As discussed in Proposal to remove XML literals from the language, we all agreed that the current symbol-based XML literals has made the Scala XML implementation stall on the old fashioned scala-xml library, which is not desired to every participator in that thread. In this post, I will propose an alternative solution other than simply removing XML literals.
Background
Name-based for comprehension has been proven success in Scala language design. A for / yield expression will be converted to higher-order function calls to flatMap, map and withFilter methods, no matter which type signatures they are. The for comprehension can be used for either Option or List, even when List has an additional implicit CanBuildFrom parameter. Third-party libraries like Scalaz and Cats also provides Ops to allow monadic data types in for comprehension.
Name-based pattern matching is introduced by Dotty. It is greatly simplified the implementation compared to Scala 2. In addition, specific symbols in Scala library (Option, Seq) are decoupled from the Scala compiler.
Considering the success of the above name-based syntactic sugars, in order to decouple scala-xml library from Scala compiler, name-based XML literal is an obvious approach.
Goals
- Keeping source-level backward compatibility to existing symbol-based XML literals in most use cases of
scala-xml - Allowing schema-aware XML literals, i.e. static type varying according to tag names, similar to the current TypeScript and Binding.scala behavior.
- Schema-aware XML literals should be understandable by both the compiler and IDE (e.g. no white box macros involved)
- Existing libraries like ScalaTag should be able to support XML literals by adding a few simple wrapper classes. No macro or metaprogramming knowledge is required for library authors.
- The compiler should expose as less as possible number of special names, in case of being intolerably ugly.
Non-goals
- Embedding fully-featured standard XML in Scala.
- Allowing arbitrary tag names and attribute names (or avoiding reserved word).
- Distinguishing lexical differences, e.g.
<a b = "c"></a>vs<a b="c"/>.
The proposal
Lexical Syntax
Kept unchanged from Scala 2.12
XML literal translation
Scala compiler will translate XML literal to Scala AST before type checking. The translation rules are:
Self-closing tags without prefixes
<tag-name />
will be translated to
xml.tags.`tag-name`()
Self-closing tags with some prefixes
<prefix-1:tag-name />
will be translated to
xml.tags.`prefix-1`.`tag-name`()
Attributes
<tag-name attribute-1="value"
attribute-2={ f() }
prefix-2:attribute-3={"value"} />
will be translated to
xml.tags.`tag-name`(
xml.attributes.`attribute-1`(xml.text("value")),
xml.attributes.`attribute-2`(xml.interpolation(f())),
xml.attributes.`prefix-2`.`attribute-3`(xml.interpolation("value"))
)
CDATA
<![CDATA[ raw ]]> will be translated to xml.text(" raw ") if -Xxml:coalescing flag is on, or xml.cdata(" raw ") if the flag is turned off as -Xxml:-coalescing.
Process instructions
<?xml-stylesheet type="text/xsl" href="sty.xsl"?>
will be translated to
xml.processInstructions.`xml-stylesheet`("type=\"text/xsl\" href=\"sty.xsl\"")
Child nodes
<tag-name attribute-1="value">
text & hexadecimal reference & decimal reference
<child-1/>
<!-- my comment -->
{ math.random }
<![CDATA[ raw ]]>
</tag-name>
will be translated to
xml.tags.`tag-name`(
xml.attributes.`attribute-1`(xml.text("value")),
xml.text("""
text """),
xml.entities.amp,
xml.text(""" hexadecimal reference """),
xml.entities.AMP,
xml.text(""" decimal reference
"""),
xml.tags.`child-1`(),
xml.text("""
"""),
xml.comment(" my comment "),
xml.text("""
"""),
xml.interpolation(math.random),
xml.text("""
"""),
xml.cdata(" raw "), // or xml.text(" raw ") if `-Xxml:coalescing` flag is set
xml.text("""
""")
)
Note that hexadecimal references and decimal references will be unescaped and translated to xml.text() automatically, while entity references are translated to fields in xml.entities.
XML library vendors
An XML library vendor should provide a package or object named xml, which contains the following methods or values:
- tags
- attributes
- entities
- processInstructions
- text
- comment
- cdata
- interpolation
An XML library user can switch different implementations by importing different xml packages or objects. scala.xml is used by default when no explicit import is present.
In a schema-aware XML library like Binding.scala, its tags, attributes, processInstructions and entities methods should return factory objects that contain all the definitions of available tag names and attribute names. An XML library user can provide additional tag names and attribute names in user-defined implicit classes for tags and attributes.
In a schema-less XML library like scala-xml, its tags, attributes, processInstructions and entities should return builders that extend scala.Dynamic in order to handle tag names and attribute names in selectDynamic or applyDynamic.
Known issues
Name clash
<toString/> or <foo toString="bar"/> will not compile due to name clash to Any.toString.
- Compilation error is the desired behavior in a schema-aware XML library as long as
toStringis not a valid name in the schema. Fortunately, unlike JSX,<div class="foo"></div>should compile becauseclassis a valid method name. - A schema-less XML library user should instead explicit construct
new Elem("toString").
White space only text
Alternative approach
XML initialization can be implemented in a special string interpolation as xml"<x/>". The pros and cons of these approaches are list in the following table:
| symbol-based XML literals in Scala 2.12 | name-based XML literals in this proposal | xml string interpolation | |
|---|---|---|---|
| XML is parsed by ... | compiler | compiler | library, IDE, and other code browsers including Github, Jekyll (if syntax highlighting is wanted) |
| Is third-party schema-less XML library supported? | No, unless using white box macros | Yes | Yes |
| Is third-party schema-aware XML library supported? | No, unless using white box macros | Yes | No, unless using white box macros |
| How to highlight XML syntax? | By regular highlighter grammars | By regular highlighter grammars | By special parsing rule for string content |
| Can presentation compiler perform code completion for schema-aware XML literals? | No | Yes | No |