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:
“Translate a book a dozen times from one language to another, and what becomes of its style? Most books would be worn out and disappear in this ordeal. The pen which wrote it is soon destroyed, but the poem survives.”
—Henry David Thoreau (18171862)
“Syntax is the study of the principles and processes by which sentences are constructed in particular languages. Syntactic investigation of a given language has as its goal the construction of a grammar that can be viewed as a device of some sort for producing the sentences of the language under analysis.”
—Noam Chomsky (b. 1928)
“The reader uses his eyes as well as or instead of his ears and is in every way encouraged to take a more abstract view of the language he sees. The written or printed sentence lends itself to structural analysis as the spoken does not because the readers eye can play back and forth over the words, giving him time to divide the sentence into visually appreciated parts and to reflect on the grammatical function.”
—J. David Bolter (b. 1951)