I have no practical experience with Xtext, so I will talk mostly about MPS.
LWB
Both Xtext and MPS are language workbenches, so they have their own schema used to metamodel the abstract syntax (structure of concepts), some way to define the concrete syntax (notations) and some way to define generators (M2M or M2T transformations) or less usually interpreters. Then they have provide the IDE itself with highlighting, smart actions like refactoring and contextual error fixes, advanced search and navigation (go to declaration etc.), checking for errors (type errors, static code analysis, checking of defined constraints & rules, checking cardinality, dataflow analysis), ... So yes, lots of options for validation. I have mentioned things, that are in MPS, not sure if Xtext provides everything. However, all of these features are organised in so-called aspects which you can check out in a summary table which shortly describes each aspect.
Projectional editor
As you have mentioned, MPS uses a projectional editor. You directly manipulate the AST, parser-based post-IntelliJ smart IDEs are able to provide you with intelligent actions like recaftoring and go to declaration etc. only because they parse the language in memory and construct an AST behind the scenes anyway. Projectional editors skip the parsing step.
Dodging ambiguity
It uses no parser at all, so all of the downsides of having a parser are gone. First of all, the language developer does not need to be an expert in syntax analysis, so you don't need to hire them specifically. But the best win is to have infinite language composability. This is achieved, as you have mentioned, by totally avoiding ambiguities which could appear in grammars (MPS does not use a grammar, but a model). Let's say you use language A and language B. For demonstration, let's say both languages extend BaseLanguage (abbr. BL, the MPS-equivalent of Java) and they have both defined a statement to log. Concept a logs to stderr and b logs to a file. However, both a and b have an identical concrete syntax (i.e. editor definition in MPS) which just says log. Now if you had a parser and it encounters the token log it cannot decide from which language the concept is, so it's ambiguous - not even a look-ahead parser can do it. In a projectional editor this cannot happen, because only the projection is identical and under the hood the AST has an instance of either a or b (you can think of it as always using the whole FQN of a class in Java, just the package is hidden in the IDE, so you can use identically named classes from different packages). The "ambiguity" is resolved at the time of writing by the user: when he writes log a dropdown menu appears clearly showing that one of them is a and the other is b (maybe even shows a description which would say "Log to file" / "Log to stderr").
Modularity
Consequently, MPS has very good modularity, composability and extensibility of languages. You have mentioned
allows for multiple languages to be extended for the language you are making [...]
why would someone want to extend multiple language
You need to differentiate between using a language and extending it
(if you are interested more Völter talks about 4 kinds of composition techniques regarding languages: referencing, extension, reuse and embedding). Using a lanugage is just the ability to write programs in it. If you extend a language, it's kind of like inheritance, you add new concepts to it, f.e. create a new type of Java (BL) statement. And it has been done in the standard languages shipped with MPS too. You have for example the checkedDots language which extends BL with an operation .? which is null-safe (similarly to null-conditional operator ?. in C#). So why extend a language? Because you can use new constructs, add new functionality or syntactic sugar. Another ready-to-use language in BL is the tuples language, which has both indexed and named tuples. Then there is the collections language, which kind-of replaces the Java Stream API. All of these little languages are extensions which you can start using with a simple Ctrl+L. You could also embed another language to your language - use a regex inside an SQL statement inside your Java code.
Generation
Another kind of language dependency in MPS is to have a "generation target" language. Generators in MPS work in a way that you transform your language sentence (i.e. model) into another MPS language. You can invent your own little language, or implement LOLcode and setup the generator to transform it into valid Java code. However, this language must already exist in MPS, so you cannot generate it to Python, if there is no Python implementation in MPS. The other alternative is to generate text (M2T), this way you could theoretically generate Python source code, or just print the LOLcode as-is.
Multiple notations
The second great difference in projectional vs. parser-based editors is that the latter inherently supports only textual notation. Maybe there are some external tools you can use. On the other side, MPS provides textual, tabular, symbolic (math symbols) and graphical (diagrams) notations. There is a possibility to swap your view from one notation to another, per concept or for the whole "file" (program).
Drawbacks
It's not all roses though. Projectional editors have some limitations, or challenges to solve. There is an analysis of challenges in projectional editors which points out mainly usability and infrastructure integration. They are mostly solved in MPS, f.e. regarding infrastructure you have a good VCS diff/merge tool. For automatic/cmd builds there is a language that generates Ant. Gradle or Maven does not work with MPS directly, but through Ant. Regarding usability "MPS takes a
while to get used to, but then its usability is comparable to ParEs."3 You should use a language called GrammarCells (available through MPS-extensions or mbeddr.platform) which makes it easy to build good editors (mainly for arithemtic expressions), otherwise by default you must enter concepts in prefix order (+ first, not the number). Comments in MPS cannot be placed willy-nilly. Cannot establish references to non-existing nodes... (see the Table 1. in 3)
MPS currently does not have a web-based version. There are some planned, though. Jetbrains works on WebMPS, then there is modelix.
Portability
Generally, you are stuck to working in MPS. By default it is not really portable, unless you explicitly define generators which produce portable output. If you want to input a program , you can code a paste-handler where you could put your parser, or you can change the format in which the AST is stored (from XML to maybe directly your language, but this would again require a parser to read). I am currently working on a solution which enables to import an MPS language from a YAJCo model (model-based parser generator, where the input is not a grammar, but Java classes representing the semantic model). Then you can import a sentence (file) which creates and populates a model (AST). From the program in MPS you can generate Java source code which fills the original Java classes if you need it.
BTW the mbeddr project has implemented importing from ECore check here
Dictionary
- M2M = model to model
- M2T = model to text
