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SML (Standard ML) |
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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 (Revised) in 1997). LanguageStandard ML is a mostly functional programming language. Programs written in Standard ML mostly consist of expressions whose values are to be calculated. 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:
fun factorial n =
if n = 0 then 1 else n * factorial (n-1)
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:
fun factorial 0 = 1
| factorial n = n * factorial (n - 1)
This can be rewritten using a case statement like this:
val rec factorial =
fn n => case n of 0 => 1
| n => n * factorial (n - 1)
Here, the keyword val introduces a binding of an identifier to a value, fn introduces the definition of an anonymous function, and case introduces a sequence of patterns and corresponding expressions. Using a local function, this function can be rewritten to use tail recursion:
fun factorial n =
let
fun tail_fact p 0 = p
| tail_fact p m = tail_fact (p * m) (m - 1)
in
tail_fact 1 n
end
The value of a let-expression is that of the expression between in and end. Article source: http://en.wikipedia.org/wiki/SML_(programming_language) Go back to Programming Articles home page
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