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CVL commands

  • require exp - assume that exp is true at this point (i.e., the tool will only consider executions in which exp holds). For example, require e.msg.sender == admin would ignore any cases where the caller is not the admin.

  • assert exp - check if exp is true, and output a counterexample if there is an input for which it is false. For example, assert newBalance == oldBalance + amount will check that a balance always equals the correct value after a transfer (or will report an error, such as when an account transfers to itself and this assertion doesn't hold). The optional string argument is displayed when the assertion is violated. 

  • foo@withrevert(args) or invoke foo(args)

    - simulate a function named foo with arguments args allowing it to revert.

  • foo(args)or sinvoke foo(args)

    - simulate a function named foo with arguments args and assume that it does not revert. This syntax is equivalent to:

foo@withrevert(arg); // same as invoke foo(arg) require !lastReverted;

Boolean operators that do not exist in Solidity

  • Implication: => 
    A => B evaluates to true if either A is false or B is true. For example, assert e.sender != admin => lastReverted could check that if the caller is not the admin, a given function must revert in all cases.

  • Bi-directional implication: <=>
     A <=> B evaluates to true if and only if A => B && B => A. For example,  assert e.sender != admin <=> lastReverted checks that if the caller is not the admin, a given function reverts and that if the function reverted, it must be the case that the sender was not the admin (basically saying that this is the only reason it would revert).

If Then Else (ITE) expressions

  • CVL supports If Then Else (ITE) expressions that can be used at any place where an expression is expected (e.g., right-hand side of an assignment statement).

  • It uses the syntax cond ? e1 : e2 where cond is a boolean expression and e1 and e2 are arbitrary expressions of the same type.

  • It is a type error if cond is not of type bool (e.g. 5 ? 1 : 0) or if e1 and e2 are of different types (e.g. true ? 1 : false).

  • Here are a few examples showing the use of ITE expressions:

methods { inc() envfree dec() envfree } // In definitions definition ABOVE_TEN(uint256 x) returns bool = x > 10 ? true : false; definition SIGNED_INT_TO_MATHINT(uint256 x) returns mathint = x >= 2^255 ? x - 2^256 : x; rule checkITE(mathint r) { // In assignment expressions uint x = 4 > 5 ? 24 : 42; uint y = SIGNED_INT_TO_MATHINT(2^255); bool f = ABOVE_TEN(10); uint z = r > 5 ? inc() : dec(); // In assert assert z == 11; bool a; bool b; assert (z > y ? 111 : 222) == ((a && b) ? 333 : 111); // In forall exp assert forall uint256 x. ((a || b) ? 100 : 500) * x == 100 * x; }

Note: When cond evaluates to true, only expression e1 is executed, i.e., only the then branch is executed while e2 is ignored. Similarly, when cond evaluates to false, only expression e2 is executed, i.e., only the else branch is executed.