If I have something like this in my grammar:
grammar G {
token tab-indent(Int $level) {
# Using just ** $level would require <!before \t> to have the same effect, so use a code block for simplicity.
\t+ <?{ $/.chars == $level }>
}
}
is there some way to directly get the value of $level in the corresponding action method tab-indent($/)?
Right now I redo $/.chars there too, which works, but doesn't seem ideal, especially in more complex situations, where the value of the parameter can be less easy to deduce from the matching text.
Does anyone know of a better way to do this? Thanks in advance!
You can use a dynamic variable to pass information to the methods of an action class.
grammar G {
token TOP {
<tab-indent(2)> $<rest> = .*
}
token tab-indent(Int $level) {
:my $*level = $level; # has to be in this scope, not in a block
\t ** {$level}
}
}
class A {
method tab-indent($/) {
say '$*level = ', $*level;
}
}
say G.parse( actions => A.new, "\t\t\t" );
$*level = 2
｢ ｣
tab-indent => ｢ ｣
rest => ｢ ｣
Related
The example for sym shows * (WhateverCode) standing in for a single symbol
grammar Foo {
token TOP { <letter>+ }
proto token letter {*}
token letter:sym<P> { <sym> }
token letter:sym<e> { <sym> }
token letter:sym<r> { <sym> }
token letter:sym<l> { <sym> }
token letter:sym<*> { . }
}.parse("I ♥ Perl", actions => class {
method TOP($/) { make $<letter>.grep(*.<sym>).join }
}).made.say; # OUTPUT: «Perl»
It will, however, fail if we use it to stand in for a symbol composed of several letters:
grammar Foo {
token TOP { <action>+ % " " }
proto token action {*}
token action:sym<come> { <sym> }
token action:sym<bebe> { <sym> }
token action:sym<*> { . }
}.parse("come bebe ama").say; # Nil
Since sym, by itself, does work with symbols with more than one character, how can we define a default sym token that matches a set of characters?
Can * be used in sym tokens for more than one character? ... The example for sym shows * (WhateverCode) standing in for a single symbol
It's not a WhateverCode or Whatever.1
The <...> in foo:sym<...> is a quote words constructor, so the ... is just a literal string.
That's why this works:
grammar g { proto token foo {*}; token foo:sym<*> { <sym> } }
say g.parse: '*', rule => 'foo'; # matches
As far as P6 is concerned, the * in foo:sym<*> is just a random string. It could be abracadabra. I presume the writer chose * to represent the mental concept of "whatever" because it happens to match the P6 concept Whatever. Perhaps they were being too cute.
For the rest of this answer I will write JJ instead of * wherever the latter is just an arbitrary string as far as P6 is concerned.
The * in the proto is a Whatever. But that's completely unrelated to your question:
grammar g { proto token foo {*}; token foo:sym<JJ> { '*' } }
say g.parse: '*', rule => 'foo'; # matches
In the body of a rule (tokens and regexes are rules) whose name includes a :sym<...> part, you can write <sym> and it will match the string between the angles of the :sym<...>:
grammar g { proto token foo {*}; token foo:sym<JJ> { <sym> } }
say g.parse: 'JJ', rule => 'foo'; # matches
But you can write anything you like in the rule/token/regex body. A . matches a single character:
grammar g { proto token foo {*}; token foo:sym<JJ> { . } }
say g.parse: '*', rule => 'foo'; # matches
It will, however, fail if we use it to stand in for a symbol composed of several letters
No. That's because you changed the grammar.
If you change the grammar back to the original coding (apart from the longer letter:sym<...>s) it works fine:
grammar Foo {
token TOP { <letter>+ }
proto token letter {*}
token letter:sym<come> { <sym> }
token letter:sym<bebe> { <sym> }
token letter:sym<JJ> { . }
}.parse(
"come bebe ama",
actions => class { method TOP($/) { make $<letter>.grep(*.<sym>).join } })
.made.say; # OUTPUT: «comebebe»
Note that in the original, the letter:sym<JJ> token is waiting in the wings to match any single character -- and that includes a single space, so it matches those and they're dealt with.
But in your modification you added a required space between tokens in the TOP token. That had two effects:
It matched the space after "come" and after "bebe";
After the "a" was matched by letter:sym<JJ>, the lack of a space between the "a" and "m" meant the overall match failed at that point.
sym, by itself, does work with symbols with more than one character
Yes. All token foo:sym<bar> { ... } does is add:
A multiple dispatch alternative to foo;
A token sym, lexically scoped to the body of the foo token, that matches 'bar'.
how can we define a default sym token that matches a set of characters?
You can write such a sym token but, to be clear, because you don't want it to match a fixed string it can't use the <sym> in the body.(Because a <sym> has to be a fixed string.) If you still want to capture under the key sym then you could write $<sym>= in the token body as Håkon showed in a comment under their answer. But it could also be letter:whatever with $<sym>= in the body.
I'm going to write it as a letter:default token to emphasize that it being :sym<something> doesn't make any difference. (As explained above, the :sym<something> is just about being an alternative, along with other :baz<...>s and :bar<...>s, with the only addition being that if it's :sym<something>, then it also makes a <sym> subrule available in the body of the associated rule, which, if used, matches the fixed string 'something'.)
The winning dispatch among all the rule foo:bar:baz:qux<...> alternatives is chosen according to LTM logic among the rules starting with foo. So you need to write such a token that does not win as a longest token prefix but only matches if nothing else matches.
To immediately go to the back of the pack in an LTM race, insert a {} at the start of the rule body2:
token letter:default { {} \w+ }
Now, from the back of the pack, if this rule gets a chance it'll match with the \w+ pattern, which will stop the token when it hits a non-word character.
The bit about making it match if nothing else matches may mean listing it last. So:
grammar Foo {
token TOP { <letter>+ % ' ' }
proto token letter {*}
token letter:sym<come> { <sym> } # matches come
token letter:sym<bebe> { <sym> } # matches bebe
token letter:boo { {} \w**6 } # match 6 char string except eg comedy
token letter:default { {} \w+ } # matches any other word
}.parse(
"come bebe amap",
actions => class { method TOP($/) { make $<letter>.grep(*.<sym>).join } })
.made.say; # OUTPUT: «comebebe»
that just can't be the thing causing it ... "come bebe ama" shouldn't work in your grammar
The code had errors which I've now fixed and apologize for. If you run it you'll find it works as advertised.
But your comment prodded me to expand my answer. Hopefully it now properly answers your question.
Footnote
1 Not that any of this has anything to do with what's actually going on but... In P6 a * in "term position" (in English, where a noun belongs, in general programming lingo, where a value belongs) is a Whatever, not a WhateverCode. Even when * is written with an operator, eg. +* or * + *, rather than on its own, the *s are still just Whatevers, but the compiler automatically turns most such combinations of one or more *s with one or more operators into a sub-class of Code called a WhateverCode. (Exceptions are listed in a table here.)
2 See footnote 2 in my answer to SO "perl6 grammar , not sure about some syntax in an example".
The :sym<...> contents are for the reader of your program, not for the compiler, and are used to distinguish multi tokens of otherwise identical names.
It just so happened that programmers started to write grammars like this:
token operator:sym<+> { '+' }
token operator:sym<-> { '-' }
token operator:sym</> { '/' }
To avoid duplicating the symbols (here +, -, /), a special rule <sym> was introduced that matches whatever is inside :sym<...> as a literal, so you can write the above tokens as
token operator:sym<+> { <sym> }
token operator:sym<-> { <sym> }
token operator:sym</> { <sym> }
If you don't use <sym> inside the regex, you are free to write anything you want inside :sym<...>, so you can write something like
token operator:sym<fallback> { . }
Maybe like this:
grammar Foo {
token TOP { <action>+ % " " }
proto token action {*}
token action:sym<come> { <sym> }
token action:sym<bebe> { <sym> }
token action:sym<default> { \w+ }
}.parse("come bebe ama").say;
Output:
｢come bebe ama｣
action => ｢come｣
sym => ｢come｣
action => ｢bebe｣
sym => ｢bebe｣
action => ｢ama｣
When a BUILD phaser is called, it overrides default attribute assignment in Perl6. Suppose we have to use that BUILD phaser, like we do in this module (that's where I met this problem). What's the way of assigning values to attributes in that phase?
I have used this
class my-class {
has $.dash-attribute;
submethod BUILD(*%args) {
for %args.kv -> $k, $value {
self."$k"( $value );
}
}
};
my $my-instance = my-class.new( dash-attribute => 'This is the attribute' );
And I get this error
Too many positionals passed; expected 1 argument but got 2
Other combinations of $!or $., direct assignment, declaring the attribute as rw (same error) yield different kind of errors. This is probably just a syntax issue, but I couldn't find the solution. Any help will be appreciated.
There are two things wrong in your example, the way I see it. First of all, if you want an attribute to be writeable, you will need to mark it is rw. Secondly, changing the value of an attribute is done by assignment, rather than by giving the new value as an argument.
So I think the code should be:
class my-class {
has $.dash-attribute is rw;
submethod BUILD(*%args) {
for %args.kv -> $k, $value {
self."$k"() = $value;
}
}
};
my $my-instance = my-class.new( dash-attribute => 'attribute value' );
dd $my-instance;
# my-class $my-instance = my-class.new(dash-attribute => "attribute value")
You could do it the same way the object system normally does it under the hood for you.
(not recommended)
class C {
has $.d;
submethod BUILD ( *%args ){
for self.^attributes {
my $short-name = .name.substr(2); # remove leading ｢$!｣
next unless %args{$short-name}:exists;
.set_value( self, %args{$short-name} )
}
}
}
say C.new(d => 42)
C.new(d => 42)
Not sure whether grammars are meant to do such things: I want tokens to be defined in runtime (in future — with data from a file). So I wrote a simple test code, and as expected it wouldn't even compile.
grammar Verb {
token TOP {
<root>
<ending>
}
token root {
(\w+) <?{ ~$0 (elem) #root }>
}
token ending {
(\w+) <?{ ~$0 (elem) #ending }>
}
}
my #root = <go jump play>;
my #ending = <ing es s ed>;
my $string = "going";
my $match = Verb.parse($string);
.Str.say for $match<root>;
What's the best way of doing such things in Perl 6?
To match any of the elements of an array, just write the name of the array variable (starting with a # sigil) in the regex:
my #root = <go jump play>;
say "jumping" ~~ / #root /; # Matches ｢jump｣
say "jumping" ~~ / #root 'ing' /; # Matches ｢jumping｣
So in your use-case, the only tricky part is passing the arrays from the code that creates them (e.g. by parsing data files), to the grammar tokens that need them.
The easiest way would probably be to make them dynamic variables (signified by the * twigil):
grammar Verb {
token TOP {
<root>
<ending>
}
token root {
#*root
}
token ending {
#*ending
}
}
my #*root = <go jump play>;
my #*ending = <ing es s ed>;
my $string = "going";
my $match = Verb.parse($string);
say $match<root>.Str;
Another way would be to pass a Capture with the arrays to the args adverb of method .parse, which will pass them on to token TOP, from where you can in turn pass them on to the sub-rules using the <foo(...)> or <foo: ...> syntax:
grammar Verb {
token TOP (#known-roots, #known-endings) {
<root: #known-roots>
<ending: #known-endings>
}
token root (#known) {
#known
}
token ending (#known) {
#known
}
}
my #root = <go jump play>;
my #ending = <ing es s ed>;
my $string = "going";
my $match = Verb.parse($string, args => \(#root, #ending));
say $match<root>.Str; # go
The approach you were taking could have worked but you made three mistakes.
Scoping
Lexical variable declarations need to appear textually before the compiler encounters their use:
my $foo = 42; say $foo; # works
say $bar; my $bar = 42; # compile time error
Backtracking
say .parse: 'going' for
grammar using-token {token TOP { \w+ ing}}, # Nil
grammar using-regex-with-ratchet {regex TOP {:ratchet \w+ ing}}, # Nil
grammar using-regex {regex TOP { \w+ ing}}; # ｢going｣
The regex declarator has exactly the same effect as the token declarator except that it defaults to doing backtracking.
Your first use of \w+ in the root token matches the entire input 'going', which then fails to match any element of #root. And then, because there's no backtracking, the overall parse immediately fails.
(Don't take this to mean that you should default to using regex. Relying on backtracking can massively slow down parsing and there's typically no need for it.)
Debugging
See https://stackoverflow.com/a/19640657/1077672
This works:
my #root = <go jump play>;
my #ending = <ing es s ed>;
grammar Verb {
token TOP {
<root>
<ending>
}
regex root {
(\w+) <?{ ~$0 (elem) #root }>
}
token ending {
(\w+) <?{ ~$0 (elem) #ending }>
}
}
my $string = "going";
my $match = Verb.parse($string);
.Str.say for $match<root>;
outputs:
go
In Edit distance: Ignore start/end, I offered a Perl 6 solution to a fuzzy fuzzy matching problem. I had a grammar like this (although maybe I've improved it after Edit #3):
grammar NString {
regex n-chars { [<.ignore>* \w]**4 }
regex ignore { \s }
}
The literal 4 itself was the length of the target string in the example. But the next problem might be some other length. So how can I tell the grammar how long I want that match to be?
Although the docs don't show an example or using the $args parameter, I found one in S05-grammar/example.t in roast.
Specify the arguments in :args and give the regex an appropriate signature. Inside the regex, access the arguments in a code block:
grammar NString {
regex n-chars ($length) { [<.ignore>* \w]**{ $length } }
regex ignore { \s }
}
class NString::Actions {
method n-chars ($/) {
put "Found $/";
}
}
my $string = 'The quick, brown butterfly';
loop {
state $from = 0;
my $match = NString.subparse(
$string,
:rule('n-chars'),
:actions(NString::Actions),
:c($from++),
:args( \(5) )
);
last unless ?$match;
}
I'm still not sure about the rules for passing the arguments though. This doesn't work:
:args( 5 )
I get:
Too few positionals passed; expected 2 arguments but got 1
This works:
:args( 5, )
But that's enough thinking about this for one night.
I have a string which looks like that:
width=13
height=15
name=Mirek
I want to turn it into hash (using Perl 6). Now I do it like that:
my $text = "width=13\nheight=15\nname=Mirek";
my #lines = split("\n", $text);
my %params;
for #lines {
(my $k, my $v) = split('=', $_);
%params{$k} = $v;
}
say %params.perl;
But I feel there should exist more concise, more idiomatic way to do that. Is there any?
In Perl, there's generally more than one way to do it, and as your problem involves parsing, one solution is, of course, regexes:
my $text = "width=13\nheight=15\nname=Mirek";
$text ~~ / [(\w+) \= (\N+)]+ %% \n+ /;
my %params = $0>>.Str Z=> $1>>.Str;
Another useful tool for data extraction is comb(), which yields the following one-liner:
my %params = $text.comb(/\w+\=\N+/)>>.split("=").flat;
You can also write your original approach that way:
my %params = $text.split("\n")>>.split("=").flat;
or even simpler:
my %params = $text.lines>>.split("=").flat;
In fact, I'd probably go with that one as long as your data format does not become any more complex.
If you have more complex data format, you can use grammar.
grammar Conf {
rule TOP { ^ <pair> + $ }
rule pair {<key> '=' <value>}
token key { \w+ }
token value { \N+ }
token ws { \s* }
}
class ConfAct {
method TOP ($/) { make (%).push: $/.<pair>».made}
method pair ($/) { make $/.<key>.made => $/.<value>.made }
method key ($/) { make $/.lc }
method value ($/) { make $/.trim }
}
my $text = " width=13\n\theight = 15 \n\n nAme=Mirek";
dd Conf.parse($text, actions => ConfAct.new).made;