When comparing Standard ML vs Reason ML, the Slant community recommends Reason ML for most people. In the question“What are the best languages for learning functional programming?” Reason ML is ranked 13th while Standard ML is ranked 14th.
Ranked in these QuestionsQuestion Ranking
Pro Powerful module system
The module system that Standard ML uses gives the programmer the power to define custom data types whose internal implementation is invisible to other programmers using the module.
Pro Implementing laziness is trivial
Since mutability is only confined to a special type of reference cells, implementing laziness in SML can be done in only 20 lines of code.
Pro Enforces distinction between data and computations
Since it uses strict evaluation, it enforces distinction between data and computations which in turn enables you to use induction on algebraic data types as a reasoning principle.
Pro Great exception system
Secret messages can be sent across distant parts of a program without possibility of being intercepted by unintended recipients in the middle.
Pro Superior type inference
Ocaml type inference is so smart that you never have to repeat yourself and keep code very clean, type errors also are very pleasant
Pro Immutability with escape hatches
reason includes true immutability, but it has escape hatches to let you use mutations in exceptional cases.
The same reasonml code can compile to js (eg. run on browsers or node.js, use any lib in npm), or compile to assembly thru ocaml (unless of course you load js externals), running on any device, with C-comparable (or better) performance.
Pro JSX syntax natively supoorted
reason was created by the creator of react, for developers already using JSX to template web or native UIs this results very familiar
Con Not very popular outside academia
SML is mostly used in academia and doesn't have many uses in industry. While it's a good language for learning functional programming concepts, the language itself won't be very useful.
Con A standard async syntax is pending
Async syntax is not standard across native/js projects and in both cases a bit awkward for non-ocaml devs. Currently this is reasonml most voted issue in their GitHub repo so hopefully, there's news soon.