The Fortress Language Specification - CiteSeerX

13.10 Assignments
Syntax:
Expr ::= Expr AssignOp Expr
AssignOp ::= := | Op=
An assignment expression consists of a left-hand side indicating one or more variables or a subscripted expression (as
described in Section 34.7) to be updated, an assignment token, and a right-hand-side expression.
The assignment token may be ‘ := ’, to indicate ordinary assignment; or may be any operator (other than ‘ : ’ or ‘ = ’
or ‘ < ’ or ‘ > ’) followed by ‘ = ’ with no intervening whitespace, to indicate compound (updating) assignment. A
compound assignment is a syntactic sugar; for example, x += e is a shorthand for x := x + e . An assignment
expression evaluates its right-hand-side expression and binds its left-hand side to the value of the right-hand-side. An
assignment expression performs a memory write operation.
A left-hand side of an assignment expression may be a single variable, a subscripted expression, or n variables using
tuple notation. If tuple notation is used, then the right-hand side must be an expression which ultimately evaluates to a
tuple of length n or a function application that returns a tuple of n values. Variables updated in assignment expressions
must be already declared. The value of an assignment expression is () .
Here are some examples:
x := f(0)
c ij := c ij + a ik b kj
(a, b, c) := (b, c, a) (∗ Permute a, b, and c ∗)
x += 1
(x, y) += (δ x , δ y )
myBag = myBag ∪ newItems
myBag∪ = newItems
13.10.1 Definite Assignment
References to uninitialized variables are statically forbidden. As with the Java Programming Language, this static
constraint is ensured with a specific conservative flow analysis. In essence, an initialization of a variable must occur
on every possible execution path to each reference to a variable. Variable initialization is performed in a local scope
analogously to the rules for top-level initialization of simple components, defined in Section 22.6.
13.11 Do Expressions
Syntax:
Flow ::= Do
Do ::= do BlockElem ∗ end
BlockElem ::= Expr[ , GeneratorList]
| LocalVarFnDecl
A do expression consists of the special reserved word do , a series of expressions, a generated expressions (described
in Section 13.11.1) local variable declarations, or local function declarations (block expression), and the special reserved
word end . The last of the block expression must not be a local declaration. A do expression evaluates its
subexpressions and local declarations in order. The value and type of a do expression is the value and type of the last
expression in the block expression. Each do expression introduces a new scope. Some compound expressions have
clauses that are implicitly block expressions.
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