ML programming language

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ML is a general-purpose functional programming language developed by Robin Milner and others in the late 1970s at the University of Edinburgh, whose syntax is inspired by ISWIM. Historically, ML stands for metalanguage as it was conceived to develop proof tactics in the LCF theorem prover (the language of which ML was the metalanguage is pplambda, a combination of the first-order predicate calculus and the simply-typed polymorphic lambda-calculus). It is known for its use of the Hindley-Milner type inference algorithm, which can infer the types of most values without requiring the extensive annotation often criticised in languages such as Java.

ML is often referred to as an impure functional language, because it permits side-effects, and therefore imperative programming, unlike purely functional programming languages such as Haskell.

Features of ML include a call-by-value evaluation strategy, first class functions, automatic memory management through garbage collection, parametric polymorphism, static typing, type inference, algebraic data types, pattern matching, and exception handling.

Unlike Haskell, ML uses eager evaluation, which means that all subexpressions are always evaluated. One result of this is that you cannot use infinite lists per se. However, lazy evaluation and hence infinite lists can be simulated, through use of anonymous functions.

Today there are several languages in the ML family; the two major dialects are Standard ML and Caml, but others exist, including F# - an open research project that targets the Microsoft .NET platform. Ideas from ML have influenced numerous other languages, such as Haskell, Cyclone, and Nemerle.

ML's strengths are mostly applied in language design and manipulation (compilers, analyzers, theorem provers), but it is a general-purpose language also used in bioinformatics, financial systems, and applications including a genealogical database, a peer-to-peer client/server program, etc.

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Examples of ML

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Anatomy of a ML function

The "Hello World" of functional languages is the factorial function. Expressed as pure ML:

fun fac : (fn: int -> int) 0 = 1
 | fac n = n * fac (n-1);

This describes the factorial as a recursive function, with a single terminating base case. It is similar to the descriptions of factorials found in mathematics textbooks. Much of ML code is similar to mathematics in facility and syntax.

The first line of the factorial function shown is optional, and describes the types of this function. It can be read as the function fac (fac) has type (:) function from integer to integer (fn: int -> int). That is, it takes an integer as an argument, and returns another integer. Rewritten without the unnecessary type annotation, it looks like:

fun fac 0 = 1
 |  fac n = n * fac(n-1);

The second line relies on pattern matching, an important part of ML programming. Note that parameters of a function are not in parentheses but separated by spaces. When the function's argument is 0 (zero) it will return the integer 1 (one). For all other cases the second line is tried. This is the recursion, and executes the function again until the base case is reached.

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