When comparing SML vs F#, the Slant community recommends F# for most people. In the question“What are the best functional programming languages for programming beginners?” F# is ranked 8th while SML is ranked 19th. The most important reason people chose F# is:
F#'s syntax tends to be terse while remaining very readable and easy to understand without being a chore to write.
Specs
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Pros
Pro Interpreter supports defining functions
SML comes with a powerful code interpreter in which you can both define and use functions.
Pro Concise syntax
F#'s syntax tends to be terse while remaining very readable and easy to understand without being a chore to write.
Pro Easier transition from other paradigms
Since F# is not a purely functional language, it lends itself to being more easily picked up by programmers that have experience with other paradigms.
Pro .NET Interoperability
Since F# runs on the Common Language Runtime or CLR, it has access to the entire .NET Framework, as well as libraries written in other .NET languages such as C#, VB.NET, and C++/CLI.
Pro Multiplatform, it runs on .Net Core
Forget Xamarin and Mono. F# now runs on the multiplatform .Net Core!
Pro You can run F# in the browser
Thanks to a project like Fable, you can write and run your client app in F# and run it over JavaScript. It allows you to share code between your server and client.
Pro Natively supported by Visual Studio
This is a .Net language natively supported by Visual Studio. Though it is not as tooled up as C# the support is still substantial. In particular, C# deployment scenarios can be enabled for it with small C# wrapper projects. Integration with Visual Studio provides: IntelliSense, debugging, projects an other features.
Pro Fall into the pit of success
F# directs you into a workflow where the right way is the path of least resistance.
Coming from a C# background, its restrictions might feel arbitrary at first (e.g., what do you mean my code has to be in dependency order!? Arbitrary alphabetical or bust!), but you'll soon realize that your code is cohesive, concise and consistent in a way that it never was before - and you can compile and run with confidence!
Pro F# is supported by Xamarin
Thanks to Xamarin, F# is a functional language that you can use to build for iOS, Android and Windows.
Cons
Con Functions can have side effects
For example, it's possible in SML to have a function that returns the sum of two numbers but which also does some printing in the middle. This makes it hard to whether a function has a side effect or whether it mutates some global state.
Con Has no ad-hoc polymorphism (á la Type Classes)
You have generics, you have interfaces, you have inheritance, you have a lot of things at your disposal but you don't have Type Clases. They can be emulated using some clever constructs but there's nothing like having the real thing.
Con Infested with OOP
Still relying on .NET OOP libraries.