We are sometimes asked to modify MLton to change the language it compiles. In short, we are conservative about making such changes. There are a number of reasons for this.
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The Definition of Standard ML is an extremely high standard of specification. The value of the Definition would be significantly diluted by changes that are not specified at an equally high level, and the dilution increases with the complexity of the language change and its interaction with other language features.
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The SML community is small and there are a number of SML implementations. Without an agreed-upon standard, it becomes very difficult to port programs between compilers, and the community would be balkanized.
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Our main goal is to enable programmers to be as effective as possible with MLton/SML. There are a number of improvements other than language changes that we could spend our time on that would provide more benefit to programmers.
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The more the language that MLton compiles changes over time, the more difficult it is to use MLton as a stable platform for serious program development.
Despite these drawbacks, we have extended SML in a couple of cases.
We allow these language extensions because they provide functionality that is impossible to achieve without them or have non-trivial community support. The Definition does not define a foreign function interface. So, we must either extend the language or greatly restrict the class of programs that can be written. Similarly, the Definition does not provide a mechanism for namespace control at the module level, making it impossible to deliver packaged libraries and have a hope of users using them without name clashes. The ML Basis system addresses this problem. We have also provided a formal specification of the ML Basis system at the level of the Definition.