Standard ML - Language

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:

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)

or as a lambda function:

val rec factorial = fn 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 in a more efficient tail recursive style.

fun factorial n = let 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.

Read more about this topic:  Standard ML

Famous quotes containing the word language:

    It is difficult for a woman to define her feelings in language which is chiefly made by men to express theirs.
    Thomas Hardy (1840–1928)

    If when a businessman speaks of minority employment, or air pollution, or poverty, he speaks in the language of a certified public accountant analyzing a corporate balance sheet, who is to know that he understands the human problems behind the statistical ones? If the businessman would stop talking like a computer printout or a page from the corporate annual report, other people would stop thinking he had a cash register for a heart. It is as simple as that—but that isn’t simple.
    Louis B. Lundborg (1906–1981)

    Experiment is necessary in establishing an academy, but certain principles must apply to this business of art as to any other business which affects the artis tic sense of the community. Great art speaks a language which every intelligent person can understand. The people who call themselves modernists today speak a different language.
    Robert Menzies (1894–1978)