The Fortress Language Specification - CiteSeerX

The loop body reads a i and sum , then adds them and writes the result back to sum ; this will appear to occur
atomically with respect to all other threads (including the other iterations of the loop body). Note in particular that the
atomic expression will appear atomic with respect to reads and writes not in atomic expressions of a i and sum .
13.24 Spawn Expressions
Syntax:
Flow ::= spawn Expr
A thread can be created by a spawn expression. A spawn expression consists of the special reserved word spawn
followed by an expression. A spawn expression spawns a thread which evaluates its subexpression in parallel with
any succeeding evaluation. The value of a spawn expression is the spawned thread and the type of the expression is
the Thread trait.
The Thread trait has the following methods:
• The val method returns the value computed by the subexpression of the spawn expression. If the thread has
not yet completed execution, the invocation of val blocks until it has done so.
• The wait method waits for a thread to complete, but does not return a value.
• The ready method returns true if a thread has completed, and returns false otherwise.
• The stop method attempts to terminate a thread as described in Section 32.6.
In the absence of sufficient parallel resources, an attempt is made to run the subexpression of the spawn expression
before continuing succeeding evaluation (so long as we have not specified a region for evaluation as described in
Section 32.7, and we are not currently evaluating an atomic expression as described in Section 13.23). We can
imagine that it is actually the rest of the evaluation after the parallel block which is spawned off in parallel. This is
a subtle technical point, but makes the sequential execution of parallel code simpler to understand, and avoids subtle
problems with the asymptotic stack usage of parallel code [19, 11].
13.25 Throw Expressions
Syntax:
Flow ::= throw Expr
A throw expression consists of the special reserved word throw followed by a subexpression. The subexpression
must have the type Exception (see Chapter 14). A throw expression evaluates its subexpression to an exception
value and throws the exception value; the expression completes abruptly and has BottomType.
The type Exception has exactly two direct mutually exclusive subtypes, CheckedException and UncheckedException.
Every CheckedException that is thrown must be caught or forbidden by an enclosing try expression (see Section
13.26), or it must be declared in the throws clause of an enclosing functional declaration (see Section 12.1).
Similarly, every CheckedException declared to be thrown in the static type of a functional called must be either
caught or forbidden by an enclosing try expression, or declared in the throws clause of an enclosing functional
declaration.
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