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[REBOL] Design by contract

From: dankelg8:cs:man:ac at: 7-Feb-2001 14:13

Here's a Design By Contract script that I hacked together yesterday. For those who don't know, Design By Contract is a software pattern used to enforce preconditions and postconditions of a function. For example, in Rebol there's no way currently to define your own types. A function that does vector multiplication can look like this: vector-mul: func [v1 [block!] v2 [block!]] [...] The problem here is that a block can contain anything. DBC offers a solution to this: vector-mul: cfunc [v1 [block!] v2 [block!]] [ <preconditions> ][ <function body> ][ <postconditions> ] Here's what it could look like: vector-mul: cfunc [a [block!] b [block!]] [ (length? a) = (length? b) fold func [x y] [(number? x) and y] true a fold func [x y] [(number? x) and y] true b ][ <function body> ][ (length? cres) = (length? a) ] fold checks if all the elements of the block are numbers. cres is the result of evaluating the function body. If either the pre- or postcondition fails, an error will be thrown.
>> vector-mul [1 2 none 4] [3 4 5 6]
** User Error: Precondition violated! ** Near: vector-mul [1 2 none 4] [3 4 5 6] Fold above is defined like: fold: func [ combine [any-function!] init [any-type!] series [series!] /local res ][ res: init foreach element series [res: combine element res] res ] Preconditions and postconditions can be turned off such that cfunc will return a normal func. So if speed is critical, the tests can be done while debugging. Anyway, here's the code. Hopefully someone of you have got a better version or have ideas of how to make it better. There are a few problems with this one, e.g. cres is in the global context Usage: contract/enforce my-func: cfunc .... as above REBOL [ Title: "Design by contract" Date: 6-feb-2001 Example: [ my-squareroot: cfunc [ x [number!] ][ x >= 0 ][ square-root x ][ (cres * cres) = x ] ] ] contract: make object! [ pre-check: post-check: false enforce: does [pre-check: post-check: true] set 'cfunc func [ arguments [block!] pre-conditions [block!] body [block!] post-conditions [block!] /local new-body new-args ][ if pre-check [ insert body compose/deep [if not all [(pre-conditions)] [throw make error! "Precondition violated!"]] ] if post-check [ body: compose/deep [cres: do [(body)]] append body compose/deep [ if not all [(post-conditions)] [throw make error! "Postcondition violated!"] return cres ] ] if all [any [pre-check post-check] not find/only arguments [catch]] [insert/only arguments [catch]] func arguments body ] ]