[mercury-users] Typeclass question

[Fergus Henderson]

On 19-May-2004, Anthony Lee SENYARD <anthls at cs.mu.OZ.AU> wrote:
> On Wed, May 19, 2004 at 01:01:16PM +1000, Fergus Henderson wrote:
> > 	:- type list_of_float ---> +(list(float)).
> > 
> > 	:- instance mytype(list_of_float, float) where [
> > 		(lookup(+L, X, Y) :- list__index0_det(L, X, Y))
> > 	].
> > 
> > Then instead of calling lookup(Foo, ...) you would call lookup(+Foo, ...).
> 
> I've been wanting to do the same thing myself and tried out the
> above solution with:
...
> main -->
>          { MyArray = array__array([1.0, 1.0, 0.0, 2.0]) },
>          { test__lookup(+(MyArray), 2, Result) },
>          io__write(Result).
> 
> ------------------------------------------------------------
> and got the following error message:
> 
> test.m:047: In predicate `test.main/2':
> test.m:047:   type error: unsatisfied typeclass constraint(s):
> test.m:047:   test.mytype((test.array_of_float), T).
> test.m:047: In predicate `test.main/2':
> test.m:047:   warning: unresolved polymorphism.
> test.m:047:   The variable with an unbound type was:
> test.m:047:       Result: T
> test.m:047:   The unbound type variable(s) will be implicitly
> test.m:047:   bound to the builtin type `void'.
> For more information, try recompiling with `-E'.
> 
> Did I do something wrong?  Thanks for any help....

Mercury's type class system doesn't have functional dependencies,
so the compiler can't infer that calling lookup on a value of type
array_of_float will give you back a result of type float.
You need to disambiguate the types, e.g. by using an explicit type
annotation.  For example, change the call to io__write to

          io__write(Result `with_type` float).

or

          io__write_float(Result).

-- 
Fergus Henderson                    |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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