The Fortress Language Specification - CiteSeerX

halve(x : R64) : R64 = x/2
Predictably, 32-bit precision floats are written R32 .
64-bit integers are denoted by the type Z64 . 32-bit integers by the type Z32 , and infinite precision integers by the
type Z .
2.6 Functions in Fortress
Fortress allows recursive, and mutually recursive function definitions. Here is a simple definition of a factorial
function in Fortress:
factorial(n) =
if n = 0 then 1
else n factorial(n − 1) end
Note the juxtaposition of parameter n with the recursive call factorial(n − 1) . In Fortress, as in mathematics, multiplication
is represented through juxtaposition. By default, two expressions of numeric type that are juxtaposed represent
a multiplication. On the other hand, juxtaposition of an expression with a function type with another expression
to its right represents function application, as in the following example:
sin x
2.6.1 Keyword Parameters
Functions in Fortress can be defined to take keyword arguments by providing default values for parameters. In the
following example:
makeColor(red : Z64 = 0,green : Z64 = 0,blue : Z64 = 0) =
if 0 ≤ red ≤ 255 ∧ 0 ≤ green ≤ 255 ∧ 0 ≤ blue ≤ 255
then Color(red,green,blue)
else throw Error end
the function makeColor takes three keyword arguments, all of which default to 0 . If we call it as follows:
makeColor(green = 255)
the arguments red and blue are both given the value 0 .
There are some other aspects of this example worth mentioning. For example, the body of this function consists of an
if expression. The test of the if expression checks that all three parameters have values between 0 and 255 . The
boolean operator ≤ is chained: An expression of the form x ≤ y ≤ z is equivalent to the expression x ≤ y ∧ y ≤ z ,
just as in mathematical notation. The then clause provides an example of a constructor call in Fortress, and the else
clause shows us an example of a throw expression in Fortress.
2.6.2 Varargs Parameters
It is also possible to define functions that take a variable number of arguments. We have already seen such a function:
Executable.run . Here is another:
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