Support guarded ALT with boolean pre-conditions

[associate-1]

Difficulty: Medium — Parser must handle expr & prefix on ALT cases; codegen must conditionally enable/disable select cases (e.g., by setting channels to nil when the guard is false).

Summary

Occam ALT cases can have boolean guard pre-conditions using the & operator. A guard that evaluates to FALSE disables that ALT branch. Currently the parser does not support this syntax.

Occam syntax

ALT
  running & in ? x
    ... only considered if running is TRUE
  (NOT paused) & tim ? AFTER timeout
    ... only considered if paused is FALSE
  TRUE & stop ? any
    ... always considered

Proposed Go mapping

select {
case x = <-in:   // wrap in: if running { ... }
case <-time.After(...):  // wrap in: if !paused { ... }
case any = <-stop:       // always active
}

For reflect.Select-based ALT replicators, disabled guards can be handled by setting the channel to nil (which makes a select case block forever, effectively disabling it).

Affected files

• bar.occ (lines 327, 338, 352, 399, 402)

Work required

1. Parser: Before parsing a channel input in ALT, check for expr & guard prefix
2. AST: Add a Guard expression field to ALT case nodes
3. Codegen: Conditionally enable/disable select cases based on guard value

Part of #43.