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contract-prover

Overview

This package contains a tool to verify and add contracts in FlatCurry programs by proving contracts with an SMT solver. If a proof is successful, no contract check will be performed at run time, otherwise a dynamic (strict) contract check will be added. The static verification of contracts has the advantage that the resulting program will run more efficiently compared to a program with dynamic contract checking only.

A detailed description of the ideas of this tool can be found in the journal paper:

M. Hanus: Combining Static and Dynamic Contract Checking for Curry, Fundamenta Informaticae 173(4), pp. 285-314, 2020, DOI 10.3233/FI-2020-1925

The tool is invoked via

> curry-contracts <Curry module>

This analyzes the FlatCurry code of the module, attempts to prove the contracts in this module (unless option --add is set), and adds dynamic contract checking if a proof is not successful. Finally, the FlatCurry program is replaced by the transformed version (unless option --target=NONE is set). Hence, this tool might be integrated into the compilation chain of a Curry system. In addition to the transformation of the FlatCurry program, successful proofs will be stored in files so that they can be re-used by other tools. For instance, if the postcondition of an operation f defined in module M is verified, a file PROOF_M_f_SatisfiesPostCondition.smt is generated. This file contains the SMT script of this proof.

The directory examples contains various examples where the contract prover can eliminate all contracts at compile time.

Dynamic contracts

As mentioned above, the tool adds checks for contracts at run time if it cannot be verified that these contracts always hold. This is done by transforming the FlatCurry code of the modules by inserting Curry code which checks the contracts when the correspondings operations are invoked at run time. When a contract is not satisfied, this checking code terminates the program with an error message about the violated contract. The error message also shows the arguments (and result in case of a violated postcondition) of the operation, if this is possible with the operation Prelude.show (i.e., if the types are not polymorphic and not functional). Thus, for user-defined types occurring in contracts, it is assumed that instances of class Show exist for these types.

Web Demo Installation

If you want to try the contract prover on simple programs via a web interface, you can use a Web Demo Installation of the prover.

Implementation

In contrast to the first contract prover described in LOPSTR 2017 paper, which tried to remove contracts added by the Curry preprocessor, this version tries to verify contracts before they are added to the Curry program and adds dynamic checks only for unverified contracts (see the auxiliary operations defined in include/ContractChecker.curry).

The strategy is as follows:

  1. For each postcondition f'post, try to verify it. If this is not successful, a dynamic check is added to f.

  2. For each function call (f args), where a preconditon f'pre exists, try to verify this precondition in the given context of the call. If it cannot be verified, transform the function call into

    checkPreCond (f args) (f'pre args) "f" (args)

    See include/ContractChecker.curry for the definition of checkPreCond.

Notes:

  • Contracts can also be stored in separate files. When checking a module m, if there is a Curry module m_SPEC in the load path for module m or in the package directory include, the contents of m_SPEC is added to m before it is checked.

  • Contracts for operators can also be specified by operations named by op_xh1...hn', where each hi is a two digit hexadecimal number and the name of the operator corresponds to the ord values of h1...hn. For instance, a precondition for the operator !! can be named op_x2121'pre. To generate such names automatically, one can use the option --name of the tool.

Directories of the package:

  • examples: some examples (and test suite)

  • include: an include file for the SMT solver and a small Curry program containing operations which perform dynamic contract checking for unverified contracts

  • src: source code of the implementation