The Fortress Language Specification - CiteSeerX

The body of each iteration is run in its own implicit thread. The expressions in the generator list can each be considered
to run in a separate implicit thread. Together these implicit threads form a thread group.
Some common Generators include:
l : u
Any range expression
a.indices The index set of an array a
{0, 1, 2, 3} The elements of an aggregate expression
sequential(g) A sequential version of generator g
The generator sequential(g) forces the iterations using distinct values from g to be performed in order. Every
generator has an associated natural order which is the order obtained by sequential . For example, a sequential for
loop starting at 1 and going to n can be written as follows:
for i ← sequential(1 :n) do
· · ·
end
Given a multidimensional array, the indices generator returns a tuple of values, which can be bound by a tuple of
variables to the left of the arrow:
(i, j) ← my2DArray.indices
The parallelism of a loop on this generator follows the spatial distribution (discussed in Section 32.5) of my2DArray
as closely as possible.
The order of nesting of generators need not imply anything about the relative order of nesting of iterations. In most
cases, multiple generators can be considered equivalent to multiple nested loops. However, the compiler will make an
effort to choose the best possible iteration order it can for a multiple-generator loop; there may be no such guarantee for
nested loops. Thus loops with multiple generators are preferable in general. Note that the early termination behavior
of nested looping is subtly different from a single multi-generator loop; see Section 32.6.
13.18 Summations and Other Reduction Expressions
Syntax:
Flow ::= Accumulator
∑ ∏
[[ GeneratorList ]] Expr
Accumulator ::= | | BIG Op
A reduction expression begins with a big operator such as ∑ or ∏ followed by an optional generator list (described
in Section 13.17), followed by a subexpression. A complete list of these operators are described in Section 16.8.1.
When a generator list is provided, generators produce values and bind the values to the identifiers that are used in the
subexpression. Most generators, unlike the sequential generator, may execute each evaluation of the subexpression
in a separate implicit thread. The value of a reduction expression is the result of the operation over the values of the
subexpressions. The type of a reduction expression is the return type of the big operator used.
A reduction expression with a generator list:
∑ [v1 ← g 1 , v 2 ← g 2 , . . .]e
is equivalent to the following code:
do
result = 0
for v 1 ← g 1 ,v 2 ← g 2 , . . . do
result += e
end
103