Standard ML
| Paradigm | multi-paradigm: functional, imperative |
|---|---|
| Typing discipline | strong, static, inferred |
| Filename extensions | .sml |
| Major implementations | |
| MLKit, MLton, MLWorks, Moscow ML, Poly/ML, SML/NJ, MLj, SML.NET | |
| Dialects | |
| Alice, Concurrent ML, Dependent ML | |
| Influenced by | |
| ML, Hope | |
| Influenced | |
| Elm, F*, OCaml, Rust, Scala | |
Standard ML (SML) is a general-purpose, modular, functional programming language with compile-time type checking and type inference. It is popular among compiler writers and programming language researchers, as well as in the development of theorem provers.
SML is a modern descendant of the ML programming language used in the Logic for Computable Functions (LCF) theorem-proving project. It is distinctive among widely used languages in that it has a formal specification, given as typing rules and operational semantics in The Definition of Standard ML (1990, revised and simplified as The Definition of Standard ML (Revised) in 1997).
Standard ML is a functional programming language with some impure features. Programs written in Standard ML consist of expressions to be evaluated, as opposed to statements or commands, although some expressions return a trivial "unit" value and are only evaluated for their side-effects.
Like all functional programming languages, a key feature of Standard ML is the function, which is used for abstraction. For instance, the factorial function can be expressed as:
A Standard ML compiler is required to infer the static type int -> int of this function without user-supplied type annotations. I.e., it has to deduce that n is only used with integer expressions, and must therefore itself be an integer, and that all value-producing expressions within the function return integers.
The same function can be expressed with clausal function definitions where the if-then-else conditional is replaced by a sequence of templates of the factorial function evaluated for specific values, separated by '|', which are tried one by one in the order written until a match is found:
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