[Haskell-beginners] Re: Enforcing Monad Laws

[Brandon S Allbery KF8NH]

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On 7/2/10 04:04 , Heinrich Apfelmus wrote:
> Jorden M wrote:
>> C++ `Concepts', which almost made it into the C++0x standard, are
>> roughly similar to Haskell type classes. The proposal for concepts in
>> C++ had a feature called axioms, which allow the programmer to specify
>> semantics on the functions the concept contains. This allows for
>> enforcing things such as the Monad Laws, as well as letting the
>> compiler make certain optimizations it may not have been able to make
>> without axiomatic guarantees.
> 
> I have a hard time imagining that axioms are being used to prove properties
> about programs in a language such as C++... :) Any pointers?

I just took a look at it, and Concepts look to me like Haskell
contexts/constraints; an example is:

> concept LessThanComparable<typename T> {
>   bool operator<(const T& x, const T& y);
> }
> 
> template<typename T>
> requires LessThanComparable<T>
> const T& min(const T& x, const T& y) {
>   return x < y? x : y;
> }

which is obviously just an Ord costraint on min().

(My initial thought before looking at this was that it was programming by
contract, or possibly just making invariants explicit; the latter wouldn't
actually prove anything, though.)

- -- 
brandon s. allbery     [linux,solaris,freebsd,perl]      allbery at kf8nh.com
system administrator  [openafs,heimdal,too many hats]  allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university      KF8NH
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