo}
RFC 237: Hashes Should Interpolate in Double-Quoted Strings
I agree with this RFC in principle, but we can't define the default
hash stringifier in terms of variables that are going away in Perl 6,
so the RFC's proposal of using $" is right out.
All objects should have a method by which they produce readable output.
How this may be overridden by user preference is open to debate.
Certainly, dynamic scoping has its problems. But lexical override of an
object's preferences is also problematic. Individual object properties
appear to give a decent way out of this. More on that below.
[Update: Hash values by default interpolate with tabs between key and
value, and with newline between pairs. But you can give it as specific
format with the ".as" method.]
On "printf" formats, I don't see any way to dwim that %d isn't an
array, so we'll just have to put formats into single quotes in general.
Those format strings that also interpolate variables will be able to
use the new "\qq{$var}" feature.
[Update: Since hash interpolations require braces now, "printf" formats
are safe again (unless they happen to be followed by curlies).]
Note for those who are thinking we should just stick with Perl 5
interpolation rules: We have to allow "%" to introduce interpolation
now because individual hash values are no longer named with $foo{$bar},
but rather %foo{$bar}. So we might as well allow interpolation of
complete hashes.
RFC 251: Interpolation of Class Method Calls
Class method calls are relatively rare (except for constructors, which
will be rarely interpolated). So rather than scanning for identifiers
that might introduce a class, I think we should just depend on
expression interpolation instead:
"There are $(Dog.numdogs) dogs."
[Update: That's now done with closure interpolation.]
RFC 252: Interpolation of Subroutines
I think subroutines should interpolate, provided they're introduced
with the funny character. (On the other hand, how hard is "$(sunset
$date)" or "@(sunset $date)"? On the gripping hand, I like the
consistency of "&" with "$", "@" and "%".)
I think the parens are required, since in Perl 6, scalar &sub will just
return a reference, and require parens if you really want to deref the
sub ref. (It's true that a subroutine can be called without parens when
used as a list operator, but you can't interpolate those without a
funny character.)
For those worried about the use of "&" for signature checking
suppression, we should point out that "&" will no longer be the way to
suppress signature checking in Perl 6, so it doesn't matter.
RFC 327: "\v" for Vertical Tab
I think the opportunity cost of not reserving "\v" for future use is
too high to justify the small utility of retaining compatibility with a
feature virtually nobody uses anymore. For instance, I almost used "\v"
and "\V" for switching into and out of verbatim (single-quote) mode,
until I decided to unify that with quoting syntax and use "\qq{}" and
"\q{}" instead.
RFC 328: Single quotes don't interpolate \' and \\
I think hyperquotes will be possible with a declaration of your quoting
rules, so we're not going to change the basic single-quote rules
(except for supporting "\q").
[Update: There are adverbial modifiers now that can do hyperquoting.
See S2.]
Other Decisions About Literals
Scoping of \L et al.
I'd like to get rid of the gratuitously ugly "\E" as an end-of-scope
marker. Instead, if any sequence such as "\L", "\U" or "\Q" wishes to
impose a scope, then it must use curlies around that scope:
"\L{stuff}", "\U{stuff}" or "\Q{stuff}". Any literal curlies contained
in stuff must be backslashed. (Curlies as syntax (such as for
subscripts) should nest correctly.)
[Update: Those constructs are now gone entirely. Use closure
interpolation to interpolate the value of an expression.]
Bareword Policy
There will be no barewords in Perl 6. Any bare name that is a declared
package name will be interpreted as a class object that happens to
stringify to the package name. All other bare names will be interpreted
as subroutine or method calls. For nonstrict applications, undefined
subroutines will autodefine themselves to return their own name. Note
that in "${name}" and friends, the name is considered autoquoted, not a
bareword.
[Update: The "${name}" construct is gone. Use closure interpolation to
disambiguate expression interpolations: "{$name}text". The old brace
syntax is now reserved for hard dereferences only. Use "$::($name)"
for symbolic dereferences.]
Weird brackets
Use of brackets to disambiguate
"${foo[bar]}"
from
"${foo}[bar]"
will no longer be supported. Instead, the expression parser will always
grab as much as it can, and you can make it quit at a particular point
by interpolating a null string, specified by "\Q":
"$foo\Q[bar]"
[Update: That's gone too. Just use closure interpolation to
disambiguate.]
Special tokens
Special tokens will turn into either POD directives or lexically scoped
OO methods under the "MY" pseudo-package:
Old New
------
__LINE__ MY.line
__FILE__ MY.file
__PACKAGE__ MY.package
__END__ =begin END (or remove)
__DATA__ =begin DATA
[Update: The first three are now "$?LINE", "$?FILE", and "$?PACKAGE".
There are other such variables too. See S2.]
Heredoc Syntax
I think heredocs will require quotes around any identifier, and we need
to be sure to support "<< qq(END)" style quotes. Space is now allowed
before the (required) quoted token. Note that custom quoting is now
possible, so if you define a fancy "qh" operator for your fancy
hyperquoting algorithm, then you could say "<<qh(END)".
It is still the case that you can say "<<""" to grab everything up to
the next blank line. However, Perl 6 will consider any line containing
only spaces, tabs, etc., to be blank, not just the ones that
immediately terminate with newline.
Context
In Perl 5, a lot of contextual processing was done at run-time, and
even then, a given function could only discover whether it was in void,
scalar or list context. In Perl 6, we will extend the notion of context
to be more amenable to both compile-time and run-time analysis. In
particular, a function or method can know (theoretically even at
compile time) when it is being called in:
Void context
Scalar context
Boolean context
Integer context
Numeric context
String context
Object context
List context
Flattening list context (true list context).
Non-flattening list context (list of scalars/objects)
Lazy list context (list of closures)
Hash list context (list of pairs)
(This list isn't necessarily exhaustive.)
Each of these contexts (except maybe void) corresponds to a way in
which you might declare the parameters of a function (or the left side
of a list assignment) to supply context to the actual argument list (or
right side of a list assignment). By default, parameters will supply
object context, meaning individual parameters expect to be aliases to
the actual parameters, and even arrays and hashes don't do list context
unless you explicitly declare them to. These aren't cast in stone yet
(or even Jello), but here are some ideas for possible parameter
declarations corresponding to those contexts:
Scalar context
Boolean context bit $arg
Integer context int $arg
Numeric context num $arg
String context str $arg
Object context $scalar, %hash, Dog @canines, &foo
List context
Flattening list context *@args
Non-flattening list context $@args
Lazy list context &@args
Hash list context *%args
[Update: Some of these are bogus. See S6 for the most recent
formulation.]
(I also expect unary * to force flattening of arrays in rvalue
contexts. This is how we defeat the type signature in Perl 6, instead
of relying on the initial ampersand. So instead of Perl 5's
"&push(@list)", you could just say "push *@list", and it wouldn't
matter what "push"'s parameter signature said.)
It's also possible to define properties to modify formal arguments,
though that can get clunky pretty quickly, and I'd like to have a
concise syntax for the common cases, such as the last parameter
slurping a list in the customary fashion. So the signature for the
built-in "push" could be
sub push (@array, *@pushees);
Actually, the signature might just be "(*@pushees)", if "push" is
really a method in the "ARRAY" class, and the object is passed
implicitly:
class ARRAY;
sub .push (*@pushees);
sub .pop (;int $numtopop);
sub .splice (int $offset, int $len, *@repl);
But we're getting ahead of ourselves.
[Update: Indeed, methods are declared with a "method" keyword, and
without the dot. Also the built-in class is "Array", not "ARRAY". See
A12 and S12 for much more about classes and methods.]
By the way, all function and method parameters (other than the object
itself) will be considered read-only unless declared with the "rw"
property. (List assignments will default the other way.) This will
prevent a great deal of the wasted motion current Perl implementations
have to go through to make sure all function arguments are valid
lvalues, when most of them are in fact never modified.
Hmm, we're still getting ahead of ourselves. Back to contexts.
References are now transparent to boolean context
References are no longer considered to be "always true" in Perl 6. Any
type can overload its "bit()" casting operator, and any type that
hasn't got a "bit()" of its own inherits one from somewhere else, if
only from class UNIVERSAL. The built-in bit methods have the expected
boolean semantics for built-in types, so arrays are still true if they
have something in them, strings are true if they aren't "" or "0", etc.
Lists
RFC 175: Add "list" keyword to force list context (like "scalar")
Another RFC rescued from the compost pile. In Perl 6, type names will
identify casting functions in general. (A casting function merely
forces context -- it's a no-op unless the actual context is different.)
In Perl 6, a list used in a scalar context will automatically turn
itself into a reference to the list rather than returning the last
element. (A subscript of "[-1]" can always be used to get the last
element explicitly, if that's actually desired. But that's a rarity, in
practice.) So it works out that the explicit list composer:
[1,2,3]
is syntactic sugar for something like:
scalar(list(1,2,3));
Depending on whether we continue to make a big deal of the list/array
distinction, that might actually be spelled:
scalar(array(1,2,3));
Other casts might be words like "hash" (supplying a pairlist context)
and "objlist" (supplying a scalar context to a list of expressions).
Maybe even the optional "sub" keyword could be considered a cast on a
following block that might not otherwise be considered a closure in
context. Perhaps "sub" is really spelled "lazy". In which case, we
might even have a "lazylist" context to supply a lazy context to a list
of expressions.
[Update: Most of these aren't necessary. Just use the appropriate
method.]
And of course, you could use standard casts like "int()", "num()", and
"str()", when you want to be explicit about such contexts at compile
time. (Perl 5 already has these contexts, but only at run time.) Note
also that, due to the relationship between unary functions and methods,
"$foo.int", "$foo.num", and "$foo.str" will be just a different way to
write the same casts.
Lest you worry that your code is going to be full of casts, I should
point out that you won't need to use these casts terribly often because
each of these contexts will typically be implied by the signature of
the function or method you're calling. (And Perl will still be
autoconverting for you whenever it makes sense.) More on that in
Apocalypse 6, Subroutines. If not sooner.
So, while boolean context might be explicitly specified by writing:
if (bit $foo)
or
if ($foo.bit)
you'd usually just write it as in Perl 5:
if ($foo)
Other Decisions about Lists
Based on some of what we've said, you can see that we'll have the
ability to define various kinds of lazily generated lists. The specific
design of these operators is left for subsequent Apocalypses, however.
I will make one observation here, that I think some of the proposals
for how array subscripts are generated should be generalized to work
outside of subscripts as well. This may place some constraints on the
general use of the ":" character in places where an operator is
expected, for instance.
[Update: And indeed, we now have adverbs that look like :foo(1).]
As mentioned above, we'll be having several different kinds of list
context. In particular, there will be a hash list context that assumes
you're feeding it pairs, and if you don't feed it pairs, it will assume
the value you feed it is a key, and supply a default value. There will
likely be ways to get hashes to default to interesting values such as 0
or 1.
[Update: there isn't really a hash list context.]
In order to do this, the "=>" operator has to at least mark its left
operand as a key. More likely, it actually constructs a pair object in
Perl 6. And the "{ foo => $bar }" list composer will be required to use
"=>" (or be in a hashlist context), or it will instead be interpreted
as a closure without a "sub". (You can always use an explicit "sub" or
"hash" to cast the brackets to the proper interpretation.)
I've noticed how many programs use "qw()" all over the place (much more
frequently than the input operator, for instance), and I've always
thought "qw()" was kind of ugly, so I'd like to replace it with
something prettier. Since the input operator is using up a pair of
perfectly good bracketing characters for little syntactic gain, we're
going to steal those and make them into a qw-like list composer. In
ordinary list context, the following would be identical:
@list = < foo $bar %baz blurch($x) >;
@list = qw/ foo $bar %baz blurch($x) /; # same as this
@list = ('foo', '$bar', '%baz', 'blurch($x)'); # same as this
But in hashlist context, it might be equivalent to this:
%list = < foo $bar %baz blurch($x) >;
%list = (foo => 1, '$bar' => 1, '%baz' = 1, blurch => $x); # same as this
[Update: There is also a version with French quotes "XX" that does
interpolation before splitting into words.]
Files
Basically, file handles are just objects that can be used as iterators,
and don't belong in this chapter anymore.
RFC 034: Angle Brackets Should Not Be Used for File Globbing
Indeed, they won't be. In fact, angle brackets won't be used for input
at all, I suspect. See below. Er, above.
[Update: Angle brackets don't directly do input. There is a unary "="
that does the equivalent. For an argument it takes a filehandle,
interator, filename, or list of any of those, and returns lines. There
is a corresponding "lines" list operator.]
RFC 051: Angle Brackets Should Accept Filenames and Lists
There is likely to be no need for an explicit input operator in Perl 6,
and I want the angles for something else. I/O handles are a subclass of
iterators, and I think general iterator variables will serve the
purpose formerly served by the input operator, particularly since they
can be made to do the right Thing in context. For instance, to read
from standard input, it will suffice to say
while ($STDIN) { ... }
and the iterator will know it should assign to $_, because it's in a
Boolean context.
[Update: No, this confuses two different meanings of boolean. Use "for
=$IN {...}" instead.]
I read this RFC more as requesting a generic way to initialize an
iterator according to the type of the iterator. The trick in this case
is to prevent the re-evaluation of the spec every time -- you don't
want to reopen the file every time you read a line from it, for
instance. There will be standard ways to suppress evaluation in Perl 6,
both from the standpoint of the caller and the callee. In any case, the
model is that an anonymous subroutine is passed in, and called only
when appropriate. So an iterator syntax might prototype its argument to
be an anonymous sub, or the user might explicitly pass an anonymous
sub, or both. In any event, the "sub" keyword will be optional in Perl
6, so things like:
while (file {LIST}) { ... }
can be made to defer evaluation of LIST to the appropriate moment (or
moments, if LIST is in turn generating itself on the fly). For
appropriate parameter declarations I suppose even the brackets could be
scrapped.
Properties
Variables and values of various types have various kinds of data
attributes that are naturally associated with them by virtue of their
type. You know a dog comes equipped with a wag, hopefully attached to a
tail. That's just part of doghood.
Many times, however, you want the equivalent of a Post-It(r) note, so
you can temporarily attach bits of arbitrary information to some
unsuspecting appliance that (though it wasn't designed for it) is
nevertheless the right place to put the note. Similarly, variables and
values in Perl 6 allow you to attach arbitrary pieces of information
known as "properties." In essence, any object in Perl can have an
associated hash containing these properties, which are named by the
hash key.
Some of these properties are known at compile time, and don't actually
need to be stored with the object in question, but can actually be
stored instead in the symbol table entry for the variable in question.
(Perl still makes it appear as though these values are attached to the
object.) Compile-time properties can therefore be attached to variables
of any type.
[Update: Compile-time properties are now known as "traits".]
Run-time properties really are associated with the object in question,
which implies some amount of overhead. For that reason, intrinsic data
types like "int" and "num" may or may not allow run-time properties.
In cases where it is allowed, the intrinsic type must generally be
promoted to its corresponding object type (or wrapped in an object that
delegates back to the original intrinsic for the actual value). But you
really don't want to promote an array of a million bits to an array of
a million objects just because you had the hankering to put a sticky
note on one of those bits, so in those cases it's likely to be
disallowed, or the bit is likely to be cloned instead of referenced, or
some such thing.
Properties may also be attached to subroutines.
In general, you don't set or clear properties directly -- instead you
call an accessor method to do it for you. If there is no method of that
name, Perl will assume there was one that just sets or clears a
property with the same name as the method. However, using accessor
methods to set or clear properties allows us to define synthetic
properties. For instance, there might be a real "constant" property
that you could attach to a variable. Certain variables (such as those
in a function prototype) might have "constant" set by default. In that
case, setting a synthetic property such as "rw" might clear the
underlying "constant" property.
A property may be attached to the foregoing expression by means of the
"is" keyword. Here's a compile-time property set on a variable:
my int $pi is constant = 3;
Here's a run-time property set on a return value:
return 0 is true;
[Update: run-time properties are set with "but" rather than "is".]
Whether a property is applied to a variable at compile time or a value
at run-time depends on whether it's in lvalue or rvalue context.
(Variable declarations are always in lvalue context even when you don't
assign anything to them.)
The ""is"" works just like the ""."" of a method call, except that the
return value is the object on the left, not the return value of the
method, which is discarded.
As it happens, the ""is"" is optional in cases where an operator is
already expected. So you might see things like:
my int $pi constant = 3;
return 0 true;
In this case, the methods are actually being parsed as postfix
operators. (However, we may make it a stricture that you may omit the
"is" only for predeclared property methods.)
[Update: the "is" or "but" is never optional.]
Since these actually are method calls, you can pass arguments in
addition to the object in question:
my int @table is dim(366,24,60);
[Update: that should be "@table[^366;^24;^60]" according to S9...]
Our examples above are assuming an argument of "(1)":
my int $pi is constant(1) = 3;
return 0 is true(1);
Since the ""is"" is optional in the common cases, you can stack
multiple properties without repeating the ""is"."
my int $pi is shared locked constant optimize($optlevel) = 3;
[Update: the "is" is no longer optional.]
(Note that these methods are called on the $pi variable at compile
time, so it behooves you to make sure everything you call is defined.
For instance, $optlevel needs to be known at compile-time.)
Here are a list of property ideas stolen from Damian. (I guess that
makes it intellectual property theft.) Some of the names have been
changed to protect the (CS) innocent.
# Subroutine attributes...
sub name is rw { ... } # was lvalue
my sub rank is same { ... } # was memoized
$snum = sub is optimize(1) { ... }; # "is" required here
# Variable attributes...
our $age is constant = 21; # was const
my %stats is private;
my int @table is dim(366,24,60);
$arrayref = [1..1000000] is computed Purpose('demo of anon var attrs');
sub choose_rand (@list is lazy) { return $list[rand @list] }
# &@list notation is likely
$self = $class.bless( {name=>$name, age=>$age} is Initialized );
# Reference attributes...
$circular = \$head is weak;
# Literal attributes...
$name = "Damian" is Note("test data only");
$iohandle = open $filename is dis(qw/para crlf uni/) or die;
$default = 42 is Meaning(<<OfLife);
The Answer
OfLife
package Pet is interface;
class Dog inherits('Canine') { ... }
print $data{key is NoteToSelf('gotta get a better name for this key')};
(I don't agree with using properties for all of these things, but it's
pretty amazing how far into the ground you can drive it.)
Property names should start with an identifier letter (which includes
Unicode letters and ideographs). The parsing of the arguments (if any)
is controlled by the signature of the method in question. Property
method calls without a "." always modify their underlying property.
[Update: the signature has nothing to do with how they're parsed. If a
property has an argument it must follow without an intervening space.
However, other brackets are allowed, and using "will" rather than "is"
allows you to specify a closure with an intervening space.]
If called as an ordinary method (with a "."), the property value is
returned without being modified. That value could then be modified by a
run-time property. For instance, "$pi.constant" would return 1 rather
than the value of $pi, so we get:
return $pi.constant is false; # "1 but false" (not possible in Perl 5)
On the other hand, if you omit the dot, something else happens:
return $pi constant is false; # 3 but false (and 3 is now very constant)
[Update: The dot form is only for access of properties and traits that
are already there.]
Here are some more munged Damian examples:
if (&name.rw) { ... }
$age++ unless $age.constant;
$elements = return reduce $^ * $^, *@table.dim;
last if ${self}.Initialized;
print "$arrayref.Purpose() is not $default.Meaning()\n";
print %{$self.X}; # print hash referred to by X attribute of $self
print %{$self}.X; # print X attribute of hash referred to by $self
print %$self.X; # print X attribute of hash referred to by $self
As with the dotless form, if there is no actual method corresponding to
the property, Perl pretends there's a rudimentary one returning the
actual property.
[Update: Actually, they are real method calls now, and properties and
traits are really mixins. See A12.]
Since these methods return the properties (except when overridden by
dotless syntax), you can temporize a property just as you can any
method, provided the method itself allows writing:
temp $self.X = 0;
Note that
temp $self is X = 0;
would assign to 0 to $self instead. (Whether it actually makes sense to
set the compile-time X property at run time on the $self variable is
anybody's guess.)
Note that by virtue of their syntax, properties cannot be set by
interpolation into a string. So, happily:
print "My $variable is foobar\n";
does not attempt to set the "foobar" property on $variable.
The ""is"" keyword binds with the same precedence as ".", even when
it's not actually there.
[Update: Actually, "but" now has the precedence of other non-chaining
operators, such as "..". Since "is" occurs only in declarations, it is
not subject to operator precedence at all.]
Note that when you say "$foo.bar", you get $foo's compile-time property
if there is one (which is known at compile time, duh). Otherwise it's
an ordinary method call on the value (which looks for a run-time
property only if a method can't be found, so it shouldn't impact
ordinary method call overhead.)
[Update: No, that's always an ordinary method call now. Use
"variable($foo).bar" to get at a property of the variable, or to call
any other method on the variable object rather than the referent of
$foo.]
To get to the properties directly without going through the method
interface, use the special "btw" method, which returns a hash ref to
the properties hash.
$foo.btw{constant}
Note that synthetic properties won't show up there!
[Update: there is no such hash anymore, and synthetic properties are
simply synthetic methods.]
None of the property names in this Apocalypse should be taken as final.
We will decide on actual property names as we proceed through the
series.
Well, that's it for Apocalypse 2. Doubtless there are some things I
should have decided here that I didn't yet, but at least we're making
progress. Well, at least we're moving in some direction or other. Now
it's time for us to dance the Apocalypso, in honor of Jon Orwant and
his new wife.
perl v5.14.0 2006-02-28 Perl6::Bible::A02(3)
