Rails spinoffs - Nov 2006 - pattern help for custom template - strings

I''ve got a small function that I''m developing (maybe
you''ll recognize it):

code:

  sprintf: function(replacement) {
    var escapedString = this.replace(/\\/g, ''\\\\'');
    var values = $A(arguments);
    Template.SprintfPattern = /(^|.|\r|\n)(@\(s\)(.*?))/;
    var sprintfTemplate = new Template(escapedString, 
Template.SprintfPattern);
    return sprintfTemplate.evaluate(values);
  }


unit test:

    testStrinSprintf: function() { with(this) {
      assertEqual(''mary had a litte lamb'', ''@s had a
@s
@s''.sprintf([''mary'',''little'',''lamb'']));
    }},


However, it is not returning the expected value. It is only returning 
the original string. 

Any help with this would be greatly appreciated!

Thanks,
Ian


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Hey Ian,
OK, you''re not using Template for what it''s designed to do. 
Template is
a class that plucks properties out of an object, using a custom syntax
to identify property names amidst the pattern text.
It is NOT intended for stuff such as "%s" or "@s", where
"s" would mean
"string" instead of "property s".
You can simulate this, but in a very shallow manner, by using only
numerical indices, without conversion characters, and passing an array
as argument to the evaluate function.  Since the array has a
numerically-named property for each cell, you''re good to go.  But that
won''t take you further.
Besides, \-escaping seems unnecessary for your needs. your detection
pattern is incorrect:
Ian Tyndall a écrit :
> Template.SprintfPattern = /(^|.|\r|\n)(@\(s\)(.*?))/;
First, you don''t *need* to pollute the Template object with a custom
pattern property (which is specific to your needs, not global).  Store
it as a constant in your own object instead (String, here), in order to
avoid recomputing the regex on each call.
Second, a Template detection pattern must isolate *3 groups*:
  1. The character/boundary immediately before the special sequence
  2. The delimiter(s) of the special sequence (defaults to #{...})
  3. The property name inside the special sequence (usually uses
     (.*?), which is a reluctantly-quantified generic grab).
Your regex doesn''t do that at all.  The beginning is alright:
  1. (^|.|\r|\n)
This is virtually constant (there''s never a reason to change it,
AFAIK).
 The second part ic clearly wrong though, as your second group is:
  2. (@\(s\)(.*?))
Which basically says that your special sequences start by "@(s)", and
have no end delimiter.  It also provides the third group:
  3. (.*?)
Which is correct, but misplaced.  To actually get this to work on your
example, you''d have to go like this:
  "@s0 had a @s1 @s2".sprintf(''mary'',
''little'', ''lamb'');
Note that I *do not* bracket the arguments in an array: there are three
arguments here.  Since your method wraps them in an array already, using
$A, that''s enough (and that''s more convenient for the calling
code).  If
you use a manual array bracketing, you''ll end up with a single
argument.
Also note that the "s" part in your detection pattern becomes
irrelevant, since you can''t apply conversion on the go.  You could as
well get rid of it.  Here''s a simpler version:
Object.extend(String.prototype, {
  SPRINTF_PATTERN: /(^|.|\r|\n)(@(\d+))/,
  sprintf: function() {
    var obj = $A(arguments);
    return new Template(this, this.SPRINTF_PATTERN).evaluate(obj);
  }
});
As for the unit test:
  testStrinSprintf: function() { with(this) {
    assertEqual(''mary had a litte lamb'',
      ''@0 had a @1
@2''.sprintf(''mary'',''little'',''lamb''));
  }},
Just remember that you WON''T be able to emulate sprintf behavior
(insofar as formatting and converting is concerned) using Template.  The
mechanism is only intended at property injection in a patterned text.
''HTH
-- 
Christophe Porteneuve aka TDD
tdd-x+CfDp/qHev2eFz/2MeuCQ@public.gmane.org
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Christophe Porteneuve wrote:
>Hey Ian,
>
>OK, you''re not using Template for what it''s designed to
do.  Template is
>a class that plucks properties out of an object, using a custom syntax
>to identify property names amidst the pattern text.
>
>It is NOT intended for stuff such as "%s" or "@s", where
"s" would mean
>"string" instead of "property s".
>
>You can simulate this, but in a very shallow manner, by using only
>numerical indices, without conversion characters, and passing an array
>as argument to the evaluate function.  Since the array has a
>numerically-named property for each cell, you''re good to go.  But
that
>won''t take you further.
>
>Besides, \-escaping seems unnecessary for your needs. your detection
>pattern is incorrect:
>
>Ian Tyndall a écrit :
>  
>
>>Template.SprintfPattern = /(^|.|\r|\n)(@\(s\)(.*?))/;
>>    
>>
>
>First, you don''t *need* to pollute the Template object with a
custom
>pattern property (which is specific to your needs, not global).  Store
>it as a constant in your own object instead (String, here), in order to
>avoid recomputing the regex on each call.
>
>Second, a Template detection pattern must isolate *3 groups*:
>
>  1. The character/boundary immediately before the special sequence
>  2. The delimiter(s) of the special sequence (defaults to #{...})
>  3. The property name inside the special sequence (usually uses
>     (.*?), which is a reluctantly-quantified generic grab).
>
>Your regex doesn''t do that at all.  The beginning is alright:
>
>  1. (^|.|\r|\n)
>
>This is virtually constant (there''s never a reason to change it,
AFAIK).
> The second part ic clearly wrong though, as your second group is:
>
>  2. (@\(s\)(.*?))
>
>Which basically says that your special sequences start by "@(s)",
and
>have no end delimiter.  It also provides the third group:
>
>  3. (.*?)
>
>Which is correct, but misplaced.  To actually get this to work on your
>example, you''d have to go like this:
>
>  "@s0 had a @s1 @s2".sprintf(''mary'',
''little'', ''lamb'');
>
>Note that I *do not* bracket the arguments in an array: there are three
>arguments here.  Since your method wraps them in an array already, using
>$A, that''s enough (and that''s more convenient for the
calling code).  If
>you use a manual array bracketing, you''ll end up with a single
argument.
>
>Also note that the "s" part in your detection pattern becomes
>irrelevant, since you can''t apply conversion on the go.  You could
as
>well get rid of it.  Here''s a simpler version:
>
>Object.extend(String.prototype, {
>  SPRINTF_PATTERN: /(^|.|\r|\n)(@(\d+))/,
>  sprintf: function() {
>    var obj = $A(arguments);
>    return new Template(this, this.SPRINTF_PATTERN).evaluate(obj);
>  }
>});
>
>As for the unit test:
>
>  testStrinSprintf: function() { with(this) {
>    assertEqual(''mary had a litte lamb'',
>      ''@0 had a @1
@2''.sprintf(''mary'',''little'',''lamb''));
>  }},
>
>Just remember that you WON''T be able to emulate sprintf behavior
>(insofar as formatting and converting is concerned) using Template.  The
>mechanism is only intended at property injection in a patterned text.
>
>''HTH
>
>  
>
Thanks for the great help and explanation!

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