gistfile1.txt

contractStack <- array!
withContract <- { |impl|
    { |this., args...|
        contractStack.push (object "require" array! "ensure" array! "body" null)
        try {
            impl.apply this args...
            (last contractStack).require.forEach { |f| f! }
            out <- (last contractStack).body!
            (last contractStack).ensure.forEach { |f| f out }
            out
        } finally {
            contractStack.pop!
        }
    }
}
require <- { |impl| (last contractStack).require.push impl }
ensure <- { |impl| (last contractStack).ensure.push impl }
body <- { |impl| (last contractStack).body := impl }

abs <- withContract { |n|
    require { instanceof n number }
    ensure { |out| >= out 0 }
    body {
        if (< n 0) { - n } { n }
    }
}