Value of An Assignment
In some programming languages, an assignment statement returns a value, while in others it does not.
In most expression-oriented programming languages (for example, C), the assignment statement returns the assigned value, allowing such idioms as x = y = a, in which the assignment statement y = a returns the value of a, which is then assigned to x. In a statement such as while (f = read) {…}, the return value of a function is used to control a loop while assigning that same value to a variable.
In other programming languages, Scheme for example, the return value of an assignment is undefined and such idioms are invalid.
In Haskell, there is no variable assignment; but operations similar to assignment (like assigning to a field of an array or a field of a mutable data structure) usually evaluate to unit, the value of the unit type, which is typically the type of an expression that is evaluated purely for its side effects.
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