Author: larry
Date: Sat Jan 5 17:24:55 2008
New Revision: 14481
Modified:
doc/trunk/design/syn/S02.pod
Log:
Removal of false indication of correspondence between fatarrow and adverbial
special forms,
and much clarification of special adverbial forms, requested by Trey Harris++
Modified: doc/trunk/design/syn/S02.pod
==============================================================================
--- doc/trunk/design/syn/S02.pod (original)
+++ doc/trunk/design/syn/S02.pod Sat Jan 5 17:24:55 2008
@@ -12,9 +12,9 @@
Maintainer: Larry Wall <[EMAIL PROTECTED]>
Date: 10 Aug 2004
- Last Modified: 1 Jan 2008
+ Last Modified: 5 Jan 2008
Number: 2
- Version: 123
+ Version: 124
This document summarizes Apocalypse 2, which covers small-scale
lexical items and typological issues. (These Synopses also contain
@@ -2140,46 +2140,87 @@
a => $$a :$$a
a => @$$a :@$$a (etc.)
a => %foo<a> %foo<a>:p
- '' => $x :($x)
- '' => <x> :<x>
- '' => ($x,$y) :($x,$y)
- '' => [$x,$y] :[$x,$y]
- '' => {$x => $y} :{$x => $y}
The fatarrow construct may be used only where a term is expected
because it's considered an expression in its own right, since the
-fatarrow is parsed as a normal infix operator (even when autoquoting an
-identifier on its left). The adverbial forms are considered special
-tokens and are recognized in various positions in addition to term
-position. In particular, when used where an infix would be expected
-they modify the previous operator, ignoring the intervening term or
-parenthesized argument. The form is also used to rename parameter
-declarations and to modify the meaning of various quoting forms.
-When appended to an identifier, the adverbial syntax is used to
-generate variants of that identifier; this syntax is used for
-naming operators such as C<< infix:<+> >> and multiply dispatched
-grammatical rules such as statement_control:if. When so used the
-adverb is considered part of the name, so C<< infix:<+> >> and C<<
-infix:<-> >> are two different operators. Likewise C<< prefix:<+>
->> is different from C<< infix:<+> >>.
-
-Either fatarrow or adverbial pair notation may be used to pass named arguments
as
-terms to a function or method. After a call with parenthesized arguments,
-only adverbial syntax may be used to pass additional arguments. This is
typically
-used to pass an extra block:
+fatarrow itself is parsed as a normal infix operator (even when
+autoquoting an identifier on its left). Because the left side is a
+general expression, the fatarrow form may be used to create a Pair
+with I<any> value as the key. On the other hand, when used as above
+to generate C<Pair> objects, the adverbial forms are restricted to
+the use of identifiers as keys. You must use the fatarrow form to
+generate a C<Pair> where the key is not an identifier.
+
+Despite that restriction, it's possible for other things to
+come between a colon and its brackets; however, all of the possible
+non-identifier adverbial keys are reserved for special syntactical
+forms. Perl 6 currently recognizes decimal numbers and the null key.
+In the following table the first and second columns do I<not> mean
+the same thing:
+
+ Simple pair DIFFERS from which means
+ =========== ============ ===========
+ 2 => <101010> :2<101010> radix literal 0b101010
+ 8 => <123> :8<123> radix literal 0o123
+ 16 => <123> :16<deadbeef> radix literal 0xdeadbeef
+ 16 => <123> :16($somevalue) radix conversion function
+ '' => $x :($x) arglist or signature literal
+ '' => ($x,$y) :($x,$y) arglist or signature literal
+ '' => <x> :<x> identifier extension
+ '' => «x» :«x» identifier extension
+ '' => [$x,$y] :[$x,$y] identifier extension
+ '' => { .say } :{ .say } adverbial block
+
+All of the adverbial forms (including the normal ones with
+identifier keys) are considered special tokens and are recognized
+in various positions in addition to term position. In particular,
+when used where an infix would be expected they modify the previous
+operator, ignoring the intervening term or parenthesized argument:
+
+ 1 .. 100 :by(3) # count to 100 by threes
+
+Within declarations the form is used to rename parameter declarations:
+
+ sub foo ( :externalname($myname) ) {...}
+
+Adverbs modify the meaning of various quoting forms:
+
+ q:x 'cat /etc/passwd'
+
+When appended to an identifier (that is, in postfix position),
+the adverbial syntax is used to generate unique variants of that
+identifier; this syntax is used for naming operators such as C<<
+infix:<+> >> and multiply-dispatched grammatical rules such as
+C<statement_control:if>. When so used, the adverb is considered an
+integral part of the name, so C<< infix:<+> >> and C<< infix:<-> >>
+are two different operators. Likewise C<< prefix:<+> >> is different
+from C<< infix:<+> >>. (The notation also has the benefit of grouping
+distinct identifiers into easily accessible sets; this is how the
+standard Perl 6 grammar knows the current set of infix operators,
+for instance.)
+
+Either fatarrow or adverbial pair notation may be used to pass
+named arguments as terms to a function or method. After a call with
+parenthesized arguments, only the adverbial syntax may be used to pass
+additional arguments. This is typically used to pass an extra block:
find($directory) :{ when not /^\./ }
-This actually falls out from the preceding rules because the adverbial block
is in
-operator position, so it modifies the "find operator".
-
-Note that as usual the C<{...}> form can indicate either a closure or a hash
-depending on the contents. It does I<not> indicate a subscript despite being
-parsed as one.
-
-Note also that the C<< <a b> >> form is not a subscript and is therefore
-equivalent not to C<.{'a','b'}> but rather to C<('a','b')>. Bare C<< <a> >>
-turns into C<('a')> rather than C<('a',)>.
+This just naturally falls out from the preceding rules because the
+adverbial block is in operator position, so it modifies the "find
+operator". (Parens aren't considered an operator.)
+
+Note that (as usual) the C<{...}> form (either identifier-based
+or special) can indicate either a closure or a hash depending on
+the contents. It does I<not> always indicate a subscript despite
+being parsed as one. (The function to which it is passed can I<use>
+the value as a subscript if it chooses, however.)
+
+Note also that the C<< <a b> >> form is not a subscript and is
+therefore equivalent not to C<.{'a','b'}> but rather to C<('a','b')>.
+Bare C<< <a> >> turns into C<('a')> rather than C<('a',)>. (However,
+as with the other bracketed forms, the value may end up being used
+as a subscript depending on context.)
Two or more adverbs can always be strung together without intervening
punctuation anywhere a single adverb is acceptable. When used as
@@ -2196,7 +2237,8 @@
The negated form (C<:!a>) and the sigiled forms (C<:$a>, C<:@a>,
C<:%a>) never take an argument and don't care what the next character is.
-They are considered complete.
+They are considered complete. These forms require an identifier to
+serve as the key.
The other forms of adverb (including the bare C<:a> form) I<always>
look for an immediate bracketed argument, and will slurp it up.
@@ -2214,10 +2256,11 @@
the brackets may be separated from their initial C<:foo> with either
unspace or dot (or both), but nothing else.
-Regardless of syntax, adverbs used as named arguments generally show
-up as optional named parameters to the function in question--even
-if the function is an operator or macro. The function in question
-neither knows nor cares how weird the original syntax was.
+Regardless of syntax, adverbs used as named arguments (in either term
+or infix position) generally show up as optional named parameters to
+the function in question--even if the function is an operator or macro.
+The function in question neither knows nor cares how weird the original
+syntax was.
=item *
