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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'' (1990, revised and simplified as ''The Definition of Standard ML (Revised)'' in 1997). ==Language== 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: 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: | factorial n = n * factorial (n - 1) This can be rewritten using a case statement like this: fn n => case n of 0 => 1 | n => n * factorial (n - 1) or as a lambda function: * 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 in a more efficient tail recursive style. fun lp (0, acc) = acc | lp (m, acc) = lp (m-1, m *acc) in lp (n, 1) end (The value of a let-expression is that of the expression between in and end.) The encapsulation of an invariant-preserving tail-recursive tight loop with one or more accumulator parameters inside an invariant-free outer function, as seen here, is a common idiom in Standard ML, and appears with great frequency in SML code. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Standard ML」の詳細全文を読む スポンサード リンク
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