[mercury-users] boolean expressions from semidet predicates

[Peter Hawkins]

Jonathan Morgan wrote:

> On 5/16/06, Peter Schachte <schachte at csse.unimelb.edu.au> wrote:
> What I don't like about languages like C++ and Mercury, when compared
> to a language like Haskell, is that there is no good way (that I know
> of) to define a function to work with all types that define a given
> operator (or set of operators, like those defining equality, ordering
> and numberic operations, etc.).  C++ is worse, because it uses a
> single operator (+) to mean at least two different things (addition
> and string concatenation).
>
> If in Mercury I write the following and use type-inference I get
> complaints about type ambiguity between int and float, but if I remove
> either of them then it will specialise the definition to act either
> over ints, or over floats.  However, if I define the equivalent
> Haskell version (with implicit importing of the Prelude), type
> inference gives me f :: Num a => a -> a -> a.  While such a system
> requires the programmer to always use the same basic operators to mean
> the same thing (unlike the practise in C++) it does give constrained
> genericity for my function.

You _can_ do the same thing in Mercury using the typeclass system 
(modulo the "deriving" automatic thing haskell does for you). Nothing 
prevents the creation of a Num typeclass which has the appropriate 
operations and with int and float as members --- it's just not the way 
things are done at the moment. I assume we don't do this because 
typeclasses aren't optimized very well at the moment and this would 
require lots of type class dictionaries to be passed around.

Cheers,
Peter
--------------------------------------------------------------------------
mercury-users mailing list
post:  mercury-users at cs.mu.oz.au
administrative address: owner-mercury-users at cs.mu.oz.au
unsubscribe: Address: mercury-users-request at cs.mu.oz.au Message: unsubscribe
subscribe:   Address: mercury-users-request at cs.mu.oz.au Message: subscribe
--------------------------------------------------------------------------