Mobile Safe Ambients

Abstract

Two forms of interferences are individuated in Cardelli and Gordon's Mobile Ambients (MA): plain interferences, which are similar to the interferences one finds in CCS and π-calculus; and grave interferences, which are more dangerous and may be regarded as programming errors. To control interferences, the MA movement primitives are modified; the resulting calculus is called Mobile Safe Ambients (SA). The modification also has computational significance. In the MA interaction rules, an ambient may enter, exit, or open another ambient. The second ambient undergoes the action; it has no control on when the action takes place. In SA this is rectified: any movement takes place only if both participants agree. Existing type systems for MA can be easily adapted to SA. The type systems for controlling mobility, however, appear to be more powerful in SA, in that (i) type systems for MA may give more precise information when transplanted onto SA, and (ii) new type systems may be defined. Two type systems are presented that remove all grave interferences. Other advantages of SA are: a useful algebraic theory; programs sometimes more robust (they require milder conditions for correctness) and/or simpler. All these points are illustrated in several examples.