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digitalmars.D.learn - Contracts inheritance

reply "Eyyub" <eyyub.pangearaion gmail.com> writes:
Hai,

After watching Walter's video at Lang.NEXT, I have wanted to know 
how contracts inheritance works.

In the following code, I don't understand why foo.bar(2) 
works...but with the sames contracts in the foo function it 
doesn't work.

http://paste.pocoo.org/show/3Ab5IiQk6hTiJ0jAFZWv/

Thanks
Apr 13 2012
parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 04/13/2012 03:07 PM, Eyyub wrote:

 Hai,

 After watching Walter's video at Lang.NEXT, I have wanted to know how
 contracts inheritance works.

 In the following code, I don't understand why foo.bar(2) works...but
 with the sames contracts in the foo function it doesn't work.

 http://paste.pocoo.org/show/3Ab5IiQk6hTiJ0jAFZWv/

 Thanks


Here is the for convenience:

import std.stdio;

interface IFoo
{
	void bar(int a)
	in
	{
		assert(a != 1);
	}
}

class Foo : IFoo
{
	this()
	{}
	
	override void bar(int a)
	in
	{
		assert(a != 2);
	}
	body
	{
		writeln(a); // 2
	}
}

void foo(int a)
in
{
		assert(a == 2);
		assert(a < 2);
}
body
{
	writeln(a);
}
		
		
void main()
{
	foo(2); // don't pass
	Foo foo2 = new Foo;
	foo2.bar(2); // pass
}

foo(2) cannot work because of the second assert in the 'in' contract.

foo2.bar(2) passes because passing a single 'in' contract is sufficient. 
The 'in' contract of IFoo.bar() requires that a != 1 and it is satisfied 
for 2 so bar() can be called with argument 2.

Ali
Apr 13 2012
parent reply "Eyyub" <Eyyub.pangearaion gmail.com> writes:
On Friday, 13 April 2012 at 22:23:25 UTC, Ali Çehreli wrote:

 On 04/13/2012 03:07 PM, Eyyub wrote:

 Hai,

 After watching Walter's video at Lang.NEXT, I have wanted to 
 know how
 contracts inheritance works.

 In the following code, I don't understand why foo.bar(2) 
 works...but
 with the sames contracts in the foo function it doesn't work.

 http://paste.pocoo.org/show/3Ab5IiQk6hTiJ0jAFZWv/

 Thanks


 Here is the for convenience:

 import std.stdio;

 interface IFoo
 {
 	void bar(int a)
 	in
 	{
 		assert(a != 1);
 	}
 }

 class Foo : IFoo
 {
 	this()
 	{}
 	
 	override void bar(int a)
 	in
 	{
 		assert(a != 2);
 	}
 	body
 	{
 		writeln(a); // 2
 	}
 }

 void foo(int a)
 in
 {
 		assert(a == 2);
 		assert(a < 2);
 }
 body
 {
 	writeln(a);
 }
 		
 		
 void main()
 {
 	foo(2); // don't pass
 	Foo foo2 = new Foo;
 	foo2.bar(2); // pass
 }

 foo(2) cannot work because of the second assert in the 'in' 
 contract.

 foo2.bar(2) passes because passing a single 'in' contract is 
 sufficient. The 'in' contract of IFoo.bar() requires that a != 
 1 and it is satisfied for 2 so bar() can be called with 
 argument 2.

 Ali


Mhh ok thanks.
So, why overrides a contract if the assert is not checked at 
runtime ?

Eyyub,
Apr 16 2012
parent Mike Wey <mike-wey example.com> writes:
On 04/16/2012 08:15 PM, Eyyub wrote:

 On Friday, 13 April 2012 at 22:23:25 UTC, Ali Çehreli wrote:

 On 04/13/2012 03:07 PM, Eyyub wrote:

 Hai,

 After watching Walter's video at Lang.NEXT, I have wanted to know how
 contracts inheritance works.

 In the following code, I don't understand why foo.bar(2) works...but
 with the sames contracts in the foo function it doesn't work.

 http://paste.pocoo.org/show/3Ab5IiQk6hTiJ0jAFZWv/

 Thanks


 Here is the for convenience:

 import std.stdio;

 interface IFoo
 {
 void bar(int a)
 in
 {
 assert(a != 1);
 }
 }

 class Foo : IFoo
 {
 this()
 {}

 override void bar(int a)
 in
 {
 assert(a != 2);
 }
 body
 {
 writeln(a); // 2
 }
 }

 void foo(int a)
 in
 {
 assert(a == 2);
 assert(a < 2);
 }
 body
 {
 writeln(a);
 }


 void main()
 {
 foo(2); // don't pass
 Foo foo2 = new Foo;
 foo2.bar(2); // pass
 }

 foo(2) cannot work because of the second assert in the 'in' contract.

 foo2.bar(2) passes because passing a single 'in' contract is
 sufficient. The 'in' contract of IFoo.bar() requires that a != 1 and
 it is satisfied for 2 so bar() can be called with argument 2.

 Ali


 Mhh ok thanks.
 So, why overrides a contract if the assert is not checked at runtime ?

 Eyyub,


A contract of an override can only lower the restrictions.
So when the in contract for IFoo.bar passes there is no need to check 
the contract of Foo.bar since it must accept anything that IFoo.bar 
whould accept.
When the contract for IFoo.bar fails then the contract for Foo.bar is 
checked, since it may accept more then IFoo.bar.

-- 
Mike Wey
Apr 16 2012