Re: ambiguity of grammar for font shorthand?
Hi Tony, Tony Graham wrote: On Mon, Sep 21 2009 23:30:17 +0100, jonathan.levin...@intersystems.com wrote: ... If inherit is allowed to be a value then the grammar truly becomes ambiguous since each of these can have the value inherit and we don?t know which ones are omitted and must take the value normal. 'inherit' doesn't mix with other values [1]. AFAIK, this is true even for shorthands taken from CSS2. Well the point you’re referring to says that ‘inherit’ can’t be mixed with other operations in an expression. Technically speaking the shorthand is not an expression. And, anyway, the point also says that the ‘from-parent()’ function can be used instead, which leads to the same issue. That said, your point made me look at the introduction of section 7.31, “Shorthand Properties”: http://www.w3.org/TR/2006/REC-xsl11-20061205/#d0e33965 which says that “One cannot mix ‘inherit’ with other subproperty values as it would not be possible to specify the subproperty to which ‘inherit’ applied”. While this is not always true as we found out, that avoids the problem... ... Except when the ‘normal’ keyword is used, which applies to all three style/variant/weight properties, and may also lead to ambiguous values. If the value is 'inherit', the individual properties for which the shorthand is a shorthand individually inherit [2]. Regards, Tony Graham tony.gra...@menteithconsulting.com Director W3C XSL FO SG Invited Expert Menteith Consulting Ltd XML Guild member XML, XSL and XSLT consulting, programming and training Registered Office: 13 Kelly's Bay Beach, Skerries, Co. Dublin, Ireland Registered in Ireland - No. 428599 http://www.menteithconsulting.com -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- xmlroff XSL Formatter http://xmlroff.org xslide Emacs mode http://www.menteith.com/wiki/xslide Unicode: A Primer urn:isbn:0-7645-4625-2 [1] http://www.w3.org/TR/xsl11/#d0e5479 [2] http://www.w3.org/TR/xsl11/#shortexpan Vincent
Re: ambiguity of grammar for font shorthand?
On 23/09/2009, at 8:18 PM, Vincent Hennebert wrote: Hi Tony, Tony Graham wrote: On Mon, Sep 21 2009 23:30:17 +0100, jonathan.levin...@intersystems.com wrote: ... If inherit is allowed to be a value then the grammar truly becomes ambiguous since each of these can have the value inherit and we don?t know which ones are omitted and must take the value normal. 'inherit' doesn't mix with other values [1]. AFAIK, this is true even for shorthands taken from CSS2. Well the point you’re referring to says that ‘inherit’ can’t be mixed with other operations in an expression. Technically speaking the shorthand is not an expression. And, anyway, the point also says that the ‘from-parent()’ function can be used instead, which leads to the same issue. That said, your point made me look at the introduction of section 7.31, “Shorthand Properties”: http://www.w3.org/TR/2006/REC-xsl11-20061205/#d0e33965 which says that “One cannot mix ‘inherit’ with other subproperty values as it would not be possible to specify the subproperty to which ‘inherit’ applied”. While this is not always true as we found out, that avoids the problem... When is it not true? ... Except when the ‘normal’ keyword is used, which applies to all three style/variant/weight properties, and may also lead to ambiguous values. Font shorthand implicitly sets _all_ of these values to normal, doesn't it? If the value is 'inherit', the individual properties for which the shorthand is a shorthand individually inherit [2]. Regards, Tony Graham tony.gra...@menteithconsulting.com Director W3C XSL FO SG Invited Expert Menteith Consulting Ltd XML Guild member XML, XSL and XSLT consulting, programming and training Registered Office: 13 Kelly's Bay Beach, Skerries, Co. Dublin, Ireland Registered in Ireland - No. 428599 http:// www.menteithconsulting.com -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- xmlroff XSL Formatter http:// xmlroff.org xslide Emacs mode http://www.menteith.com/wiki/ xslide Unicode: A Primer urn:isbn: 0-7645-4625-2 [1] http://www.w3.org/TR/xsl11/#d0e5479 [2] http://www.w3.org/TR/xsl11/#shortexpan Vincent
Re: ambiguity of grammar for font shorthand?
Hi, Also, in your message you said we could ignore a value for font of caption, icon, etc., as the standard tells us to do, but the standard discusses these values and their relation to system fonts. Was this an oversight on your part or am I mis-reading the spec? [1] [1] http://www.w3.org/TR/2001/REC-xsl-20011015/slice7.html#font The spec says: XSL modifications to the CSS definition: In XSL the font property is a pure shorthand property. System font characteristics, such as font-family, and font-size, may be obtained by the use of the system-font function in the expression language. If I read this correctly the system font shorthands namely: caption, icon, menu, message-box, small-caption, status-bar are not allowed in XSL. Best Regards Alex signature.asc Description: This is a digitally signed message part
Re: ambiguity of grammar for font shorthand?
Hi, I think it is probably the case that in the context of the font short hand – the font properties cannot take the value of inherit, since this renders the grammar irreducibly ambiguous. While such an exclusion is not mentioned in the spec, it makes sense that inherit must be excluded for the reason I’ve just given. Once, I've written a CSS Minifier (shrinks CSS files). There I also didn't allow inherit for individual properties inside the font shorthand. So I would give the user a good error message here. I don't think that there are many documents out there, which actually use inherit for individual properties inside the font shorthand. Best Regards Alex signature.asc Description: This is a digitally signed message part
Re: ambiguity of grammar for font shorthand?
Hi Jonathan, Jonathan Levinson wrote: Hi Vincent, As I read the grammar for the font shorthand it is ambiguous, though not fatally so as long as one excludes the value of inherit from individual properties in the font short hand. For instance the first optional argument is font-style, font-weight, and font-variant, each of which is optional and can occur in any order. All can have the value normal. So if the value for the font shorthand is normal 10pt Arial we do not know which of these three is being set to normal even though it is harmless and the omitted values will be set to normal since that is their initial value. Actually not: the default value is inherited. If somewhere up in the hierarchy the font-weight was set to bold, then we don’t know if that ‘normal’ in the font property means that font-weight must be reset to normal or if it applies to another property. This example you’re mentioning is truly ambiguous. If inherit is allowed to be a value then the grammar truly becomes ambiguous since each of these can have the value inherit and we don't know which ones are omitted and must take the value normal. I think it is probably the case that in the context of the font short hand - the font properties cannot take the value of inherit, since this renders the grammar irreducibly ambiguous. While such an exclusion is not mentioned in the spec, it makes sense that inherit must be excluded for the reason I've just given. Excluding inherit for good is a bit too restrictive IMO. I think we should try to resolve all non-ambiguous cases, like: normal normal bold inherit bold italic inherit inherit inherit inherit etc. Some truly ambiguous values: normal normal (which one is inherited?) normal bold inherit (which one is normal, which one inherited?) normal (which one is normal, which one inherited?) etc. A good “exercise” would be to identify all cases that are ambiguous. In which case an error would be thrown with a “the value is ambiguous”-like message. Prima facie, the grammar (eliminating inherit) looks LL(1) since parsing from left to right one can always tell what property one is parsing except for the case when one of the first three is assigned normal and there are no further values unique to the properties of the first three. In this case, one has a special rule (outside the grammar) to arbitrarily pick one of the optional properties in the first optional argument as the bearer of normal, while the rest receive their initial values of normal. Actually, a “simple” regular expression might be enough. The java.util.regex package can do wonder. See attached Java file: there will always be 6 matching groups, some of them possibly being null. The first three are for style/variant/weight, then font-size, then line-height, then font families. Some magic would have to be implemented to identify the first 3 groups. Also, the regex for the individual properties would have to be refined: “\\w+” is actually wrong for font-weight. One could imagine to re-use a regex defined for each sub-property. However, an LL parser would probably be superior in error handling. The regular expression would just fail to match, and there’s not much that can be said about why it fails. An LL parser would probably be able to tell, say, that the error lies in the declaration of the font-size property. I think a good error handling is important, especially to beginners. I’ve found myself ranting against such meaningless error messages that don’t tell you at all what your error could be. There is a special case where the value of font is inherit and that works fine. Since we are testing if the single token is inherit, we can handle that special case in a recursive descent parser. We create a tokenizer which breaks on space and see if the one token returned is inherit. Also, in your message you said we could ignore a value for font of caption, icon, etc., as the standard tells us to do, but the standard discusses these values and their relation to system fonts. Was this an oversight on your part or am I mis-reading the spec? [1] See Alexander’s answer about that. I'm not sure we have to go to the complexity of parsing the font short hand in a recursive descent manner. I've updated the open issue (47709) to give my reasons why and a solution to the problem of more than two fonts separated by commas. The overly complex code I analyzed looks to me like a tokenizer not a parser, and while it could be better written (and more understandable) it seems to be doing an adequate job of tokenizing, unless I'm still missing something. There are still cases that it doesn’t handle: spaces around the slash, different order in style/variant/weight. Vincent /* * Licensed to the Apache Software Foundation (ASF) under one or more * contributor license agreements. See the NOTICE file distributed with * this work for additional information regarding
RE: ambiguity of grammar for font shorthand?
Hi Vincent, You make excellent points, however for font-style, font-variant and font-weight the initial value (the default value) is normal, not inherit. http://www.w3.org/TR/2001/REC-xsl-20011015/slice7.html#font-style http://www.w3.org/TR/2001/REC-xsl-20011015/slice7.html#font-variant http://www.w3.org/TR/2001/REC-xsl-20011015/slice7.html#font-weight This is a minor detail, but important if our discussion is used as the basis for building a recursive descent parser. Best Regards, Jonathan S. Levinson Senior Software Developer Object Group InterSystems 617-621-0600 -Original Message- From: Vincent Hennebert [mailto:vhenneb...@gmail.com] Sent: Tuesday, September 22, 2009 7:20 AM To: fop-dev@xmlgraphics.apache.org Subject: Re: ambiguity of grammar for font shorthand? Hi Jonathan, Jonathan Levinson wrote: Hi Vincent, As I read the grammar for the font shorthand it is ambiguous, though not fatally so as long as one excludes the value of inherit from individual properties in the font short hand. For instance the first optional argument is font-style, font-weight, and font-variant, each of which is optional and can occur in any order. All can have the value normal. So if the value for the font shorthand is normal 10pt Arial we do not know which of these three is being set to normal even though it is harmless and the omitted values will be set to normal since that is their initial value. Actually not: the default value is inherited. If somewhere up in the hierarchy the font-weight was set to bold, then we don’t know if that ‘normal’ in the font property means that font-weight must be reset to normal or if it applies to another property. This example you’re mentioning is truly ambiguous. If inherit is allowed to be a value then the grammar truly becomes ambiguous since each of these can have the value inherit and we don't know which ones are omitted and must take the value normal. I think it is probably the case that in the context of the font short hand - the font properties cannot take the value of inherit, since this renders the grammar irreducibly ambiguous. While such an exclusion is not mentioned in the spec, it makes sense that inherit must be excluded for the reason I've just given. Excluding inherit for good is a bit too restrictive IMO. I think we should try to resolve all non-ambiguous cases, like: normal normal bold inherit bold italic inherit inherit inherit inherit etc. Some truly ambiguous values: normal normal (which one is inherited?) normal bold inherit (which one is normal, which one inherited?) normal (which one is normal, which one inherited?) etc. A good “exercise” would be to identify all cases that are ambiguous. In which case an error would be thrown with a “the value is ambiguous”-like message. Prima facie, the grammar (eliminating inherit) looks LL(1) since parsing from left to right one can always tell what property one is parsing except for the case when one of the first three is assigned normal and there are no further values unique to the properties of the first three. In this case, one has a special rule (outside the grammar) to arbitrarily pick one of the optional properties in the first optional argument as the bearer of normal, while the rest receive their initial values of normal. Actually, a “simple” regular expression might be enough. The java.util.regex package can do wonder. See attached Java file: there will always be 6 matching groups, some of them possibly being null. The first three are for style/variant/weight, then font-size, then line-height, then font families. Some magic would have to be implemented to identify the first 3 groups. Also, the regex for the individual properties would have to be refined: “\\w+” is actually wrong for font-weight. One could imagine to re-use a regex defined for each sub-property. However, an LL parser would probably be superior in error handling. The regular expression would just fail to match, and there’s not much that can be said about why it fails. An LL parser would probably be able to tell, say, that the error lies in the declaration of the font-size property. I think a good error handling is important, especially to beginners. I’ve found myself ranting against such meaningless error messages that don’t tell you at all what your error could be. There is a special case where the value of font is inherit and that works fine. Since we are testing if the single token is inherit, we can handle that special case in a recursive descent parser. We create a tokenizer which breaks on space and see if the one token returned is inherit. Also, in your message you said we could ignore a value for font of caption, icon, etc., as the standard tells us to do, but the standard discusses these values and their relation to system fonts. Was this an oversight on your part or am I mis-reading the spec? [1
Re: ambiguity of grammar for font shorthand?
On 23/09/2009, at 12:13 AM, Jonathan Levinson wrote: Hi Vincent, You make excellent points, however for font-style, font-variant and font-weight the initial value (the default value) is normal, not inherit. http://www.w3.org/TR/2001/REC-xsl-20011015/slice7.html#font-style http://www.w3.org/TR/2001/REC-xsl-20011015/slice7.html#font-variant http://www.w3.org/TR/2001/REC-xsl-20011015/slice7.html#font-weight This is a minor detail, but important if our discussion is used as the basis for building a recursive descent parser. An important detail. When the font shorthand is encountered, all font properties (including those that cannot be defined in the shorthand) are set to their initial values.
Re: ambiguity of grammar for font shorthand?
Hi Jonathan, Jonathan Levinson wrote: Hi Vincent, You make excellent points, however for font-style, font-variant and font-weight the initial value (the default value) is normal, not inherit. http://www.w3.org/TR/2001/REC-xsl-20011015/slice7.html#font-style http://www.w3.org/TR/2001/REC-xsl-20011015/slice7.html#font-variant http://www.w3.org/TR/2001/REC-xsl-20011015/slice7.html#font-weight Sure but the properties are inheritable. The initial value is used for the root element and propagated down the FO tree. See http://www.w3.org/TR/2006/REC-xsl11-20061205/#speccomact This is a minor detail, but important if our discussion is used as the basis for building a recursive descent parser. Moreover we can easily get things wrong with the XSL-FO Recommendation, so double-checking is beneficial and welcome. snip/ Thanks, Vincent
Re: ambiguity of grammar for font shorthand?
On Mon, Sep 21 2009 23:30:17 +0100, jonathan.levin...@intersystems.com wrote: ... If inherit is allowed to be a value then the grammar truly becomes ambiguous since each of these can have the value inherit and we don?t know which ones are omitted and must take the value normal. 'inherit' doesn't mix with other values [1]. AFAIK, this is true even for shorthands taken from CSS2. If the value is 'inherit', the individual properties for which the shorthand is a shorthand individually inherit [2]. Regards, Tony Graham tony.gra...@menteithconsulting.com Director W3C XSL FO SG Invited Expert Menteith Consulting Ltd XML Guild member XML, XSL and XSLT consulting, programming and training Registered Office: 13 Kelly's Bay Beach, Skerries, Co. Dublin, Ireland Registered in Ireland - No. 428599 http://www.menteithconsulting.com -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- xmlroff XSL Formatter http://xmlroff.org xslide Emacs mode http://www.menteith.com/wiki/xslide Unicode: A Primer urn:isbn:0-7645-4625-2 [1] http://www.w3.org/TR/xsl11/#d0e5479 [2] http://www.w3.org/TR/xsl11/#shortexpan
ambiguity of grammar for font shorthand?
Hi Vincent, As I read the grammar for the font shorthand it is ambiguous, though not fatally so as long as one excludes the value of inherit from individual properties in the font short hand. For instance the first optional argument is font-style, font-weight, and font-variant, each of which is optional and can occur in any order. All can have the value normal. So if the value for the font shorthand is normal 10pt Arial we do not know which of these three is being set to normal even though it is harmless and the omitted values will be set to normal since that is their initial value. If inherit is allowed to be a value then the grammar truly becomes ambiguous since each of these can have the value inherit and we don't know which ones are omitted and must take the value normal. I think it is probably the case that in the context of the font short hand - the font properties cannot take the value of inherit, since this renders the grammar irreducibly ambiguous. While such an exclusion is not mentioned in the spec, it makes sense that inherit must be excluded for the reason I've just given. Prima facie, the grammar (eliminating inherit) looks LL(1) since parsing from left to right one can always tell what property one is parsing except for the case when one of the first three is assigned normal and there are no further values unique to the properties of the first three. In this case, one has a special rule (outside the grammar) to arbitrarily pick one of the optional properties in the first optional argument as the bearer of normal, while the rest receive their initial values of normal. There is a special case where the value of font is inherit and that works fine. Since we are testing if the single token is inherit, we can handle that special case in a recursive descent parser. We create a tokenizer which breaks on space and see if the one token returned is inherit. Also, in your message you said we could ignore a value for font of caption, icon, etc., as the standard tells us to do, but the standard discusses these values and their relation to system fonts. Was this an oversight on your part or am I mis-reading the spec? [1] [1] http://www.w3.org/TR/2001/REC-xsl-20011015/slice7.html#font I'm not sure we have to go to the complexity of parsing the font short hand in a recursive descent manner. I've updated the open issue (47709) to give my reasons why and a solution to the problem of more than two fonts separated by commas. The overly complex code I analyzed looks to me like a tokenizer not a parser, and while it could be better written (and more understandable) it seems to be doing an adequate job of tokenizing, unless I'm still missing something. Best Regards, Jonathan S. Levinson Senior Software Developer Object Group InterSystems 617-621-0600
