Contract programming

[John Colvin via Digitalmars-d-learn]

On Wednesday, 17 December 2014 at 21:17:57 UTC, John Colvin wrote:
> On Wednesday, 17 December 2014 at 21:07:12 UTC, Oleg wrote:
>> Is this behavior normal or it's bug?
>>
>> [code]
>>
>> import std.stdio;
>>
>> class A
>> {
>>    float func( float x )
>>    in
>>    {
>>        assert( x > 0 );
>>        stderr.writeln( "A.func in block" );
>>    }
>>    body
>>    {
>>        stderr.writeln( "A.func body block" );
>>        return x / 3;
>>    }
>> }
>>
>> class B : A
>> {
>>    override float func( float x )
>>    in
>>    {
>>        assert( x < 10 );
>>        stderr.writeln( "B.func in block" );
>>    }
>>    body
>>    {
>>        stderr.writeln( "B.func body block" );
>>        return x * 3;
>>    }
>> }
>>
>> void main()
>> {
>>    auto a = new B;
>>    auto v = a.func( 3.14 );
>> }
>>
>> [output]
>>
>> A.func in block
>> B.func body block
>>
>> In class B 'in' block not called, but if add 'out' block it 
>> called both for A and B classes.
>
> Only 1 of the in contracts must be satisfied and it appears 
> that in this case the A.foo one was checked first. Seeing as it 
> passed, there was no need to check the B.foo in contract. 
> Imagine it as an OR of in contracts.
>
> All out contracts must be satisfied, hence both must be run to 
> check. Images it as an AND of the out contracts.
>
> See http://dlang.org/contracts.html

A corollary of this is that you have to be careful about 
side-effects inside contracts. Normally contracts should be pure.