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Consider the following code that provides a radius of a
circle and calculates the diameter:
let radius = 25.08
let diameter = radius * 2.00
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Here is another version of the program with a negative value
for the radius:
let radius = -25.08
let diameter = radius * 2.00
The F# language provides a special function used to evaluate
an expression. The function is named assert and its syntax is:
assert expression
The expression must evaluate to a true or false
result. Here is an example:
let radius = -25.08
let diameter = radius * 2.00
assert (radius > 0.00)
When the assert() function is called, if it returns
false, the compiler displays an error dialog box and stops. Here is an example:

If the function returns true, the program proceeds normally.