doctrine of frustration of contract in english, american and iranian law (comparative study)

“pacta sunt servanda” is one of the most fundamental principles in the common law and iranian legal systems, which have been exposed to exceptions in the process of time. these exceptions are part of general doctrine of frustration. iranians exceptions to this rule are named as “ta`azzor” and “ta`assor” rules. doctrine of frustration in common law includes three subdivision theories: “impossibility of performance”, “frustration of purpose” and “impracticability” (hardship. all of these theories applied where a supervening event occurs. in english courts, only first two theories are accepted but third one is applicable in american courts. in imamieh jurisprudence and iranian law, “ta`azzor” rule in most aspects is similar to impossibility and “ta`assor” rule is somehow like impracticability. some iranian lawyers are said that we have no rule like “frustration of purpose” but i believe we can find traces of this theory in imamieh jurisprudence and according which it is part of “ta`azzor” rule.