Purely functional languages can provide an opportunity for computation to be performed in parallel, avoiding the von Neumann bottleneck of sequential one step at time execution, since values are independent of each other. Single assignment is the only form of assignment available in purely functional languages, such as Haskell, which do not have variables in the sense of imperative programming languages but rather named constant values possibly of compound nature with their elements progressively defined on-demand. Imperative assignment can introduce side effects while destroying and making the old value unavailable while substituting it with a new one, and is referred to as destructive assignment for that reason in LISP and functional programming, similar to destructive updating. Single assignment is an example of name binding and differs from assignment as described in this article in that it can only be done once, usually when the variable is created no subsequent reassignment is allowed.Īn evaluation of expression does not have a side effect if it does not change an observable state of the machine, and produces same values for same input. In functional programming, assignment is discouraged in favor of single assignment, also called initialization. x := x + 1) is disallowed in purely functional languages.
functions that do not depend on the state of some variable(s), but produce the same results for a given set of parametric inputs at any point in time. The purpose is to enforce referential transparency, i.e. However, some languages (primarily strictly functional languages) do not allow that kind of "destructive" reassignment, as it might imply changes of non-local state. Other languages define assignment as a statement (meaning that it cannot be used in an expression).Īssignments typically allow a variable to hold different values at different times during its life-span and scope. In some languages, the symbol used is regarded as an operator (meaning that the assignment statement as a whole returns a value). The second most commonly used notation is x := expr (originally ALGOL 1958, popularised by Pascal). Today, the most commonly used notation for this operation is x = expr (originally Superplan 1949–51, popularized by Fortran 1957 and C). In most imperative programming languages, the assignment statement (or expression) is a fundamental construct.
In computer programming, an assignment statement sets and/or re-sets the value stored in the storage location(s) denoted by a variable name in other words, it copies a value into the variable. For assignment of letters to disk file systems, see Drive letter assignment.