Design Docs‎ > ‎Formatting‎ > ‎MessageFormat‎ > ‎MessageFormat 2011q1‎ > ‎

Questions & Decisions

SelectFormat.applyPattern(String pattern)
  • ICU 4.6: Throws an IllegalArgumentException("Duplicate keyword error.") if there are duplicate keywords.
  • Proposal: Don't test for duplicates, just return the sub-message for the first matching keyword.
  • Rationale: Saves time, especially in one-shot use case, and saves creation of a HashSet or HashMap.
  • Decision: Proposal accepted (ICU meeting 2011-feb-16)
  • Implemented: Java
SelectFormat.applyPattern(String pattern)
  • ICU 4.6: Limits keywords to ASCII alphanumeric identifiers: [a-zA-Z][a-zA-Z0-9]*
    • Pro: Simple, stable, might be sufficient.
    • Con: Seems a little limited.
  • Note: The keyword syntax should be the same for SelectFormat & PluralFormat (see below).
  • Note: Argument names are Unicode identifiers (see below). Not sure whether they should be consistent with plural/select keywords.
  • Question: Should we support a wider set of keywords?
  • For example: [^[[:Pattern_Syntax:][:Pattern_White_Space:]]]+
    • Pro: More lenient, still stable, still does not require properties data.
    • Con: Slightly more expensive to test than just ASCII.
  • Alternative: Support all Unicode identifiers [:ID_Start:][:ID_Continue:]*
    • Pro: Common Unicode identifier syntax.
    • Con: These properties are not immutable and therefore cannot be hardcoded as fixed data. And an older ICU version (with older Unicode data) will reject identifiers from a newer Unicode version.
  • Proposal: Use Unicode "pattern identifiers": [^[[:Pattern_Syntax:][:Pattern_White_Space:]]]+
  • Decision: Proposal accepted (ICU meeting 2011-feb-16)
  • Implemented: Java
PluralFormat.applyPattern(String pattern)
  • ICU 4.6: Keywords are delimited by optional [:Pattern_White_Space:]* and then '{', but later rejected if they are not in the set of PluralRules keywords for the locale.
  • Background: The MessagePattern class will be used for parsing. It is locale-independent and cannot verify PluralRules keywords. Testing for whether keywords are in a particular set costs a bit of time.
  • Note: While normally only 6 predefined plural keywords are used, the PluralRules class technically supports Unicode identifier syntax, plus '_' in initial position: [[:ID_Start:][_]][:ID_Continue:].
    • Bug in PluralRules.isValidKeyword(String token): It uses charAt() rather than codePointAt(), so its identifier syntax is limited to BMP characters.
  • Proposal:
    • (a) Make the MessagePattern class test PluralFormat keywords for the same syntax as in SelectFormat, see above (ASCII vs. Pattern vs. Unicode).
      • Proposal: Use Unicode "pattern identifiers": [^[[:Pattern_Syntax:][:Pattern_White_Space:]]]+
    • (b) Do not test whether the keywords are in some PluralRules set. (They just won't match if they aren't.)
  • Decision: Proposal accepted (ICU meeting 2011-feb-16)
  • Implemented: Java
MessageFormat.applyPattern(String pattern)
  • ICU 4.6: Argument names (when not numbers) are Unicode identifiers [:ID_Start:][:ID_Continue:]*
  • Question: Should they be consistent with plural/select keywords?
  • Proposal: Use Unicode "pattern identifiers": [^[[:Pattern_Syntax:][:Pattern_White_Space:]]]+
  • Decision: Proposal accepted (ICU meeting 2011-feb-16)
  • Implemented: Java
MessageFormat.applyPattern(String pattern)
  • ICU 4.6: In Java, argument numbers must not contain whitespace. That is, "{ 0}" and "{1 ,number}" are rejected (by Integer.parseInt()).
  • However, in C++, Pattern_White_Space anywhere in the argument number field is allowed and ignored (by stou()).
  • In both Java and C++, the type and style fields allow leading and trailing whitespace which is ignored (unless in the style field if it's not one of the predefined keywords like "medium"). So "{2,   number   ,   medium   }" is valid.
  • Proposal: Consistently allow leading and trailing Pattern_White_Space only.
  • Decision: Proposal accepted (ICU meeting 2011-feb-16)
  • Implemented: Java
MessageFormat.applyPattern(String pattern)
  • ICU 4.6: Argument numbers can use any [:Nd:] digits, via Integer.parseInt().
  • However, if an argument number string differs from the Integer.toString() of that number (leading zeros, leading plus sign, non-ASCII digits), then format(Object[]) fails to match that argument number string to any input value.
  • format(Map<String, Object>) fails to match an argument if the string is not the same, even if the Integer.parseInt() results of the argument number and the map key would match.
  • JDK MessageFormat documents: "The ArgumentIndex value is a non-negative integer written using the digits '0' through '9'".
  • Proposal: Only allow ASCII digits in argument numbers (i.e., in argument name/number strings that start with a digit).
  • Decision: Proposal accepted in modified form (ICU meeting 2011-feb-16):
    • While parsing an argument name/number, collect a "pattern identifier".
    • Trim leading/trailing Pattern_White_Space.
    • If the identifier contains only ASCII digits, then it is an argument number and must not have leading zeros (except "0" itself).
    • Otherwise it is an argument name.
  • Implemented: Java
MessageFormat.applyPattern(String pattern)
  • ICU 4.6: Forbids mixing named and numbered arguments, although the formatting and other code is written to apply named values to numbered pattern arguments.
  • Proposal: Allow mixing named and numbered arguments, and just return true from usesNamedArguments() in those cases.
  • Decision: Proposal accepted (ICU meeting 2011-feb-16)
  • Implemented: Java
SelectFormat.format(String keyword)
  • ICU 4.6: Throws an IllegalArgumentException("Invalid formatting argument.") if the keyword is not ASCII alphanumeric. Seems unnecessary.
  • Proposal: Remove this test, and just return the "other" sub-message when there is no match.
  • Decision: Proposal rejected (ICU meeting 2011-feb-16). Continue to validate keywords (but now as "pattern identifiers" to match the parser).
  • Implemented: Java
MessageFormat.setFormat(int, Format) and siblings (setFormatByXyz(), getFormats(), ...)
  • ICU 4.6: After applyPattern(), these methods modify the resulting, internal array of Format objects.
    • For example, if format is a short-style SimpleDateFormat, then applyPattern("Level at {3,number,percent}").setFormat(0, format).toPattern() becomes "Level at {3,date,short}".
  • These methods are hard to implement, or force inefficient data structures and code.
    • They only really work with an internal data structure equivalent to the current one.
    • There is no good way to implement MessageFormat.toPattern() with these. (Just take a look at the code...!)
    • An efficient, simple implementation (e.g., based on MessagePattern) does not need to instantiate any ChoiceFormat/PluralFormat/SelectFormat objects for proper message formatting, except artificially to satisfy a getFormat() method.
  • These methods have broken semantics and implementation.
    • It seems really strange to construct a MessageFormat from a pattern and then to override some of the parsed, internal data structure.
    • It is impossible to localize/translate a sub-format that is provided in runtime code, and it is impossible to localize a sub-format (e.g., ChoiceFormat) because its text snippets lack the necessary context. But localization is the main purpose for MessageFormat.
    • The old (JDK 1.1) setFormat() etc. take an index that refers to the parsing order. With a localized string, the arguments are often rearranged, and the parsing order differs. (The ones "ByArgumentIndex" are only @since 1.4 and the ones "ByArgumentName" are ICU-specific and only exist to be parallel with "ByArgumentIndex".)
    • Format objects returned by getFormats() etc. are not cloned and can be modified, changing the behavior of the MessageFormat object that returned them.
    • All of these methods only work on top-level arguments. Arguments in nested MessageFormat objects are not visible.
      • In the current implementation, such nested MessageFormat objects do not even exist except as temporary objects while formatting the result of a choice/plural/select argument.
      • For example, applyPattern("ab {3} {2,select,other{cd{3}ef}} yz").setFormatByArgumentIndex(3, format) only affects the top-level {3}, not the nested {3}.
    • ICU4J MessageFormat.toPattern() reconstitutes the message pattern via RTTI, testing for a small set of com.ibm.icu.text.xyz classes and their standard options. It does not test for equivalent JDK java.text.xyz classes, and it has no chance of doing anything useful with an unknown subclass of java.text.Format.
  • These are hardly used even inside ICU.
    • "Even" where people might understand these APIs...
    • There is one call to setFormat(int, Format) and one class with four calls to setFormatByArgumentIndex(int, Format). (Aside from test code.)
  • The usage example in the API docs is broken.
    • ChoiceFormat does not work for plural formatting. (That's why we added PluralFormat.)
  • There are trivial workarounds for formatting.
    • The canonical example of setting a ChoiceFormat can be expressed much simpler via a MessageFormat pattern string that contains the ChoiceFormat pattern.
    • In general, simply use the sub-formatter object to format to a string, and then pass that string value into MessageFormat.
  • However, workarounds for parsing are not trivial. (Doable, but involve more thought and code.)
  • Proposal: Deprecate all forms of MessageFormat.setFormat() and getFormats(), and change them to just throw new UnsupportedOperationException().
  • Note: If we later discover that someone has an important use case for some of these, then we can un-deprecate and work on some implementation (and concentrate on those methods and use cases that are needed). Rather than spending a lot of time fully supporting questionable API.
  • Alternative: Implement most but not all of setFormat() etc.: For example, getFormat() return null for choice/plural/select arguments, and toPattern() always return the original pattern string.
  • Decision: Support setFormat() etc. but: (ICU meeting 2011-feb-16)
    • toPattern() will throw a java.lang.IllegalStateException when custom formats have been set (it already could not write a usable pattern string in all cases)
    • getFormats() etc. will return null for choice/plural/select arguments that have not been overridden (because the implementation should not have to construct those Format objects)
      • Otherwise we would have to construct these objects in getFormats() etc., and for maximum compatibility we would also have to set them at that time so that when the caller modifies the objects, the modifications are used in subsequent format()/parse() etc. calls.
  • Implemented: Java
MessageFormat implements Serializable
  • ICU 4.6: Supports serialization via standard readObject()/writeObject(), apparently backward-compatible at least a few versions. (Maybe back to when UFormat was inserted into the class hierarchy?)
  • It looks like this might be the hardest part of the "API" to be supported with a very different implementation. The MessageFormat class does not just read and write the message pattern string, it reads and writes whatever non-transient fields it has; trouble is, the old data structure would take a fair bit of convoluted code to turn into the new one.
  • At the same time, there seems to be no need to serialize these *Format classes using Java Serializable, and there seems to be consensus that the Serializable mechanism was somewhat ill-conceived and applied to too many classes.
  • Proposal: Implement readObject() and writeObject() and just have them throw java.io.NotSerializableException. (Yes, there is a nice, standard, JDK 1.1 Exception class for "when an instance is required to have a Serializable interface" but we don't actually want to implement it.)
  • If this is not ok:
    • Is it ok to break serialization compatibility in ICU 4.8? (That is, ICU 4.6/4.8 MessageFormat would not be able to deserialize each other.)
    • Is it ok to not support serialization if setFormat() or one of its siblings has been called?
  • Yoshito writes 2011-feb-16:
    • Breaking serialization backward compatibility in stable API class is not desired, but I personally can accept such breakage if this is well informed. 
    • I disagree to throw NotSerializableException always. I request -
      • Change the value of serialVersionUID.
      • readObject to check the serialVersionUID - if old one, throw an exception.
      • Implement Serializable properly for ICU4J 4.8 and maintain serialization compatibility for 4.8 and beyond.
  • Decision: Agreed in icu-design email 2011-feb-17
  • Implemented: Java (serialization works as requested by Yoshito: new data format & new ID; custom Format objects are supported; incompatible before/after ICU 4.8)
MessageFormat.applyPattern(String pattern) & MessagePattern.parse(String pattern)
  • The MessagePattern implementation imposes new limitations.
    • Pattern string length 0..0xfffff (max. 1M chars/UChars).
    • Argument numbers limited to 0..127.
    • Argument name lengths limited to 1..127.
    • Argument type name lengths limited to 1..127.
    • Choice/plural selector lengths limited to 1..127.
    • Maximum number of numeric choice/plural values: 128
    • Maximum depth of nested MessageFormat patterns: 128
  • Proposal: Accept these limitations.
  • Decision: Proposal tentatively accepted (ICU meeting 2011-feb-16).
  • Implemented: Java
  • Modified limitations: (Java branch r29636 2011-mar-11)
    • Pattern string length Integer.MAX_VALUE.
    • Argument numbers limited to 0..32767.
    • Argument name lengths limited to 1..65535.
    • Argument type name lengths limited to 1..65535.
    • Argument style text lengths limited to 0..65535.
    • Choice/plural selector lengths limited to 1..65535.
    • Maximum number of numeric choice/plural values: 32768
    • Maximum depth of nested MessageFormat patterns: 32768
Stand-alone Choice/Plural/SelectFormat.applyPattern(String pattern)
  • ICU 4.6: The ChoiceFormat, PluralFormat, and SelectFormat classes have not been enforcing that the snippets they choose/select follow full MessageFormat syntax. For example, it was possible to create a stand-alone PluralFormat object with "odd{The number {#} is odd.}".
    • This would not work as a nested pattern inside a MessageFormat because the resulting snippet "The number {#} is odd." would have failed being parsed for a temporary MessageFormat object.
  • My current work-in-progress code rejects such a pattern even in a stand-alone Choice/Plural/SelectFormat because it always requires that chosen/selected snippets are MessageFormat patterns. For example, while "odd{The number {#} is odd.}" is rejected, "odd{The number {1,number,#} is odd.}" works.
  • I intend to change this for better compatibility, that is, adding code to the new implementation to suppress full MessageFormat-syntax parsing in stand-alone Choice/Plural/SelectFormat objects. As far as I can tell, these just handle quoting/escaping and enforce that curly braces {} are used in matched pairs. I will take a closer look at the JDK ChoiceFormat behavior in particular.
  • Decision: Implement strict MessageFormat-syntax parsing (ICU meeting 2011-mar-02)
  • Implemented: Java
MessageFormat.formatToCharacterIterator() vs. # plural number
  • Background:
    • Java's Format.formatToCharacterIterator() writes a string with attribute meta data, such as which part of the result is a number fraction or, for a MessageFormat, which part of the result comes from which argument.
    • For MessageFormat, the argument's result substring also has the attributes from the argument's formatter object.
  • In the MessageFormat reimplementation, we mostly recreate current behavior. However, one question came up:
    • In a plural sub-message, a # is replaced with the formatted plural number. In ICU 4.6, neither MessageFormat nor PluralFormat wrote any attributes here. In the new code, the # will get at least a NumberFormat.Field attribute.
  • Question: Should we also write a new kind of attribute for the # replacement to make it look more like a simple {2} which gets both the MessageFormat.Field.ARGUMENT and the NumberFormat.Field(s)?
    • If so, on which class should we declare the Field, what should it be called, and what attribute value should it come with?
    • Right now, (on the development branch) we have a new MessageFormat.Field value of
    •         public static final Field PLURAL_NUMBER = new Field("plural number field");
    • Value: We currently set this same Object as the attribute value.
      • This is customary for Format.Field attributes, although MessageFormat.Field.ARGUMENT has the argument ID [e.g., Integer(0) or "first_name"] as the attribute value.
      • In MessageFormat, we could set the plural argument ID, but if we wanted to write the attribute in PluralFormat as well, we would not have that information there.
  • Proposal: We could go either way, but, given that this API seems rarely used, it might be ok to not write an additional attribute at all, and I think we should remove this from the reimplementation (on the development branch).
    • We should be able to add it later if needed.
  • Decision: Do not write such a special attribute for the # plural number. (ICU meeting 2011-mar-02)
  • Implemented: Java (that is, withdrawn from dev. branch, r29514, 20110302)
C++ ChoiceFormat internal-array getters; getLimits(), getClosures(), getFormats() [there is no ICU4J ChoiceFormat class]
  • Proposal: Deprecate the ICU C++ ChoiceFormat getters which return the internal arrays, and set count=0/returning NULL.
  • Background: With the new MessageFormat/MessagePattern work, these getters which expose the old internal data structures are hard to implement. They are const and return arrays that are owned by the ChoiceFormat object.
  • Assertion: I don't see any reasonable use case for these getters. I expect most users to use MessageFormat pattern strings with embedded choice arguments, except that they should probably use plural or select arguments at this point. The old class API docs showed creating a separate ChoiceFormat and setting it into a MessageFormat object, although that would be translation-hostile, but even then I don't see why one would access the pieces of the just-built ChoiceFormat. Assuming that users build formatters and use them for formatting and parsing, any code and time to recreate the old internal arrays when no one will call the getters would be wasted.
  • Decision: Deprecate & disable. (ICU meeting 2011-apr-13)
  • Implemented: C++
Comments