Loops: while...do Expression
while...do expression is used to perform iterative execution (looping) while a specified test condition is true.
while test-expression do body-expression
The test-expression is evaluated; if it is
true, the body-expression is executed and the test expression is evaluated again. The body-expression must have type
unit. If the test expression is
false, the iteration ends.
The following example illustrates the use of the
open System let lookForValue value maxValue = let mutable continueLooping = true let randomNumberGenerator = new Random() while continueLooping do // Generate a random number between 1 and maxValue. let rand = randomNumberGenerator.Next(maxValue) printf "%d " rand if rand = value then printfn "\nFound a %d!" value continueLooping <- false lookForValue 10 20
The output of the previous code is a stream of random numbers between 1 and 20, the last of which is 10.
13 19 8 18 16 2 10 Found a 10!
You can use
while...do in sequence expressions and other computation expressions, in which case a customized version of the
while...do expression is used. For more information, see Sequences, Asynchronous Workflows, and Computation Expressions.